








             AAnn IInnttrroodduuccttiioonn ttoo tthhee UUNNIIXX SShheellll


                        _S_. _R_. _B_o_u_r_n_e
                      Murray Hill, NJ

            _(_U_p_d_a_t_e_d _f_o_r _4_._3_B_S_D _b_y _M_a_r_k _S_e_i_d_e_n_)
           _(_F_u_r_t_h_e_r _u_p_d_a_t_e_d _b_y _P_e_r_r_y _E_. _M_e_t_z_g_e_r_)


                          _A_B_S_T_R_A_C_T


     The _s_h_e_l_l is a command programming  language  that
     provides  an  interface  to  the UNIX(R) operating
     system.  Its features include control-flow  primi-
     tives,  parameter  passing,  variables  and string
     substitution.  Constructs such as _w_h_i_l_e_,  _i_f  _t_h_e_n
     _e_l_s_e_,  _c_a_s_e and _f_o_r are available.  Two-way commu-
     nication is possible between the  _s_h_e_l_l  and  com-
     mands.   String-valued  parameters, typically file
     names or flags, may be passed  to  a  command.   A
     return code is set by commands that may be used to
     determine control-flow, and  the  standard  output
     from a command may be used as shell input.

     The _s_h_e_l_l can modify the environment in which com-
     mands run.  Input and output can be redirected  to
     files,  and  processes  that  communicate  through
     `pipes' can be invoked.   Commands  are  found  by
     searching  directories  in  the  file  system in a
     sequence that can be defined by  the  user.   Com-
     mands can be read either from the terminal or from
     a file, which  allows  command  procedures  to  be
     stored for later use.





-----------
  This  paper  was updated in 2010 to reflect most
features of modern POSIX shells, which all  follow
the  design  of  S.R.  Bourne's  original  v7 Unix
shell.  Among these are ash, bash, ksh and others.
Typically  one  of  these  will  be  installed  as
/bin/sh on a modern system.  It does not  describe
the  behavior of the c shell (csh).  If it's the c
shell (csh) you're interested in, a good place  to
begin  is  William Joy's paper "An Introduction to
the C shell" (USD:4).









USD:3-2                    An Introduction to the UNIX Shell


11..00 IInnttrroodduuccttiioonn

The  shell is both a command language and a programming lan-
guage that provides an interface to the UNIX operating  sys-
tem.   This  memorandum  describes,  with examples, the UNIX
shell.  The  first  section  covers  most  of  the  everyday
requirements  of terminal users.  Some familiarity with UNIX
is an advantage when reading this section; see, for example,
"UNIX for beginners".1 Section 2 describes those features of
the shell primarily intended for  use  within  shell  proce-
dures.    These  include  the  control-flow  primitives  and
string-valued variables provided by the shell.  A  knowledge
of  a programming language would be a help when reading this
section.  The last section describes the more advanced  fea-
tures  of  the shell.  References of the form "see _p_i_p_e (2)"
are to a section of the UNIX manual.2

11..11 SSiimmppllee ccoommmmaannddss

Simple commands consist of one or more  words  separated  by
blanks.   The  first  word  is the name of the command to be
executed; any remaining words are passed as arguments to the
command.  For example,

          who

is  a command that prints the names of users logged in.  The
command

          ls -l

prints a list of files in the current directory.  The  argu-
ment  _-_l  tells _l_s to print status information, size and the
creation date for each file.

11..22 IInnppuutt oouuttppuutt rreeddiirreeccttiioonn

Most commands produce output on the standard output that  is
initially  connected  to  the  terminal.  This output may be
sent to a file by writing, for example,

          ls -l >file

The notation _>_f_i_l_e is interpreted by the shell  and  is  not
passed  as  an  argument to _l_s_.  If _f_i_l_e does not exist then
the shell creates it; otherwise  the  original  contents  of
_f_i_l_e  are  replaced  with the output from _l_s_.  Output may be
appended to a file using the notation

          ls -l >>file

In this case _f_i_l_e is also created if  it  does  not  already
exist.










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The  standard  input  of  a command may be taken from a file
instead of the terminal by writing, for example,

          wc <file

The command _w_c reads its standard input (in this case  redi-
rected from _f_i_l_e) and prints the number of characters, words
and lines found.  If only the number of  lines  is  required
then

          wc -l <file

could be used.

11..33 PPiippeelliinneess aanndd ffiilltteerrss

The  standard  output of one command may be connected to the
standard input of another by writing  the  `pipe'  operator,
indicated by ||, as in,

          ls -l || wc

Two commands connected in this way constitute a _p_i_p_e_l_i_n_e and
the overall effect is the same as

          ls -l >file; wc <file

except that no _f_i_l_e is used.  Instead the two _p_r_o_c_e_s_s_e_s  are
connected  by  a pipe (see _p_i_p_e(2)) and are run in parallel.
Pipes are unidirectional and synchronization is achieved  by
halting _w_c when there is nothing to read and halting _l_s when
the pipe is full.

A _f_i_l_t_e_r is a command that reads its standard input,  trans-
forms  it in some way, and prints the result as output.  One
such filter, _g_r_e_p_, selects from its input those  lines  that
contain some specified string.  For example,

          ls || grep old

prints  those lines, if any, of the output from _l_s that con-
tain the string _o_l_d_.  Another useful filter  is  _s_o_r_t.   For
example,

          who || sort

will print an alphabetically sorted list of logged in users.

A pipeline may consist of more than two commands, for  exam-
ple,

          ls || grep old || wc -l

prints  the  number  of  file names in the current directory









USD:3-4                    An Introduction to the UNIX Shell


containing the string _o_l_d_.

11..44 BBaacckkggrroouunndd ccoommmmaannddss

To execute a command (or pipeline) the shell  normally  cre-
ates the new _p_r_o_c_e_s_s_e_s and waits for them to finish.  A com-
mand may be run without waiting for it to finish.  For exam-
ple,

          cc pgm.c &

calls  the C compiler to compile the file _p_g_m_._c_.  The trail-
ing && is an operator that instructs the shell  not  to  wait
for  the  command  to  finish.  To help keep track of such a
process the shell reports its job  number  (see  below)  and
process  id  following its creation.  Such a command is said
to be running in the _b_a_c_k_g_r_o_u_n_d.   By  contrast,  a  command
executed  without  the  && is said to be running in the _f_o_r_e_-
_g_r_o_u_n_d.

A list of currently active  processes,  including  ones  not
associated with the current shell, may be obtained using the
_p_s(1) command.

11..55 FFiillee nnaammee ggeenneerraattiioonn

Many commands accept arguments which are  file  names.   For
example,

          ls -l main.c

prints information relating to the file _m_a_i_n_._c.

The shell provides a mechanism for generating a list of file
names that match a pattern.  For example,

          ls -l *.c

generates, as arguments to _l_s_, all file names in the current
directory that end in _._c_.  The character * is a pattern that
will match any string including the null string.  In general
_p_a_t_t_e_r_n_s are specified as follows.

     **       Matches  any string of characters including the
             null string.

     ??       Matches any single character.

-----------
 Even after execution, one may move commands  from
the  foreground  to the background, or temporarily
suspend their execution (which is known  as  _s_t_o_p_-
_p_i_n_g  a  command.   This is described in detail in
section 3.10 on _J_o_b _C_o_n_t_r_o_l..









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     [[......]]   Matches any one of the characters enclosed.   A
             pair  of  characters  separated by a minus will
             match any character lexically between the pair.

For example,

          [a-z]*

matches  all  names  in the current directory beginning with
one of the letters _a through _z_.

          /usr/fred/test/?

matches all names in the directory //uussrr//ffrreedd//tteesstt that  con-
sist  of  a single character.  If no file name is found that
matches the pattern then the pattern is  passed,  unchanged,
as an argument.

