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11.3 File Descriptors

Most shells, if not all (including Bash, Zsh, Ash), output traces on stderr, even for subshells. This might result in undesirable content if you meant to capture the standard-error output of the inner command:

     $ ash -x -c '(eval "echo foo >&2") 2>stderr'
     $ cat stderr
     + eval echo foo >&2
     + echo foo
     $ bash -x -c '(eval "echo foo >&2") 2>stderr'
     $ cat stderr
     + eval 'echo foo >&2'
     ++ echo foo
     $ zsh -x -c '(eval "echo foo >&2") 2>stderr'
     # Traces on startup files deleted here.
     $ cat stderr
     +zsh:1> eval echo foo >&2
     +zsh:1> echo foo

One workaround is to grep out uninteresting lines, hoping not to remove good ones.

If you intend to redirect both standard error and standard output, redirect standard output first. This works better with HP-UX, since its shell mishandles tracing if standard error is redirected first:

     $ sh -x -c ': 2>err >out'
     + :
     + 2> err $ cat err
     1> out

Don't try to redirect the standard error of a command substitution. It must be done inside the command substitution. When running ‘: `cd /zorglub` 2>/dev/null’ expect the error message to escape, while ‘: `cd /zorglub 2>/dev/null`’ works properly.

On the other hand, some shells, such as Solaris or FreeBSD /bin/sh, warn about missing programs before performing redirections. Therefore, to silently check whether a program exists, it is necessary to perform redirections on a subshell:

     $ /bin/sh -c 'nosuch 2>/dev/null'
     nosuch: not found
     $ /bin/sh -c '(nosuch) 2>/dev/null'
     $ bash -c 'nosuch 2>/dev/null'

FreeBSD 6.2 sh may mix the trace output lines from the statements in a shell pipeline.

It is worth noting that Zsh (but not Ash nor Bash) makes it possible in assignments though: ‘foo=`cd /zorglub` 2>/dev/null’.

Some shells, like ash, don't recognize bi-directional redirection (‘<>’). And even on shells that recognize it, it is not portable to use on fifos: Posix does not require read-write support for named pipes, and Cygwin does not support it:

     $ mkfifo fifo
     $ exec 5<>fifo
     $ echo hi >&5
     bash: echo: write error: Communication error on send

When catering to old systems, don't redirect the same file descriptor several times, as you are doomed to failure under Ultrix.

     ULTRIX V4.4 (Rev. 69) System #31: Thu Aug 10 19:42:23 GMT 1995
     UWS V4.4 (Rev. 11)
     $ eval 'echo matter >fullness' >void
     illegal io
     $ eval '(echo matter >fullness)' >void
     illegal io
     $ (eval '(echo matter >fullness)') >void
     Ambiguous output redirect.

In each case the expected result is of course fullness containing ‘matter’ and void being empty. However, this bug is probably not of practical concern to modern platforms.

Solaris 10 sh will try to optimize away a : command in a loop after the first iteration, even if it is redirected:

     $ for i in 1 2 3 ; do : >x$i; done
     $ ls

As a workaround, echo or eval can be used.

Don't rely on file descriptors 0, 1, and 2 remaining closed in a subsidiary program. If any of these descriptors is closed, the operating system may open an unspecified file for the descriptor in the new process image. Posix says this may be done only if the subsidiary program is set-user-ID or set-group-ID, but HP-UX 11.23 does it even for ordinary programs.

Don't rely on open file descriptors being open in child processes. In ksh, file descriptors above 2 which are opened using ‘exec n>file’ are closed by a subsequent ‘exec’ (such as that involved in the fork-and-exec which runs a program or script). Thus, using sh, we have:

     $ cat ./descrips
     #!/bin/sh -
     echo hello >&5
     $ exec 5>t
     $ ./descrips
     $ cat t

But using ksh:

     $ exec 5>t
     $ ./descrips
     $ cat t

Within the process which runs the ‘descrips’ script, file descriptor 5 is closed.

Don't rely on duplicating a closed file descriptor to cause an error. With Solaris /bin/sh, when the redirection fails, the output goes to the original file descriptor.

     $ bash -c 'echo hi >&3' 3>&-; echo $?
     bash: 3: Bad file descriptor
     $ /bin/sh -c 'echo hi >&3' 3>&-; echo $?

Fortunately, an attempt to close an already closed file descriptor will portably succeed. Likewise, it is safe to use either style of ‘n<&-’ or ‘n>&-’ for closing a file descriptor, even if it doesn't match the read/write mode that the file descriptor was opened with.

DOS variants cannot rename or remove open files, such as in ‘mv foo bar >foo’ or ‘rm foo >foo’, even though this is perfectly portable among Posix hosts.

A few ancient systems reserved some file descriptors. By convention, file descriptor 3 was opened to /dev/tty when you logged into Eighth Edition (1985) through Tenth Edition Unix (1989). File descriptor 4 had a special use on the Stardent/Kubota Titan (circa 1990), though we don't now remember what it was. Both these systems are obsolete, so it's now safe to treat file descriptors 3 and 4 like any other file descriptors.