From time to time I need to troubleshoot the execution of a command that produces verbose output. Redirecting output to a file is fundamental to analyzing the execution. However, sometimes I also need to get immediate feedback that the command is running. In that case, it is usually enough to send all output to the console and either all or part of it to a file. But what if you need to look at more than one aspect of the output and thus you need that output in more than one file?

Output to a single file

Sending output to the console and to a file is precisely the purpose of tee. Command tee reads from standard input and outputs what reads to the console and to a given file. Consider the following program that outputs to standard output and standard error.

(echo 'ok'; echo 'error' >&2)

I send standard output and error to the console and only standard output to a file by means of tee in the following way.

(echo 'ok'; echo 'error' >&2) | tee out.log

I send both outputs to the console and to the same file in the following way.

(echo 'ok'; echo 'error' >&2) 2>&1 | tee all.log

I can even send both outputs to the console and only standard error to a file like so.

(echo 'ok'; echo 'error' >&2) 3>&2 2>&1 1>&3 | tee err.log

Sometimes troubleshooting is easier when you look at standard output and error separately. How do you do that?

Output to two or more files

I can send standard output and standard error to the console and to a file by running two instances of tee. I do so by means of process substitution. To illustrate process substitution, consider the following code that sends standard error and standard output to separate files.

(echo 'ok'; echo 'error' >&2)  2> err.log > out.log

I send standard error and standard output the console and to separate files in like so.

 (echo 'ok'; echo 'error' 1>&2) 2> >(tee err.log) > >(tee out.log)
#----------------------------- ----------- -----------
# CMD ERR OUT

What the previous line of Bash does is execute commands CMD, ERR, and OUT and connect the standard error of CMD to the input of ERR and the standard output of CMD to the input of OUT.

Sometimes I still need to put error output in context. In that case, I send standard error and standard output to the console, to separate files, and to the same file like this.

(echo 'ok'; echo 'error' 1>&2) 2> >(tee err.log) > >(tee out.log) | tee all.log

When I only need to look at errors and put them in context, I do the following. I send standard error and standard output to the console and to the same file. While doing that, I send standard error to a separate file. The following Bash line illustrates the technique.

(echo 'ok'; echo 'error' 1>&2) 2> >(tee err.log) | tee all.log

Do you have any interesting techniques for redirecting output? Let me know in the comments.

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