This  mechanism  is useful both to save typing and to select
names according to some pattern.  It may  also  be  used  to
find files.  For example,

          echo /usr/fred/*/core

finds and prints the names of all _c_o_r_e files in sub-directo-
ries of //uussrr//ffrreedd..  (_e_c_h_o is a standard  UNIX  command  that
prints  its arguments, separated by blanks.)  This last fea-
ture can be expensive, requiring a scan of all  sub-directo-
ries of //uussrr//ffrreedd..

There  is  one exception to the general rules given for pat-
terns.  The character `..'  at the start of a file name  must
be explicitly matched.

          echo *

will  therefore echo all file names in the current directory
not beginning with `..'.

          echo ..*

will echo all those file names that begin  with  `..'.   This
avoids inadvertent matching of the names `..' and `....'  which
mean `the current  directory'  and  `the  parent  directory'
respectively.   (Notice  that  _l_s suppresses information for
the files `..' and `....'.)

Finally, the tilde character, `~~', may be used  to  indicate
the home directory of a user.  The `~~' at the beginning of a
path name followed by a non-alphabetic character expands  to
the  current  user's home directory.  If the `~~' is followed
by a login name, it expands to the named user's home  direc-
tory.  For example:










USD:3-6                    An Introduction to the UNIX Shell


          ls ~
          cd ~egbert/

will list the contents of the user's home directory and then
change to the home directory of the user ``egbert''.

11..66 QQuuoottiinngg

Characters that have a special meaning to the shell, such as
<<  >> ** ?? || &&,, are called metacharacters.  A complete list of
metacharacters is given in appendix B.  Any  character  pre-
ceded  by  a  \\  is _q_u_o_t_e_d and loses its special meaning, if
any.  The \\ is elided so that

          echo \?

will echo a single ??,, and

          echo \\

will echo a single \\..  To allow long strings to be continued
over more than one line the sequence \\nneewwlliinnee is ignored.

\\  is  convenient  for quoting single characters.  When more
than one character needs  quoting  the  above  mechanism  is
clumsy  and  error  prone.   A  string  of characters may be
quoted by enclosing the string between single  quotes.   For
example,

          echo xx'****'xx

will echo

          xx****xx

The  quoted  string  may  not contain a single quote but may
contain newlines, which are preserved.  This quoting  mecha-
nism is the most simple and is recommended for casual use.

A third quoting mechanism using double quotes is also avail-
able that  prevents  interpretation  of  some  but  not  all
metacharacters.   Discussion  of  the details is deferred to
section 3.5.

11..77 PPrroommppttiinngg

When the shell is used from  a  terminal  it  will  issue  a
prompt  before reading a command.  By default this prompt is
`$$ '.  It may be changed by saying, for example,

          PS1="yesdear$ "

that sets the prompt to be the string _y_e_s_d_e_a_r_$_.  If  a  new-
line  is  typed  and  further input is needed then the shell









An Introduction to the UNIX Shell                    USD:3-7


will issue the prompt `>> '.  Sometimes this can be caused by
mistyping  a  quote mark.  If it is unexpected then entering
the interrupt character (typically  CONTROL-C)  will  return
the  shell  to  read  another  command.   This prompt may be
changed by saying, for example,

          PS2=more

Entering the interrupt character may also be used to  termi-
nate most programs running as the current foreground job.

(PS1 and PS2 are _s_h_e_l_l _v_a_r_i_a_b_l_e_s, which will be described in
section 2.4 below.)

11..88 TThhee sshheellll aanndd llooggiinn

Following _l_o_g_i_n(1) the shell is called to read  and  execute
commands  typed at the terminal.  If the user's login direc-
tory contains the file ..pprrooffiillee then it is assumed  to  con-
tain  commands  and  is read by the shell before reading any
commands from the terminal.

(Most versions of the shell also specify a file that is read
and executed on start-up whether or not the shell is invoked
by login.  The ENV shell variable, described in section  2.4
below,  can  be used to override the name of this file.  See
the shell manual page for further information.)

11..99 SSuummmmaarryy


     +o    llss
          Print the names of files in the current directory.

     +o    llss >>ffiillee
          Put the output from _l_s into _f_i_l_e_.

     +o    llss || wwcc --ll
          Print  the  number  of files in the current direc-
          tory.

     +o    llss || ggrreepp oolldd
          Print those file names containing the string  _o_l_d_.

     +o    llss || ggrreepp oolldd || wwcc --ll
          Print  the number of files whose name contains the
          string _o_l_d_.

     +o    cccc ppggmm..cc &&
          Run _c_c in the background.













USD:3-8                    An Introduction to the UNIX Shell


22..00 SShheellll ssccrriippttss

The shell may be used to read and execute commands contained
in a file.  For example,

          sh file [ args ...... ]

calls  the shell to read commands from _f_i_l_e_.  Such a file is
called a _s_h_e_l_l _s_c_r_i_p_t_.  Arguments may be supplied  with  the
call and are referred to in _f_i_l_e using the positional param-
eters $$11,, $$22,, ........

For example, if the file _w_g contains

          who || grep $1

then

          sh wg fred

is equivalent to

          who || grep fred


UNIX files have three independent  attributes,  _r_e_a_d_,  _w_r_i_t_e
and  _e_x_e_c_u_t_e_.  The UNIX command _c_h_m_o_d(1) may be used to make
a file executable.  For example,

          chmod +x wg

will ensure that the file _w_g has execute status.   Following
this, the command

          wg fred

is equivalent to

          sh wg fred

This  allows  shell  scripts  and  other programs to be used
interchangeably.  In either case a new process is created to
run the command.

The  `##'  character  is  used  as a comment character by the
shell.  All characters following  the  `#'  on  a  line  are
ignored.

A  typical  modern system has several different shells, some
with differing command syntax, and it is desirable to  spec-
ify which one should be invoked when an executable script is
invoked.  If the special comment











An Introduction to the UNIX Shell                    USD:3-9


          #!/_p_a_t_h/_t_o/_i_n_t_e_r_p_r_e_t_e_r

appears as the first line in a script, it is used to specify
the  absolute  pathname  of the shell (or other interpreter)
that should be used to execute the file.   (Without  such  a
line,  //bbiinn//sshh  is assumed.)  It is best if a script explic-
itly states what shell it is intended for in this manner.

As well as providing names for  the  positional  parameters,
the number of positional parameters to a script is available
as $$##..  The name of the file being executed is available  as
$$00..

A  special  shell parameter $$** is used to substitute for all
positional parameters except $$00..  A typical use of  this  is
to provide some default arguments, as in,

          nroff -T450 -ms $*

which simply prepends some arguments to those already given.
(The variable $$@@ also expands to  ``all  positional  parame-
ters'', but is subtly different when expanded inside quotes.
See section 3.5, below.)

22..11 CCoonnttrrooll ffllooww -- ffoorr

A frequent use of shell scripts is to loop through the argu-
ments  ($$11,, $$22,, ......)  executing commands once for each argu-
ment.  An example of such a script is _t_e_l that searches  the
file //uussrr//sshhaarree//tteellnnooss that contains lines of the form

          ......
          fred mh0123
          bert mh0789
          ......

The text of _t_e_l is

          #!/bin/sh

          for i
          do
               grep $i /usr/share/telnos
          done

The command

          tel fred

prints  those  lines  in  //uussrr//sshhaarree//tteellnnooss that contain the
string _f_r_e_d_.

          tel fred bert










USD:3-10                   An Introduction to the UNIX Shell


prints those lines containing _f_r_e_d  followed  by  those  for
_b_e_r_t_.

