(autoconf.info)Shell Substitutions


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10.6 Shell Substitutions
========================

Contrary to a persistent urban legend, the Bourne shell does not
systematically split variables and back-quoted expressions, in
particular on the right-hand side of assignments and in the argument of
`case'.  For instance, the following code:

     case "$given_srcdir" in
     .)  top_srcdir="`echo "$dots" | sed 's|/$||'`" ;;
     *)  top_srcdir="$dots$given_srcdir" ;;
     esac

is more readable when written as:

     case $given_srcdir in
     .)  top_srcdir=`echo "$dots" | sed 's|/$||'` ;;
     *)  top_srcdir=$dots$given_srcdir ;;
     esac

and in fact it is even _more_ portable: in the first case of the first
attempt, the computation of `top_srcdir' is not portable, since not all
shells properly understand `"`..."..."...`"'.  Worse yet, not all
shells understand `"`...\"...\"...`"' the same way.  There is just no
portable way to use double-quoted strings inside double-quoted
back-quoted expressions (pfew!).

`$@'
     One of the most famous shell-portability issues is related to
     `"$@"'.  When there are no positional arguments, Posix says that
     `"$@"' is supposed to be equivalent to nothing, but the original
     Unix version 7 Bourne shell treated it as equivalent to `""'
     instead, and this behavior survives in later implementations like
     Digital Unix 5.0.

     The traditional way to work around this portability problem is to
     use `${1+"$@"}'.  Unfortunately this method does not work with Zsh
     (3.x and 4.x), which is used on Mac OS X.  When emulating the
     Bourne shell, Zsh performs word splitting on `${1+"$@"}':

          zsh $ emulate sh
          zsh $ for i in "$@"; do echo $i; done
          Hello World
          !
          zsh $ for i in ${1+"$@"}; do echo $i; done
          Hello
          World
          !

     Zsh handles plain `"$@"' properly, but we can't use plain `"$@"'
     because of the portability problems mentioned above.  One
     workaround relies on Zsh's "global aliases" to convert `${1+"$@"}'
     into `"$@"' by itself:

          test "${ZSH_VERSION+set}" = set && alias -g '${1+"$@"}'='"$@"'

     Zsh only recognizes this alias when a shell word matches it
     exactly; `"foo"${1+"$@"}' remains subject to word splitting.
     Since this case always yields at least one shell word, use plain
     `"$@"'.

     A more conservative workaround is to avoid `"$@"' if it is
     possible that there may be no positional arguments.  For example,
     instead of:

          cat conftest.c "$@"

     you can use this instead:

          case $# in
          0) cat conftest.c;;
          *) cat conftest.c "$@";;
          esac

     Autoconf macros often use the `set' command to update `$@', so if
     you are writing shell code intended for `configure' you should not
     assume that the value of `$@' persists for any length of time.

`${10}'
     The 10th, 11th, ... positional parameters can be accessed only
     after a `shift'.  The 7th Edition shell reported an error if given
     `${10}', and Solaris 10 `/bin/sh' still acts that way:

          $ set 1 2 3 4 5 6 7 8 9 10
          $ echo ${10}
          bad substitution

`${VAR:-VALUE}'
     Old BSD shells, including the Ultrix `sh', don't accept the colon
     for any shell substitution, and complain and die.  Similarly for
     ${VAR:=VALUE}, ${VAR:?VALUE}, etc.

`${VAR=LITERAL}'
     Be sure to quote:

          : ${var='Some words'}

     otherwise some shells, such as on Digital Unix V 5.0, die because
     of a "bad substitution".


     Solaris `/bin/sh' has a frightening bug in its interpretation of
     this.  Imagine you need set a variable to a string containing `}'.
     This `}' character confuses Solaris `/bin/sh' when the affected
     variable was already set.  This bug can be exercised by running:

          $ unset foo
          $ foo=${foo='}'}
          $ echo $foo
          }
          $ foo=${foo='}'   # no error; this hints to what the bug is
          $ echo $foo
          }
          $ foo=${foo='}'}
          $ echo $foo
          }}
           ^ ugh!

     It seems that `}' is interpreted as matching `${', even though it
     is enclosed in single quotes.  The problem doesn't happen using
     double quotes.

