Save output of command to variable and print without using tee?

Question

I am trying to find a method to save the output of a command to variable but also print it in real time. I initially found an example with tee that seemed to work;

VARIABLE=$(./build.sh | tee /dev/tty)

However the problem with this is that if I run it on a system with virtual console (i.e. git hub actions) I get an error tee: /dev/tty: No such device or address. I just want to find a way to save output to a variable and do a normal print in real time (i.e. just like echo, printf, etc.)

Answer 1

The problem with using /dev/tty is it assumes that stdout was originally attached to a tty - this is not necessarily so as demonstrated by GitHub Actions. It would also be a problem if you were redirecting stdout of the script elsewhere (a log file for instance).

What you want to pass to tee is what was stdout before the process substitution (the $(...) bit) - process substitution takes stdout in order to capture the output. Bash and other shells have the ability to manipulate file descriptors for your use case:

exec 3>&1  # Open FD 3 as a duplicate of stdout (fd 1)
# Run ./build.sh but make sure it does not have FD 3 open and tee to FD 3
VARIABLE=$(./build.sh 3>&- | tee /dev/fd/3)
exec 3>&-  # Close FD 3

Doing it this way allows the stdout of the script to be manipulated outside the script (redirecting to a logger, or /dev/null, etc), preserving the correct behaviour WRT stdout.

As pointed out by @SOUser in the comments, there is an issue when writing the output of a script to a file. The problem is that tee /dev/fd/3 truncates the file when it opens it, discarding anything written to it already. Attempting to fix this with tee -a /dev/fd/3 causes even stranger behaviour - the line to be captured gets lost entirely for some reason.

A solution to this is to write to a pipe and not a file, so instead of ./script > test.log, use ./script | cat > test.log. When done this way, tee cannot truncate the file because it is not attached to it - it is attached to a pipe to cat which cannot be truncated.

Answer 2

You could tee to stderr:

VARIABLE=$(./build.sh | tee /dev/stderr)