The ffoorr loop notation is recognized by the shell and has the
general form

          ffoorr _n_a_m_e iinn _w_1 _w_2 ......
          ddoo _c_o_m_m_a_n_d_-_l_i_s_t
          ddoonnee

A _c_o_m_m_a_n_d_-_l_i_s_t is a sequence of one or more simple  commands
separated or terminated by a newline or semicolon.  Further-
more, reserved words like ddoo and ddoonnee  are  only  recognized
following  a newline or semicolon.  _n_a_m_e is a shell variable
that is set to the words _w_1 _w_2 ...... in  turn  each  time  the
_c_o_m_m_a_n_d_-_l_i_s_t  following ddoo is executed.  If iinn _w_1 _w_2 ......  is
omitted then the loop is executed once for  each  positional
parameter; that is, iinn _$_* is assumed.

Another  example  of  the  use of the ffoorr loop is the _c_r_e_a_t_e
command whose text is

          for i do >$i; done

The command

          create alpha beta

ensures that two empty files _a_l_p_h_a and _b_e_t_a  exist  and  are
empty.   The notation _>_f_i_l_e may be used on its own to create
or clear the contents of a file.  Notice also that  a  semi-
colon (or newline) is required before ddoonnee..

22..22 CCoonnttrrooll ffllooww -- ccaassee

A  multiple way branch is provided for by the ccaassee notation.
For example,

          case $# in
               1)   cat >>$1 ;;
               2)   cat >>$2 <$1 ;;
               *)   echo 'usage: append [ from ] to' ;;
          esac

is an _a_p_p_e_n_d command.  When called with one argument as

          append file

$$## is the string _1 and the standard input is copied onto the
end of _f_i_l_e using the _c_a_t command.

          append file1 file2

appends  the contents of _f_i_l_e_1 onto _f_i_l_e_2_.  If the number of









An Introduction to the UNIX Shell                   USD:3-11


arguments supplied to _a_p_p_e_n_d is other than 1  or  2  then  a
message is printed indicating proper usage.

The general form of the ccaassee command is

          ccaassee _w_o_r_d iinn
               _p_a_t_t_e_r_n)) _c_o_m_m_a_n_d_-_l_i_s_t;;;;
               ......
          eessaacc

The  shell  attempts to match _w_o_r_d with each _p_a_t_t_e_r_n_, in the
order in which the patterns appear.  If a match is found the
associated  _c_o_m_m_a_n_d_-_l_i_s_t  is  executed  and execution of the
ccaassee is complete.  Since * is the pattern that  matches  any
string it can be used for the default case.

A  word of caution: no check is made to ensure that only one
pattern matches the case argument.  The  first  match  found
defines  the set of commands to be executed.  In the example
below the commands following the second * will never be exe-
cuted.

          case $# in
               *) ...... ;;
               *) ...... ;;
          esac


Another  example  of  the use of the ccaassee construction is to
distinguish between different forms  of  an  argument.   The
following example is a fragment of a _c_c command.

          for i
          do case $i in
                  -[ocs])     ...... ;;
                  -*)    echo "unknown flag $i" ;;
                  *.c)   /lib/c0 $i ...... ;;
                  *)     echo "unexpected argument $i" ;;
             esac
          done


To  allow  the same commands to be associated with more than
one pattern the ccaassee command provides for  alternative  pat-
terns separated by a ||.  For example,

          case $i in
               -x||-y)    ......
          esac

is equivalent to












USD:3-12                   An Introduction to the UNIX Shell


          case $i in
               -[xy])    ......
          esac


The usual quoting conventions apply so that

          case $i in
               \?)  ......

will match the character ??..

22..33 HHeerree ddooccuummeennttss

The   shell   script  _t_e_l  in  section  2.1  uses  the  file
//uussrr//sshhaarree//tteellnnooss to supply the data for _g_r_e_p_.  An  alterna-
tive  is  to  include this data within the shell script as a
_h_e_r_e document, as in,

          for i
          do grep $i <<!
             ......
             fred mh0123
             bert mh0789
             ......
          !
          done

In this example the shell takes the lines between <<<<!! and  !!
as  the standard input for _g_r_e_p_.  The string !! is arbitrary,
the document being terminated by a line that consists of the
string following <<.

Parameters are substituted in the document before it is made
available to _g_r_e_p as illustrated  by  the  following  script
called _e_d_g_.

          ed $3 <<%
          g/$1/s//$2/g
          w
          %

The call

          edg string1 string2 file

is then equivalent to the command

          ed file <<%
          g/string1/s//string2/g
          w
          %

and  changes  all occurrences of _s_t_r_i_n_g_1 in _f_i_l_e to _s_t_r_i_n_g_2_.









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Substitution can be prevented using \ to quote  the  special
character $$ as in

          ed $3 <<+
          1,\$s/$1/$2/g
          w
          +

(This  version of _e_d_g is equivalent to the first except that
_e_d will print a ?? if there are no occurrences of the  string
$$11..)   Substitution  within a _h_e_r_e document may be prevented
entirely by quoting the terminating string, for example,

          grep $i <<'end'
          ......
          end

The document is presented without modification to _g_r_e_p_.   If
parameter  substitution  is  not required in a _h_e_r_e document
this latter form is more efficient.

22..44 SShheellll vvaarriiaabblleess

The  shell provides string-valued variables.  Variable names
begin with a letter  and  consist  of  letters,  digits  and
underscores.   Variables may be given values by writing, for
example,

          user=fred box=m000 acct=mh0000

which assigns values to the variables uusseerr,, bbooxx and aacccctt..  A
variable  may be set to the null string by saying, for exam-
ple,

          null=

The value of a variable is substituted by preceding its name
with $$; for example,

          echo $user

will echo _f_r_e_d_.

Variables may be used interactively to provide abbreviations
for frequently used strings.  For example,

          b=/usr/fred/bin
          mv pgm $b

will move the file _p_g_m from the  current  directory  to  the
directory   //uussrr//ffrreedd//bbiinn..    A  more  general  notation  is
-----------
Also known as _e_n_v_i_r_o_n_m_e_n_t _v_a_r_i_a_b_l_e_s,, sseeee  _e_n_v_i_r_o_n_-
_m_e_n_t((77))..









USD:3-14                   An Introduction to the UNIX Shell


available for parameter (or variable) substitution, as in,

          echo ${user}

which is equivalent to

          echo $user

and is used when the parameter name is followed by a  letter
or digit.  For example,

          tmp=/tmp/ps
          ps a >${tmp}a

will direct the output of _p_s to the file //ttmmpp//ppssaa,, whereas,

          ps a >$tmpa

would  cause  the  value  of the variable ttmmppaa to be substi-
tuted.

Except for $$?? the following are set initially by the  shell.
$$?? is set after executing each command.

     $$??      The  exit status (return code) of the last com-
             mand executed as a decimal string.   Most  com-
             mands  return  a  zero exit status if they com-
             plete successfully, otherwise a  non-zero  exit
             status  is  returned.   Testing  the  value  of
             return codes is dealt with later under  iiff  and
             wwhhiillee commands.

     $$##      The  number  of positional parameters (in deci-
             mal).  Used, for example, in the _a_p_p_e_n_d command
             to check the number of parameters.

     $$$$      The  process number of this shell (in decimal).
             Since process  numbers  are  unique  among  all
             existing  processes,  this string is frequently
             used to generate unique temporary  file  names.
             For example,

                       ps a >/tmp/ps$$
                       ......
                       rm /tmp/ps$$


     $$!!      The  process  number of the last process run in
             the background (in decimal).

     $$--      The current shell flags, such as --xx and --vv..

Some variables have a  special  meaning  to  the  shell  and
should be avoided for general use.









