On Jul 26, 1:55pm, Wayne Conrad wrote:
> "&>" is really a shorthand for "2>&1 >". These two commands do the same thing:
>
> foo &> logfile # bash-specific shorthand for...
> foo 2>&1 > logfile # this, which works in both bash and ksh
>
> I hope this isn't too much (or too confusing).
This is incorrect (but an easy mistake to make). The command:
foo &>logfile
is actually equivalent to:
foo >logfile >&1
The command that you listed, ``foo 2>&1 >logfile'' actually routes
stderr to the current stdout and routes stdout to logfile. The following
Perl script may be used to illustrate the difference:
--- test-stdout-stderr ---
#!/usr/bin/perl -w
print STDOUT "stdout\n";
print STDERR "stderr\n";
--- end test-stdout-stderr ---
Now lets try some different tests:
# No redirection
ocotillo:ptests$ ./test-stdout-stderr
stdout
stderr
# Redirect stdout to logfile.
ocotillo:ptests$ ./test-stdout-stderr >logfile
stderr
ocotillo:ptests$ cat logfile
stdout
# Redirect stderr to logfile.
ocotillo:ptests$ ./test-stdout-stderr 2>logfile
stdout
ocotillo:ptests$ cat logfile
stderr
# Redirect stdout to logfile and then redirect stderr to (the now
# redirected) stdout.
ocotillo:ptests$ ./test-stdout-stderr >logfile 2>&1
ocotillo:ptests$ cat logfile
stderr
stdout
# Redirect stderr to stdout and redirect stdout to logfile. Note
# that this *does not* cause stderr to be redirected to logfile;
# a copy of the descriptor is made prior to performing the second
# redirection.
ocotillo:ptests$ ./test-stdout-stderr 2>&1 >logfile
stderr
ocotillo:ptests$ cat logfile
stdout
# Like the above, but pipe the ``stderr redirected to stdout''
# portion to a command which'll save this output to logfile2.
ocotillo:ptests$ ./test-stdout-stderr 2>&1 >logfile | cat >logfile2
ocotillo:ptests$ cat logfile
stdout
ocotillo:ptests$ cat logfile2
stderr
If all of this confuses you, just think about how you'd do the
equivalent of the above redirections in C using open() and dup2(). In
fact, to really understand this stuff it would be a valuable exercise
to code the above examples in C...
Kevin
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