`${VAR=EXPANDED-VALUE}'
     On Ultrix, running

          default="yu,yaa"
          : ${var="$default"}

     sets VAR to `M-yM-uM-,M-yM-aM-a', i.e., the 8th bit of each char
     is set.  You don't observe the phenomenon using a simple `echo
     $var' since apparently the shell resets the 8th bit when it
     expands $var.  Here are two means to make this shell confess its
     sins:

          $ cat -v <<EOF
          $var
          EOF

     and

          $ set | grep '^var=' | cat -v

     One classic incarnation of this bug is:

          default="a b c"
          : ${list="$default"}
          for c in $list; do
            echo $c
          done

     You'll get `a b c' on a single line.  Why?  Because there are no
     spaces in `$list': there are `M- ', i.e., spaces with the 8th bit
     set, hence no IFS splitting is performed!!!

     One piece of good news is that Ultrix works fine with `:
     ${list=$default}'; i.e., if you _don't_ quote.  The bad news is
     then that QNX 4.25 then sets LIST to the _last_ item of DEFAULT!

     The portable way out consists in using a double assignment, to
     switch the 8th bit twice on Ultrix:

          list=${list="$default"}

     ...but beware of the `}' bug from Solaris (see above).  For safety,
     use:

          test "${var+set}" = set || var={VALUE}

`${#VAR}'
`${VAR%WORD}'
`${VAR%%WORD}'
`${VAR#WORD}'
`${VAR##WORD}'
     Posix requires support for these usages, but they do not work with
     many traditional shells, e.g., Solaris 10 `/bin/sh'.

     Also, `pdksh' 5.2.14 mishandles some WORD forms.  For example if
     `$1' is `a/b' and `$2' is `a', then `${1#$2}' should yield `/b',
     but with `pdksh' it yields the empty string.

``COMMANDS`'
     Posix requires shells to trim all trailing newlines from command
     output before substituting it, so assignments like `dir=`echo
     "$file" | tr a A`' do not work as expected if `$file' ends in a
     newline.

     While in general it makes no sense, do not substitute a single
     builtin with side effects, because Ash 0.2, trying to optimize,
     does not fork a subshell to perform the command.

     For instance, if you wanted to check that `cd' is silent, do not
     use `test -z "`cd /`"' because the following can happen:

          $ pwd
          /tmp
          $ test -z "`cd /`" && pwd
          /

     The result of `foo=`exit 1`' is left as an exercise to the reader.

     The MSYS shell leaves a stray byte in the expansion of a
     double-quoted command substitution of a native program, if the end
     of the substitution is not aligned with the end of the double
     quote.  This may be worked around by inserting another pair of
     quotes:

          $ echo "`printf 'foo\r\n'` bar" > broken
          $ echo "`printf 'foo\r\n'`"" bar" | cmp - broken
          - broken differ: char 4, line 1

`$(COMMANDS)'
     This construct is meant to replace ``COMMANDS`', and it has most
     of the problems listed under ``COMMANDS`'.

     This construct can be nested while this is impossible to do
     portably with back quotes.  Unfortunately it is not yet
     universally supported.  Most notably, even recent releases of
     Solaris don't support it:

          $ showrev -c /bin/sh | grep version
          Command version: SunOS 5.10 Generic 121005-03 Oct 2006
          $ echo $(echo blah)
          syntax error: `(' unexpected

     nor does IRIX 6.5's Bourne shell:
          $ uname -a
          IRIX firebird-image 6.5 07151432 IP22
          $ echo $(echo blah)
          $(echo blah)

     If you do use `$(COMMANDS)', make sure that the commands do not
     start with a parenthesis, as that would cause confusion with a
     different notation `$((EXPRESSION))' that in modern shells is an
     arithmetic expression not a command.  To avoid the confusion,
     insert a space between the two opening parentheses.

     Avoid COMMANDS that contain unbalanced parentheses in
     here-documents, comments, or case statement patterns, as many
     shells mishandle them.  For example, Bash 3.1, `ksh88', `pdksh'
     5.2.14, and Zsh 4.2.6 all mishandle the following valid command:

          echo $(case x in x) echo hello;; esac)

`$((EXPRESSION))'
     Arithmetic expansion is not portable as some shells (most notably
     Solaris 10 `/bin/sh') don't support it.

`^'
     Always quote `^', otherwise traditional shells such as `/bin/sh'
     on Solaris 10 treat this like `|'.



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