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     $$MMAAIILL   When  used interactively the shell looks at the
             file  specified  by  this  variable  before  it
             issues  a  prompt.   If  the specified file has
             been modified since it was last looked  at  the
             shell  prints  the message _y_o_u _h_a_v_e _m_a_i_l before
             prompting for the next command.  This  variable
             is  typically  set in the file ..pprrooffiillee,, in the
             user's login directory.  For example,

                       MAIL=/usr/spool/mail/fred


     $$HHOOMMEE   The default argument for the _c_d  command.   The
             current  directory is used to resolve file name
             references that do not begin with a //,,  and  is
             changed using the _c_d command.  For example,

                       cd /usr/fred/bin

             makes the current directory //uussrr//ffrreedd//bbiinn..

                       cat wn

             will  print on the terminal the file _w_n in this
             directory.  The command _c_d with no argument  is
             equivalent to

                       cd $HOME

             This  variable is also typically set in the the
             user's login profile.

     $$PPWWDD    The current working directory. Set  by  the  _c_d
             ccoommmmaanndd..

     $$PPAATTHH   A  list  of  directories  that contain commands
             (the _s_e_a_r_c_h _p_a_t_h).  Each time a command is exe-
             cuted  by  the  shell  a list of directories is
             searched for an executable file.  If  $$PPAATTHH  is
             not  set  then the current directory, //bbiinn, and
             //uussrr//bbiinn are searched  by  default.   Otherwise
             $$PPAATTHH  consists of directory names separated by
             ::..  For example,

                       PATH=::/usr/fred/bin::/bin::/usr/bin

             specifies that the current directory (the  null
             string before the first ::), //uussrr//ffrreedd//bbiinn,, //bbiinn
             and //uussrr//bbiinn are to be searched in that  order.
             In this way individual users can have their own
             `private' commands that are accessible indepen-
             dently  of  the current directory.  If the com-
             mand name contains  a  //  then  this  directory
             search is not used; a single attempt is made to









USD:3-16                   An Introduction to the UNIX Shell


             execute the command.

     $$PPSS11    The primary shell prompt  string,  by  default,
             `$$ '.

     $$PPSS22    The  shell prompt when further input is needed,
             by default, `>> '.

     $$IIFFSS    The set of characters used by _b_l_a_n_k _i_n_t_e_r_p_r_e_t_a_-
             _t_i_o_n (see section 3.5).

     $$EENNVV    The  shell  reads  and executes the commands in
             the file specified by this variable when it  is
             initially  started.   Unlike the ..pprrooffiillee file,
             these commands are executed by all shells,  not
             just  the one started at login.  (Most versions
             of the shell specify a filename that is used if
             ENV  is not explicitly set. See the manual page
             for your shell.)

22..55 TThhee tteesstt ccoommmmaanndd

The _t_e_s_t  command,  although  not  part  of  the  shell,  is
intended for use by shell programs.  For example,

          test -f file

returns  zero  exit  status if _f_i_l_e exists and non-zero exit
status otherwise.  In general _t_e_s_t evaluates a predicate and
returns  the  result  as  its exit status.  Some of the more
frequently used _t_e_s_t arguments are given here,  see  _t_e_s_t(1)
for a complete specification.

          test s         true if the argument _s is not the null string
          test -f file   true if _f_i_l_e exists
          test -r file   true if _f_i_l_e is readable
          test -w file   true if _f_i_l_e is writable
          test -d file   true if _f_i_l_e is a directory

The _t_e_s_t command is known as `[[' and may be invoked as such.
For aesthetic reasons, the command ignores a  close  bracket
`]]' given at the end of a command so

          [ -f filename ]

and

          test -f filename

are  completely equivalent.  Typically, the bracket notation
is used when _t_e_s_t  is  invoked  inside  shell  control  con-
structs.











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22..66 CCoonnttrrooll ffllooww -- wwhhiillee

The  actions  of the ffoorr loop and the ccaassee branch are deter-
mined by data available to the shell.  A wwhhiillee or uunnttiill loop
and  an  iiff tthheenn eellssee branch are also provided whose actions
are determined by the exit status returned by  commands.   A
wwhhiillee loop has the general form

          wwhhiillee _c_o_m_m_a_n_d_-_l_i_s_t_1
          ddoo _c_o_m_m_a_n_d_-_l_i_s_t_2
          ddoonnee


The  value tested by the wwhhiillee command is the exit status of
the last simple command following wwhhiillee..   Each  time  round
the loop _c_o_m_m_a_n_d_-_l_i_s_t_1 is executed; if a zero exit status is
returned then _c_o_m_m_a_n_d_-_l_i_s_t_2 is executed; otherwise, the loop
terminates.  For example,

          while [ $1 ]
          do ......
             shift
          done

is equivalent to

          for i
          do ......
          done

_s_h_i_f_t is a shell command that renames the positional parame-
ters $$22,, $$33,, ...... as $$11,, $$22,, ......  and loses $$11..

Another kind of use for the  wwhhiillee//uunnttiill  loop  is  to  wait
until some external event occurs and then run some commands.
In an uunnttiill loop the termination condition is reversed.  For
example,

          until [ -f file ]
          do sleep 300; done
          _c_o_m_m_a_n_d_s

will  loop  until  _f_i_l_e exists.  Each time round the loop it
waits  for  5  minutes  before  trying  again.   (Presumably
another process will eventually create the file.)

The  most recent enclosing loop may be exited with the bbrreeaakk
command, or the rest of the body skipped and the next itera-
tion begun with the ccoonnttiinnuuee command.

The commands _t_r_u_e(1) and _f_a_l_s_e(1) return 0 and non-zero exit
statuses respectively. They are sometimes of use in  control
flow, e.g.:










USD:3-18                   An Introduction to the UNIX Shell


          while true
          do date; sleep 5
          done

is an infinite loop that prints the date and time every five
seconds.

22..77 CCoonnttrrooll ffllooww -- iiff

Also available is a general conditional branch of the form,

          iiff _c_o_m_m_a_n_d_-_l_i_s_t
          tthheenn _c_o_m_m_a_n_d_-_l_i_s_t
          eellssee _c_o_m_m_a_n_d_-_l_i_s_t
          ffii

that tests the value returned by  the  last  simple  command
following iiff..

The iiff command may be used in conjunction with the _t_e_s_t com-
mand to test for the existence of a file as in

          if [ -f file ]
          then _p_r_o_c_e_s_s _f_i_l_e
          else _d_o _s_o_m_e_t_h_i_n_g _e_l_s_e
          fi


An example of the use of iiff,, ccaassee and ffoorr  constructions  is
given in section 2.10.

A multiple test iiff command of the form

          if ......
          then ......
          else if ......
               then ......
               else if ......
                    ......
                    fi
               fi
          fi

may be written using an extension of the iiff notation as,

          if ......
          then ......
          elif ......
          then ......
          elif ......
          ......
          fi











An Introduction to the UNIX Shell                   USD:3-19


The following example is an implementation of the _t_o_u_c_h com-
mand which changes the `last modified' time for  a  list  of
files.   The command may be used in conjunction with _m_a_k_e(1)
to force recompilation of a list of files.

          #!/bin/sh

          flag=
          for i
          do case $i in
                  -c)    flag=N ;;
                  *)     if [ -f $i ]
                    then cp $i junk$$; mv junk$$ $i
                    elif [ $flag ]
                    then echo file \'$i\' does not exist
                    else >$i
                    fi
              esac
          done

The --cc flag is used in  this  command  to  force  subsequent
files  to  be  created if they do not already exist.  Other-
wise, if the file  does  not  exist,  an  error  message  is
printed.   The  shell  variable _f_l_a_g is set to some non-null
string if the --cc argument is encountered.  The commands

          cp ......; mv ......

copy the file and then overwrite  it  with  the  copy,  thus
causing the last modified date to be updated.

The sequence

          if command1
          then command2
          fi

may be written

          command1 && command2

Conversely,

          command1 |||| command2

executes  _c_o_m_m_a_n_d_2 only if _c_o_m_m_a_n_d_1 fails.  In each case the
value returned is that of the last simple command  executed.

Placing  a  `!!' in front of a pipeline inverts its exit sta-
tus, almost in the manner of the  C  operator  of  the  same
name.  Thus:












USD:3-20                   An Introduction to the UNIX Shell


          if ! [ -d $1 ]
          then
               echo $1 is not a directory
          fi

will print a message only if $1 is not a directory.

