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10.3 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 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.

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 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
foo
$ bash -x -c '(eval "echo foo >&2") 2>stderr'
$ cat stderr
+ eval 'echo foo >&2'
++ echo foo
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
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 10.8 Limitations of Shell Builtins, mv for more details.


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