How to make tee catch the stderr only in Linux?
I would like to make tee catch the stderr and log it into a file.
A brute force method by let tee listen on both stderr and stdout is okay like
cmd 2>&1 | tee -a log
But how to make tee catch the stderr only?
You can make use of “process substitution” (>(...)) to creates a FIFO to catch the STDERR and pass it to tee.
Lets take an example t.sh:
#!/bin/bash
echo hello
1>&2 echo world
It works as expected:
$ ./t.sh
hello
world
$ ./t.sh >/dev/null
world
$ ./t.sh 2>/dev/null
hello
Now, let’s redirect the STDERR to tee:
$ ./t.sh 2> >(tee /tmp/log)
hello
$ world
cat /tmp/log
world
tee prints to STDOUT by default. Let’s make it print to STDERR:
$ ./t.sh 2> >(tee /tmp/log >&2)
hello
world
$ cat /tmp/log
world
Verify it:
$ (./t.sh 2> >(tee /tmp/log >&2) ) >/dev/null
world
The “world” is not to the STDOUT anymore.
Thanks, it really helped.
I’ve tried to make logging of STDOUT and STDERR to separate files, but also have STDERR output in the terminal. This is what I came up with.
test-script.sh:
“`
#!/bin/bash
echo ‘Regular stdout’
1>&2 echo ‘Error!’
“`
Terminal:
“`
./test-script.sh > file.log 2> >(tee file-error.log >&2)
“`
Output:
“`
Error!
“`
file.log:
“`
Regular stdout
“`
file-error.log:
“`
Error!
“`