22..88 CCoommmmaanndd ggrroouuppiinngg

Commands may be grouped in two ways,

          {{ _c_o_m_m_a_n_d_-_l_i_s_t ;; }}

and

          (( _c_o_m_m_a_n_d_-_l_i_s_t ))


In  the  first  _c_o_m_m_a_n_d_-_l_i_s_t is simply executed.  The second
form executes _c_o_m_m_a_n_d_-_l_i_s_t as a separate process.  For exam-
ple,

          (cd x; rm junk )

executes  _r_m  _j_u_n_k  in  the directory xx without changing the
current directory of the invoking shell.

The commands

          cd x; rm junk

have the same effect but leave the  invoking  shell  in  the
directory xx..

22..99 SShheellll FFuunnccttiioonnss

A function may be defined by the syntax

          _f_u_n_c_n_a_m_e() {{ _c_o_m_m_a_n_d_-_l_i_s_t ;; }}

Functions  are  invoked  within a script as though they were
separate commands of the same name.   While  they  are  exe-
cuted, the positional parameters $$11,, $$22,, ...... are temporarily
set to the arguments passed to the function. For example:

          count() {
               echo $2 : $#
          }

          count a b c

would print `b : 3'.











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22..1100 DDeebbuuggggiinngg sshheellll ssccrriippttss

The shell provides  two  tracing  mechanisms  to  help  when
debugging  shell  scripts.   The first is invoked within the
script as

          set -v

(vv for verbose) and causes lines of the script to be printed
as  they  are  read.   It  is  useful to help isolate syntax
errors.  It may be invoked without modifying the  script  by
saying

          sh -v _s_c_r_i_p_t ......

where _s_c_r_i_p_t is the name of the shell script.  This flag may
be used in conjunction with the --nn flag which prevents  exe-
cution  of subsequent commands.  (Note that saying _s_e_t _-_n at
a terminal will render the terminal useless until an end-of-
file is typed.)

The command

          set -x

will  produce  an execution trace.  Following parameter sub-
stitution each command is printed as it is  executed.   (Try
these  at  the terminal to see what effect they have.)  Both
flags may be turned off by saying

          set -

and the current setting of the shell flags is  available  as
$$--.

22..1111 TThhee mmaann ccoommmmaanndd

The following is a simple implementation of the _m_a_n command,
which is used to display sections of the UNIX manual on your
terminal.  It is called, for example, as

               man sh
               man -t ed
               man 2 fork

In the first the manual section for _s_h is displayed..  Since
no section is specified, section  1  is  used.   The  second
example  will typeset (--tt option) the manual section for _e_d_.
The last prints the _f_o_r_k manual page from section  2,  which
covers system calls.













USD:3-22                   An Introduction to the UNIX Shell


          #!/bin/sh

          cd /usr/share/man

          # "#" is the comment character
          # default is nroff ($N), section 1 ($s)
          N=n s=1

          for i
          do case $i in
                  [1-9]*)     s=$i ;;
                  -t)    N=t ;;
                  -n)    N=n ;;
                  -*)    echo unknown flag \'$i\' ;;
                  *)     if [ -f man$s/$i.$s ]
                    then
                         ${N}roff -man man$s/$i.$s
                    else # look through all manual sections
                         found=no
                         for j in 1 2 3 4 5 6 7 8 9
                         do
                            if [ -f man$j/$i.$j ]
                            then
                                 man $j $i
                                 found=yes
                                 break
                            fi
                         done
                         case $found in
                              no) echo \'$i: manual page not found\'
                         esac
                    fi
             esac
          done

           FFiigguurree 11.. AA vveerrssiioonn ooff tthhee mmaann ccoommmmaanndd

33..00 KKeeyywwoorrdd ppaarraammeetteerrss

Shell  variables may be given values by assignment or when a
shell script is invoked.  An argument to a  command  of  the
form  _n_a_m_e_=_v_a_l_u_e that precedes the command name causes _v_a_l_u_e
to be assigned to  _n_a_m_e  before  execution  of  the  command
begins.   The  value  of  _n_a_m_e  in the invoking shell is not
affected.  For example,

          user=fred command

will execute _c_o_m_m_a_n_d with uusseerr set to _f_r_e_d.

The _s_e_t command may also be used to set  positional  parame-
ters from within a script.  For example,











An Introduction to the UNIX Shell                   USD:3-23


          set -- *

will set $$11 to the first file name in the current directory,
$$22 to the next, and so on.  Note that  the  first  argument,
--,  ensures  correct  treatment  when  the  first file name
begins with a -.


33..11 PPaarraammeetteerr ttrraannssmmiissssiioonn

When a command is executed both  positional  parameters  and
shell  variables  may  be  set on invocation.  Variables are
also made available implicitly to a command by specifying in
advance  that  such  parameters  are to be exported from the
invoking shell.  For example,

          export user box=red

marks the variables uusseerr and bbooxx for export (setting bbooxx  to
``red''  in  the process).  When a command is invoked copies
are made of all exportable variables (also known as _e_n_v_i_r_o_n_-
_m_e_n_t _v_a_r_i_a_b_l_e_s) for use within the invoked program.  Modifi-
cation of such variables within an invoked command does  not
affect  the  values  in the invoking shell.  It is generally
true of a shell script or other program that it cannot  mod-
ify  the state of its caller without explicit actions on the
part of the caller.

Names whose value is intended  to  remain  constant  may  be
declared  _r_e_a_d_o_n_l_y_.  The form of this command is the same as
that of the _e_x_p_o_r_t command,

          readonly name[=value] ......

Subsequent attempts to set readonly variables are illegal.

33..22 PPaarraammeetteerr ssuubbssttiittuuttiioonn

If a shell parameter is not set then the null string is sub-
stituted for it.  For example, if the variable dd is not set

          echo $d

or

          echo ${d}

will echo nothing.  A default string may be given as in

          echo ${d:-..}

which will echo the value of the variable dd if it is set and
not null and `..' otherwise.  The default string is evaluated
using the usual quoting conventions so that









USD:3-24                   An Introduction to the UNIX Shell


          echo ${d:-'*'}

will echo ** if the variable dd is not set or null.  Similarly

          echo ${d:-$1}

will echo the value of dd if it is set and not null  and  the
value (if any) of $$11 otherwise.

The notation ${d:+..} performs the inverse operation. It sub-
stitutes `..' if dd is set or not null, and otherwise  substi-
tutes null.

A  variable  may be assigned a default value using the nota-
tion

          echo ${d:=..}

which substitutes the same string as

          echo ${d:-..}

and if dd were not previously set or null then it will be set
to the string `..'.

If there is no sensible default then the notation

          echo ${d:?_m_e_s_s_a_g_e}

will  echo  the value of the variable dd if it is set and not
null, otherwise _m_e_s_s_a_g_e is printed by the shell  and  execu-
tion of the shell script is abandoned.  If _m_e_s_s_a_g_e is absent
then a standard message is printed.   A  shell  script  that
requires some variables to be set might start as follows:

          : ${user:?} ${acct:?} ${bin:?}
          ......

Colon  (::)  is  a  command that is built in to the shell and
does nothing once its arguments have been evaluated.  If any
of  the  variables  uusseerr,,  aacccctt  or bbiinn are not set then the
shell will abandon execution of the script.

33..33 CCoommmmaanndd ssuubbssttiittuuttiioonn

The standard output from a command can be substituted  in  a
similar  way  to  parameters.  The command _p_w_d prints on its
standard output the name  of  the  current  directory.   For
example,  if the current directory is //uussrr//ffrreedd//bbiinn then the
commands

          d=$(pwd)

(or the older notation d=`pwd`) is equivalent to









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          d=/usr/fred/bin


The entire string inside $(......) (or  between  grave  accents
`......`)  is  taken  as  the  command  to  be  executed and is
replaced with the output from the command.  (The  difference
between  the  $(......)  and `......` notations is that the former
may be nested, while the latter cannot be.)

