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OSXFAQ Mac OS X UNIX Tip-of-the-Day
Redirection - Redirect and Pipe Both
Monday's tip showed how we can make our scripts write error messages to standard error instead of standard out, just like 'real' Unix commands. Today, we'll look at how to negate this convention by merging the standard error from a command back into its standard out.
For example, suppose we wish to send both the standard out and standard error of 'ls' to the same file. We use the special redirection '&>'. First, we'll test an 'ls' command line that lists 'file' (which fails) and the directory 'test' that contains 'file':
$ ls test file
ls: file: No such file or directory
test:
file
$
Next, we'll redirect standard out to 'out':
$ ls test file > out
ls: file: No such file or directory
$
Finally, we'll redirect both to the same file using the special redirection '&>':
$ ls test file &> out
$
Normally, when we pipe the output of one command to another, only standard out is piped. If you need to pipe both standard out and standard error, first merge standard error into standard out, then pipe standard out as normal. Here's an example using the same 'ls' command as above. We pipe to the 'awk' command, which then prints its input lines with "Piped: " added to the beginning.
First, we pipe only standard out:
$ ls test file | awk '{print "Piped: " $0}'
ls: file: No such file or directory
Piped: test:
Piped: file
$
Next, we pipe both standard out and standard error.
$ ls test file 2>&1 | awk '{print "Piped: " $0}'
Piped: ls: file: No such file or directory
Piped: test:
Piped: file
$
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