Node:File Descriptors, Next:File System Conventions, Previous:Here-Documents, Up:Portable Shell
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 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 See Limitations of Builtins, mv for more details.