The command is written using the usual quoting  conventions,
except that inside `......` a `` must be escaped using a \\.  For
example,

          ls $(echo "$HOME")

is equivalent to

          ls $HOME

Command substitution occurs in all contexts where  parameter
substitution  occurs  (including  _h_e_r_e  documents)  and  the
treatment of the resulting text is the same in  both  cases.
This  mechanism allows string processing commands to be used
within shell scripts.  An example of such a command is _b_a_s_e_-
_n_a_m_e  which  removes  a specified suffix from a string.  For
example,

          basename main..c ..c

will print the string _m_a_i_n_.  Its use is illustrated  by  the
following fragment from a _c_c command.

          case $A in
               ......
               *..c) B=$(basename $A ..c)
               ......
          esac

that sets BB to the part of $$AA with the suffix ..cc stripped.

Here are some composite examples.

     +o    ffoorr ii iinn ``llss --tt``;; ddoo ......
          The  variable  ii  is  set to the names of files in
          time order, most recent first.

     +o    sseett ---- ``ddaattee``;; eecchhoo $$66 $$22 $$33,, $$44
          will print, e.g., _1_9_7_7 _N_o_v _1_, _2_3_:_5_9_:_5_9

33..44 AArriitthhmmeettiicc EExxppaannssiioonn

Within a $((......)) construct, integer  arithmetic  operations
are  evaluated.   (The  $  in  front  of  variable  names is
optional within $((......)).  For example:









USD:3-26                   An Introduction to the UNIX Shell


          x=5; y=1
          echo $(($x+3*2))
          echo $((y+=x))
          echo $y

will print `11', then `6', then `6' again.  Most of the con-
structs  permitted  in C arithmetic operations are permitted
though some (like `++') are not universally supported -- see
the shell manual page for details.

33..55 EEvvaalluuaattiioonn aanndd qquuoottiinngg

The  shell is a macro processor that provides parameter sub-
stitution, command substitution and file name generation for
the arguments to commands.  This section discusses the order
in which these evaluations occur and the effects of the var-
ious quoting mechanisms.

Commands are parsed initially according to the grammar given
in appendix A.  Before a command is executed  the  following
substitutions occur.

     +o    parameter substitution, e.g. $$uusseerr

     +o    command substitution, e.g. $$((ppwwdd)) or ``ppwwdd``

     +o    arithmetic expansion, e.g. $$(((($$ccoouunntt++11))))

          Only  one  evaluation occurs so that if, for exam-
          ple, the value of the variable XX is the string  _$_y
          then

                    echo $X

          will echo _$_y_.

     +o    blank interpretation

          Following  the  above  substitutions the resulting
          characters are broken into non-blank words  (_b_l_a_n_k
          _i_n_t_e_r_p_r_e_t_a_t_i_o_n).   For  this  purpose `blanks' are
          the characters of the string  $$IIFFSS.   By  default,
          this  string  consists  of blank, tab and newline.
          The null string is not regarded as a  word  unless
          it is quoted.  For example,

                    echo ''

          will pass on the null string as the first argument
          to _e_c_h_o, whereas

                    echo $null

          will call _e_c_h_o with no arguments if  the  variable









An Introduction to the UNIX Shell                   USD:3-27


          nnuullll is not set or set to the null string.

     +o    file name generation

          Each  word  is  then  scanned for the file pattern
          characters **,, ?? and  [[......]]   and  an  alphabetical
          list  of  file  names  is generated to replace the
          word.  Each such file name is a separate argument.

The  evaluations  just  described  also occur in the list of
words associated with a ffoorr loop.  Only substitution  occurs
in the _w_o_r_d used for a ccaassee branch.

As  well as the quoting mechanisms described earlier using \\
and ''......'' a third quoting mechanism is provided using double
quotes.   Within double quotes parameter and command substi-
tution occurs but file name generation and  the  interpreta-
tion  of  blanks  does not.  The following characters have a
special meaning within double quotes and may be quoted using
\\..

          $$    parameter substitution
          $$(())  command substitution
          ``    command substitution
          ""    ends the quoted string
          \\    quotes the special characters $$ `` "" \\

For example,

          echo "$x"

will  pass  the value of the variable xx as a single argument
to _e_c_h_o_.  Similarly,

          echo "$*"

will pass the positional parameters as a single argument and
is equivalent to

          echo "$1 $2 ......"

The  notation $$@@ is the same as $$** except when it is quoted.

          echo "$@"

will pass the positional parameters,  unevaluated,  to  _e_c_h_o
and is equivalent to

          echo "$1" "$2" ......


The  following  table gives, for each quoting mechanism, the
shell metacharacters that are evaluated.










USD:3-28                   An Introduction to the UNIX Shell


                          _m_e_t_a_c_h_a_r_a_c_t_e_r
                    \    $    *    `    "    '
               '    n    n    n    n    n    t
               `    y    n    n    t    n    n
               "    y    y    n    y    t    n

                    t    terminator
                    y    interpreted
                    n    not interpreted

                  FFiigguurree 22.. QQuuoottiinngg mmeecchhaanniissmmss


In cases where more than  one  evaluation  of  a  string  is
required  the  built-in command _e_v_a_l may be used.  For exam-
ple, if the variable XX has the value _$_y, and if  yy  has  the
value _p_q_r then

          eval echo $X

will echo the string _p_q_r_.

In  general  the _e_v_a_l command evaluates its arguments (as do
all commands) and treats the result as input to  the  shell.
The  input  is  read  and the resulting command(s) executed.
For example,

          wg='eval who||grep'
          $wg fred

is equivalent to

          who||grep fred

In this example, _e_v_a_l is required since there is  no  inter-
pretation  of metacharacters, such as ||, following substitu-
tion.

33..66 EErrrroorr hhaannddlliinngg

The treatment of errors detected by the shell depends on the
type  of error and on whether the shell is being used inter-
actively.  An interactive shell is one whose input and  out-
put  are  connected to a terminal.  A shell invoked with the
--ii flag is also interactive.

Execution of a command (see also 3.7) may fail  for  any  of
the following reasons.

+o    Input  output  redirection may fail.  For example, if a
     file does not exist or cannot be created.

+o    The command itself does not exist  or  cannot  be  exe-
     cuted.









An Introduction to the UNIX Shell                   USD:3-29


+o    The  command terminates abnormally, for example, with a
     "bus error" or "memory fault".  See Figure 2 below  for
     a complete list of UNIX signals.

+o    The  command terminates normally but returns a non-zero
     exit status.

In all of these cases the shell will go on  to  execute  the
next  command.   Except  for  the last case an error message
will be printed by the shell.  All  remaining  errors  cause
the  shell to exit from a script.  An interactive shell will
return to read another  command  from  the  terminal.   Such
errors include the following.

+o    Syntax errors.  e.g., if ...... then ...... done

+o    A  signal  such  as interrupt.  The shell waits for the
     current command, if any, to finish execution  and  then
     either exits or returns to the terminal.

+o    Failure of any of the built-in commands such as _c_d_.

The shell flag --ee causes the shell to terminate if any error
is detected.

     1    hangup
     2    interrupt
     3*   quit
     4*   illegal instruction
     5*   trace trap
     6*   IOT instruction
     7*   EMT instruction
     8*   floating point exception
     9    kill (cannot be caught or ignored)
     10*  bus error
     11*  segmentation violation
     12*  bad argument to system call
     13   write on a pipe with no one to read it
     14   alarm clock
     15   software termination (from _k_i_l_l (1))


                   FFiigguurree 33.. UUNNIIXX ssiiggnnaallss
Those signals marked with an asterisk produce a core dump if
not caught.  However, the shell itself ignores quit which is
the only external signal that can cause a dump.  The signals
in  this list of potential interest to shell programs are 1,
2, 3, 14 and 15.


