Capturing output

A command's output usually flashes past on the screen and it's gone. Admins need it kept: in a file to send to someone, in a log to read tomorrow, or thrown away on purpose. This lesson is about steering output wherever you want it.

Three streams

Every program is born with three “pipes” already attached:

Results and errors land on the same screen, so they look alike, but they're separate streams. That's why you can save the results and still see the errors, or the other way round.

ls /etc/hostname /nope                # one result, one error
ls /etc/hostname /nope > out.txt      # the result goes to the file… the error still shows!
ls /etc/hostname /nope 2> err.txt     # now only the error goes to a file

Redirections: the whole set

Silencing errors doesn't make them go away: ls /nope 2>/dev/null; echo $? still prints 2. Scripts test $?, not the text (lesson 13).

tee: see it and save it

df -h | tee disk.txt                     # on screen AND in the file
date | tee -a disk.txt                   # -a = add to the end
sudo dnf upgrade 2>&1 | tee upgrade.log   # keep a record of a big job, errors included
echo "hi" | sudo tee /root/note.txt      # the sudo trick from the sudo lesson

A pipe only carries stdout. That's why the upgrade line adds 2>&1 before the |, so the errors make it into the log too.

Here-documents: type a whole file

A here-document feeds several lines into a command. It's the classic way to create a config file or a script from the terminal, with no editor needed. Type the first line, press Enter, and the prompt changes to > until you type the end word:

cat > note.txt <<EOF
Made by $(whoami) on $(hostname)
Home folder: $HOME
EOF

• EOF is just a marker word. Any word works, as long as the last line matches it exactly.
• With <<EOF, $VAR and $( ) inside are filled in. With quotes, <<'EOF', everything stays exactly as typed. Use that when you're writing a script that contains $ signs.
• <<-EOF ignores leading tabs, so the text can be indented inside scripts.
• A here-string is the one-line version: tr a-z A-Z <<< "hello".

To write a root-owned file this way, combine it with tee: sudo tee /etc/motd <<'EOF' … EOF.

Record a whole session: script

script session.log      # start recording everything on screen
…                       # work normally
exit                    # stop; session.log now holds it all
script -a session.log   # -a = add to an existing recording

This is great for homework proof, bug reports and “what exactly did I type last Tuesday?” It's in util-linux on Rocky and bsdutils on Ubuntu, and preinstalled on both. Your SSH app can do this too: PuTTY has Session → Logging, and iTerm2 has automatic session logs (Linux Basics, lesson 4).

Output for later: logs

logger: write to the system log

logger -t mybackup "Backup finished OK"
journalctl -t mybackup
sudo tail /var/log/messages

logger -t mybackup "Backup finished OK"
journalctl -t mybackup
tail /var/log/syslog

Your scripts' messages end up in the same place as everything else, with a time stamp and a tag you can search for (lesson 4). Add -p user.err to mark something as an error.

Send a whole script's output to a log

Put this near the top of a script, and everything after it (results and errors) goes into the log instead of the screen:

#!/bin/bash
exec >> /home/student/nightly.log 2>&1
echo "=== started $(date) ==="
…

That's perfect for cron jobs (lesson 5), whose output would otherwise vanish into mail nobody reads.

Try it 🎥

Quick check