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C.1.4 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 or brace group:

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

Furthermore, versions of dash before 0.5.6 mistakenly truncate regular files when using ‘<>’:

$ echo a > file
$ bash -c ': 1<>file'; cat file
$ dash -c ': 1<>file'; cat file
$ rm a

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 (even if it is redirected) in a loop after the first iteration, or in a shell function after the first call:

$ for i in 1 2 3 ; do : >x$i; done
$ ls x*
$ f () { : >$1; }; f y1; f y2; f y3;
$ ls y*

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 2008 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, and the next version of Posix will allow HP-UX behavior.

If you want a file descriptor above 2 to be inherited into a child process, then you must use redirections specific to that command or a containing subshell or command group, rather than relying on exec in the shell. In ksh as well as HP-UX sh, 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):

$ echo 'echo hello >&5' >k
$ /bin/sh -c 'exec 5>t; ksh ./k; exec 5>&-; cat t
$ bash -c 'exec 5>t; ksh ./k; exec 5>&-; cat t
$ ksh -c 'exec 5>t; ksh ./k; exec 5>&-; cat t
./k[1]: 5: cannot open [Bad file number]
$ ksh -c '(ksh ./k) 5>t; cat t'
$ ksh -c '{ ksh ./k; } 5>t; cat t'
$ ksh -c '5>t ksh ./k; cat t

Don’t rely on duplicating a closed file descriptor to cause an error. With Solaris /bin/sh, failed duplication is silently ignored, which can cause unintended leaks to the original file descriptor. In this example, observe the leak to standard output:

$ 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.

On the other hand, you can’t portably use multi-digit file descriptors. Solaris ksh doesn’t understand any file descriptor larger than ‘9’:

$ bash -c 'exec 10>&-'; echo $?
$ ksh -c 'exec 9>&-'; echo $?
$ ksh -c 'exec 10>&-'; echo $?
ksh[1]: exec: 10: not found

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