-----------
 Additional signals  have  been  added  in  modern
Unix.   See  _s_i_g_v_e_c(2)  or _s_i_g_n_a_l(3) for an up-to-
date list.









USD:3-30                   An Introduction to the UNIX Shell


33..77 FFaauulltt hhaannddlliinngg

shell  scripts  normally  terminate  when  an  interrupt  is
received  from  the  terminal.   The _t_r_a_p command is used if
some cleaning up is required,  such  as  removing  temporary
files.  For example,

          trap 'rm /tmp/ps$$; exit' 2

sets  a  trap for signal 2 (terminal interrupt), and if this
signal is received will execute the commands

          rm /tmp/ps$$; exit

_e_x_i_t is another built-in command that  terminates  execution
of  a  shell script.  The _e_x_i_t is required; otherwise, after
the trap has been taken, the shell will resume executing the
script at the place where it was interrupted.

UNIX  signals can be handled in one of three ways.  They can
be ignored, in which case the signal is never  sent  to  the
process.  They can be caught, in which case the process must
decide what action to take  when  the  signal  is  received.
Lastly, they can be left to cause termination of the process
without it having to take any further action.  If  a  signal
is  being ignored on entry to the shell script, for example,
by invoking it in the background (see 3.7)  then  _t_r_a_p  com-
mands (and the signal) are ignored.

The  use  of _t_r_a_p is illustrated by this modified version of
the _t_o_u_c_h command (Figure 4).   The  cleanup  action  is  to
remove the file jjuunnkk$$$$.

          #!/bin/sh

          flag=
          trap 'rm -f junk$$; exit' 1 2 3 15
          for i
          do case $i in
                  -c)    flag=N ;;
                  *)     if test -f $i
                    then cp $i junk$$; mv junk$$ $i
                    elif test $flag
                    then echo file \'$i\' does not exist
                    else >$i
                    fi
             esac
          done


                FFiigguurree 44.. TThhee ttoouucchh ccoommmmaanndd

The  _t_r_a_p  command appears before the creation of the tempo-
rary file; otherwise it would be possible for the process to









An Introduction to the UNIX Shell                   USD:3-31


die without removing the file.

Since  there  is no signal 0 in UNIX it is used by the shell
to indicate the commands to be executed  on  exit  from  the
shell script.

A  script may, itself, elect to ignore signals by specifying
the null string as the  argument  to  trap.   The  following
fragment is taken from the _n_o_h_u_p command.

          trap '' 1 2 3 15

which  causes _h_a_n_g_u_p_, _i_n_t_e_r_r_u_p_t_, _q_u_i_t and _k_i_l_l to be ignored
both by the script and by invoked commands.

Traps may be reset by saying

          trap 2 3

which resets the traps for signals 2 and 3 to their  default
values.   A  list  of  the  current  values  of traps may be
obtained by writing

          trap


The script _s_c_a_n (Figure 5) is an example of the use of  _t_r_a_p
where there is no exit in the trap command.  _s_c_a_n takes each
directory in the current directory, prompts with  its  name,
and  then  executes  commands typed at the terminal until an
end of file or an interrupt  is  received.   Interrupts  are
ignored  while  executing  the  requested commands but cause
termination when _s_c_a_n is waiting for input.

          d=`pwd`
          for i in *
          do if test -d $d/$i
             then cd $d/$i
                  while echo "$i:"
                        trap exit 2
                        read x
                  do trap : 2; eval $x; done
             fi
          done


                 FFiigguurree 55.. TThhee ssccaann ccoommmmaanndd

_r_e_a_d _x is a built-in command that reads one  line  from  the
standard  input and places the result in the variable xx..  It
returns a non-zero exit status if either an  end-of-file  is
read or an interrupt is received.











USD:3-32                   An Introduction to the UNIX Shell


33..88 CCoommmmaanndd eexxeeccuuttiioonn

To  run  a  command  (other than a built-in) the shell first
creates a new process using the system call _f_o_r_k_.  The  exe-
cution  environment  for  the command includes input, output
and the states of signals, and is established in  the  child
process  before  the command is executed.  The built-in com-
mand _e_x_e_c is used in the rare cases when no fork is required
and simply replaces the shell with a new command.  For exam-
ple, a simple version of the _n_o_h_u_p command looks like

          trap \'\' 1 2 3 15
          exec $*

The _t_r_a_p turns off the signals specified so  that  they  are
ignored  by  subsequently created commands and _e_x_e_c replaces
the shell by the command specified.

Most forms of input output  redirection  have  already  been
described.  In the following _w_o_r_d is only subject to parame-
ter and command substitution.  No file  name  generation  or
blank interpretation takes place so that, for example,

               echo ...... >*.c

will  write its output into a file whose name is **..cc..  Input
output specifications are evaluated left to  right  as  they
appear in the command.

> _w_o_r_d      The  standard output (file descriptor 1) is sent
            to the file _w_o_r_d which is created if it does not
            already exist.

>> _w_o_r_d     The  standard  output  is sent to file _w_o_r_d_.  If
            the file exists  then  output  is  appended  (by
            seeking  to the end); otherwise the file is cre-
            ated.

< _w_o_r_d      The standard input (file descriptor 0) is  taken
            from the file _w_o_r_d_.

<< _w_o_r_d     The  standard  input  is taken from the lines of
            shell input that follow up to but not  including
            a  line  consisting  only  of  _w_o_r_d_.  If _w_o_r_d is
            quoted then no interpretation  of  the  document
            occurs.   If  _w_o_r_d  is not quoted then parameter
            and command substitution occur and \\ is used  to
            quote the characters \\ $$ `` and the first charac-
            ter of _w_o_r_d_.  In the  latter  case  \\nneewwlliinnee  is
            ignored (c.f. quoted strings).

>& _d_i_g_i_t    The  file  descriptor  _d_i_g_i_t is duplicated using
            the system call _d_u_p (2) and the result  is  used
            as the standard output.









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<& _d_i_g_i_t    The  standard  input  is  duplicated  from  file
            descriptor _d_i_g_i_t_.

<&-         The standard input is closed.

>&-         The standard output is closed.

Any of the above may be preceded by a digit  in  which  case
the  file  descriptor created is that specified by the digit
instead of the default 0 or 1.  For example,

          ...... 2>file

runs a command  with  message  output  (file  descriptor  2)
directed to _f_i_l_e_.

          ...... 2>&1

runs  a  command with its standard output and message output
merged.  (Strictly speaking file descriptor 2 is created  by
duplicating  file  descriptor 1 but the effect is usually to
merge the two streams.)

33..99 IInnvvookkiinngg tthhee sshheellll

The following flags are interpreted by the shell when it  is
invoked.   If  the  first  character  of  argument zero is a
minus, then commands are read from the file ..pprrooffiillee..

--cc _s_t_r_i_n_g
     If the --cc flag is present then commands are  read  from
     _s_t_r_i_n_g_.

--ss   If  the  --ss  flag  is present or if no arguments remain
     then commands are read from the standard input.   Shell
     output is written to file descriptor 2.

--ii   If  the  --ii  flag  is present or if the shell input and
     output are attached to a terminal  (as  told  by  _g_t_t_y)
     then this shell is _i_n_t_e_r_a_c_t_i_v_e_.  In this case TERMINATE
     is ignored (so that kkiillll 00 does not kill an interactive
     shell)  and  INTERRUPT  is  caught and ignored (so that
     wwaaiitt is interruptable).  In all cases QUIT  is  ignored
     by the shell.

33..1100 JJoobb CCoonnttrrooll

When  a command or pipeline (also known as a _j_o_b) is running
in the foreground, entering the  stop  character  (typically
CONTROL-Z  but  user settable with the _s_t_t_y(1) command) will
usually cause the job to stop.

