Shell Script – How to Capture Stdout/Stderr in File and Console

shellshell-scriptstderrstdout

I'm using bash script on AMazon Linux. When I want to redirect stderr and stdout to a file, I can run

./my_process.pl &>/tmp/out.txt

Is there a way I can redirect this output to a file and continue to see it on the console (in my shell)? How is this done?

Best Answer

Yes, by using tee:

/my_process.pl 2>&1 | tee /tmp/out.txt

Note that using &>file for redirecting both standard output and standard error to a file is an extension to the POSIX standard that is accepted by some shells. It is safer to use >file 2>&1. In this case, &> can not be used at all since we're not redirecting to a file.

In bash, one may also do

/my_process.pl |& tee /tmp/out.txt

which is equivalent of the above. In ksh93, |& means something completely different though.

Related Solutions

Bash – Show only stderr on screen but write both stdout and stderr to file

Even without any redirection, or with nothing but >logfile 2>&1, you're not guaranteed to see output in order of generation.

For starters, the stdout from the application will be line-buffered (to tty) or buffered (to a pipeline) but stderr is unbuffered, so relationships between the order of output are broken as far as a reader is concerned. Subsequent stages in any pipeline you could concoct will not get deterministically ordered access to the two streams (they are conceptually things happening in parallel, and you're always subject to the scheduler - if by the time your reader gets a slice the writer has already written to both pipes, you cannot tell which came first).

"[T]he order they happen" is only really known to the application. Ordering of output across stdout/stderr is a well-known - classic, perhaps - problem.

Linux Shell – How to Redirect stdout & stderr to Logger

You need to combine the output of STDERR and STDOUT prior to piping it to logger. Try this instead:

/home/dirname/application_name -v 2>&1 | logger &

Example

$ echo "hi" 2>&1 | logger &
[1] 26818
[1]+  Done                    echo "hi" 2>&1 | logger

$ sudo tail /var/log/messages
Apr 12 17:53:57 greeneggs saml: hi

You can use the abbreviated notation here as well, if used cautiously in a actual Bash shell (not to be confused with Dash):

$ echo "hi" |& logger &

NOTE: This is equivalent to <cmd1> 2>&1 | <cmd2>. Again only use the above when making use of an actual Bash shell interactively, would be a good way to approach it.

excerpt from ABSG

# |& was added to Bash 4 as an abbreviation for 2>&1 |.

References

Advanced Bash Scripting Guide - Chapter 20. I/O Redirection