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File Descriptors

Some file descriptors shall not be used, since some systems, admittedly arcane, use them for special purpose:

3 --- some systems may open it to /dev/tty.
4 --- used on the Kubota Titan.

Don't redirect several times the same file descriptor, 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.

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.

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

Most shells, if not all (including Bash, Zsh, Ash), output traces on stderr, even for sub-shells. This might result in undesired 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

You'll appreciate the various levels of detail...

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

Don't try to move/delete open files, such as in exec >foo; mv foo bar, see See Limitations of Builtins, mv for more details.