The jobs associated with the current shell may be listed  by
entering  the  _j_o_b_s command.  Each job has an associated _j_o_b









USD:3-34                   An Introduction to the UNIX Shell


_n_u_m_b_e_r.  Jobs that are stopped may be continued by entering

          bg %_j_o_b_n_u_m_b_e_r

and jobs may be moved to the foreground by entering

          fg %_j_o_b_n_u_m_b_e_r

If there is a sole job with a particular name (say only  one
instance  of _c_c running), _f_g and _b_g may also use name of the
command in place of the number, as in:

          bg %cc

If no `%%' clause  is  entered,  most  recently  stopped  job
(indicated  with a `+' by the _j_o_b_s command) will be assumed.
See the manual page for the shell for more details.

33..1111 AAlliiaasseess

The _a_l_i_a_s command creates a so-called shell alias, which  is
an  abbreviation  that  macro-expands  at run time into some
other command.  For example:

          alias ls="ls -F"

would cause the command sequence llss --FF to be executed  when-
ever  the  user  types  llss into the shell.  Note that if the
user types llss --aa, the shell will in fact execute llss  --FF  --aa.
The command aalliiaass on its own prints out all current aliases.
The _u_n_a_l_i_a_s command, as in:

          unalias ls

will remove an existing  alias.   Aliases  can  shadow  pre-
existing  commands, as in the example above.  They are typi-
cally used to override the interactive behavior of  commands
in  small  ways,  for  example to always invoke some program
with a favorite  option,  and  are  almost  never  found  in
scripts.

33..1122 CCoommmmaanndd LLiinnee EEddiittiinngg aanndd RReeccaallll

When  working  interactively  with  the  shell,  it is often
tedious to retype previously entered commands, especially if
they  are  complicated.  The shell therefore maintains a so-
called _h_i_s_t_o_r_y, which is stored in the file specified by the
HHIISSTTFFIILLEE environment variable if it is set.  Users may view,
edit, and re-enter previous lines of  input  using  a  small
subset  of  the  commands  of the _v_i(1) or _e_m_a_c_s(1) editors.
-----------
Technically, vi command editing is standardized by
POSIX while emacs is  not.   However,  all  modern
shells support both styles.









An Introduction to the UNIX Shell                   USD:3-35


Emacs style editing may be selected by entering

          set -o emacs

and vi style editing may be selected with

          set -o vi

The details of how command line editing works are beyond the
scope  of  this  document.   See  the  shell manual page for
details.

AAcckknnoowwlleeddggeemmeennttss

The design of the shell is based in  part  on  the  original
UNIX  shell3  and  the PWB/UNIX shell,4 some features having
been taken from both.  Similarities also exist with the com-
mand  interpreters  of the Cambridge Multiple Access System5
and of CTSS.6

I would like to thank Dennis Ritchie  and  John  Mashey  for
many  discussions during the design of the shell.  I am also
grateful to the members of the  Computing  Science  Research
Center  and to Joe Maranzano for their comments on drafts of
this document.



RReeffeerreenncceess

1.

2.

3.

4.

5.

6.






















USD:3-36                   An Introduction to the UNIX Shell


AAppppeennddiixx AA -- GGrraammmmaarr

Note: This grammar needs updating, it is obsolete.


_i_t_e_m_:          _w_o_r_d
          _i_n_p_u_t_-_o_u_t_p_u_t
          _n_a_m_e _= _v_a_l_u_e

_s_i_m_p_l_e_-_c_o_m_m_a_n_d_: _i_t_e_m
          _s_i_m_p_l_e_-_c_o_m_m_a_n_d _i_t_e_m

_c_o_m_m_a_n_d_:  _s_i_m_p_l_e_-_c_o_m_m_a_n_d
          (( _c_o_m_m_a_n_d_-_l_i_s_t ))
          {{ _c_o_m_m_a_n_d_-_l_i_s_t }}
          ffoorr _n_a_m_e ddoo _c_o_m_m_a_n_d_-_l_i_s_t ddoonnee
          ffoorr _n_a_m_e iinn _w_o_r_d ...... ddoo _c_o_m_m_a_n_d_-_l_i_s_t ddoonnee
          wwhhiillee _c_o_m_m_a_n_d_-_l_i_s_t ddoo _c_o_m_m_a_n_d_-_l_i_s_t ddoonnee
          uunnttiill _c_o_m_m_a_n_d_-_l_i_s_t ddoo _c_o_m_m_a_n_d_-_l_i_s_t ddoonnee
          ccaassee _w_o_r_d iinn _c_a_s_e_-_p_a_r_t ...... eessaacc
          iiff _c_o_m_m_a_n_d_-_l_i_s_t tthheenn _c_o_m_m_a_n_d_-_l_i_s_t _e_l_s_e_-_p_a_r_t ffii

_p_i_p_e_l_i_n_e_: _c_o_m_m_a_n_d
          _p_i_p_e_l_i_n_e || _c_o_m_m_a_n_d

_a_n_d_o_r_:    _p_i_p_e_l_i_n_e
          _a_n_d_o_r &&&& _p_i_p_e_l_i_n_e
          _a_n_d_o_r |||| _p_i_p_e_l_i_n_e

_c_o_m_m_a_n_d_-_l_i_s_t_:  _a_n_d_o_r
          _c_o_m_m_a_n_d_-_l_i_s_t ;;
          _c_o_m_m_a_n_d_-_l_i_s_t &&
          _c_o_m_m_a_n_d_-_l_i_s_t ;; _a_n_d_o_r
          _c_o_m_m_a_n_d_-_l_i_s_t && _a_n_d_o_r

_i_n_p_u_t_-_o_u_t_p_u_t_:  >> _f_i_l_e
          << _f_i_l_e
          >>>> _w_o_r_d
          <<<< _w_o_r_d

_f_i_l_e_:          _w_o_r_d
          && _d_i_g_i_t
          && _-

_c_a_s_e_-_p_a_r_t_:     _p_a_t_t_e_r_n )) _c_o_m_m_a_n_d_-_l_i_s_t ;;;;

_p_a_t_t_e_r_n_:  _w_o_r_d
          _p_a_t_t_e_r_n || _w_o_r_d

_e_l_s_e_-_p_a_r_t_:     eelliiff _c_o_m_m_a_n_d_-_l_i_s_t tthheenn _c_o_m_m_a_n_d_-_l_i_s_t _e_l_s_e_-_p_a_r_t
          eellssee _c_o_m_m_a_n_d_-_l_i_s_t
          _e_m_p_t_y

_e_m_p_t_y_:









An Introduction to the UNIX Shell                   USD:3-37


_w_o_r_d_:          a sequence of non-blank characters

_n_a_m_e_:          a sequence of letters, digits or underscores starting with a letter

_d_i_g_i_t_:         00 11 22 33 44 55 66 77 88 99


























































USD:3-38                   An Introduction to the UNIX Shell


AAppppeennddiixx BB -- MMeettaa--cchhaarraacctteerrss aanndd RReesseerrvveedd WWoorrddss

a) syntactic

     ||     pipe symbol

     &&&&    `andf' symbol

     ||||    `orf' symbol

     ;;       command separator

     ;;;;      case delimiter

     &&       background commands

     (( ))     command grouping

     <<       input redirection

     <<<<      input from a here document

     >>       output creation

     >>>>      output append



b) patterns

     **       match any character(s) including none

     ??       match any single character

     [[......]]   match any of the enclosed characters



c) substitution

     $${{......}}  substitute shell variable

     $$((......))  substitute command output

     ``......``   substitute command output

     $$((((......))))
             substitute arithmetic expression



d) quoting











An Introduction to the UNIX Shell                   USD:3-39


     \\       quote the next character

     ''......''   quote the enclosed characters except for '

     ""......""   quote the enclosed characters except for $$ ``  \\
             ""



e) reserved words

     iiff tthheenn eellssee eelliiff ffii
     ccaassee iinn eessaacc
     ffoorr wwhhiillee uunnttiill ddoo ddoonnee
     !! {{  }}













































