Use grep to search stdout, without replacing stdout

Question

At the moment, I have this in a wrapper script:

2>&1 ./update.sh | ts | tee -a ./update.log

and this in update.sh:

if apt full-upgrade -y | grep linux-headers
then
    echo
    echo Need to Fix the Capture Driver!
fi

(there's some additional logic to fix the driver automatically, but I think you get the idea)

It works, except that I don't get the full output of apt full-upgrade in the terminal or in the logfile. I only get what grep matches.

I could probably tee the full output of apt full-upgrade to a temp file, before the if, and then grep and rm the temp file, but is there a better way than that to get a complete terminal and log, and keep the logic?

Answer 1

You can't use /dev/stdout here, since that will point to the pipeline. One way might be to take a copy of the script's original stdout, and have tee print into that (I didn't test this):

exec 9>&1    # make fd 9 a copy of the current stdout
apt full-upgrade -y | tee /dev/fd/9 | grep linux-headers
exec 9>&-    # close fd 9

---

If you wanted the full output to always go to a terminal, you could use /dev/tty:

apt full-upgrade -y | tee /dev/tty | grep linux-headers

but that would make the output go to the terminal regardless of what redirections are applied to the whole script.

---

In some cases, could also use process substitution to redirect the output to a command in addition to stdout:

apt full-upgrade -y | tee >(grep linux-headers)

but this doesn't really work here since you need the exit status of grep and the process substitution doesn't make it easy to get it.