Module 3 · Text, redirection and pipes
tee, /dev/null and the standard streams recap
Split output so it goes to a file and the screen at once, throw away what you do not want, and consolidate everything you know about stdin, stdout and stderr.
What you will learn
- Use tee and tee -a to save output while still seeing it.
- Use /dev/null to discard output or errors and as an empty input.
- Combine pipes and redirections of all three streams in one command line.
A pipe sends output to one place and a redirection to another, but sometimes you want both: to watch a long command's progress on the screen *and* keep a copy in a file. That is tee, named after the T-shaped pipe fitting. Its opposite is /dev/null, the place you send output you never want to see. Between them they close the module on streams, so this lesson also pulls everything together in one table.
tee: one input, two outputs
command | tee file writes everything that flows through the pipe into file and also passes it on to stdout, so you still see it and can keep piping: make 2>&1 | tee build.log | grep -i error saves the whole build log while showing only the errors. tee -a appends instead of overwriting. You can give tee several file names to write the same data to all of them.
~% ls /etc | head -3 | tee first3.txt
fstab
group
hostname
~% cat first3.txt
fstab
group
hostname
~% echo 'run 2' | tee -a run.log
run 2
~% cat run.log
run 1
run 2
~% ls /root /nope > ok.txt 2>/dev/null
~% cat ok.txt
/root:
~% cat /dev/null | wc -c
0/dev/null: the bit bucket
/dev/null is a device file that accepts any amount of data and stores none of it; reading from it returns end-of-file immediately. Three uses come up constantly: 2>/dev/null hides error messages you have decided are noise; >/dev/null hides normal output when you only care about the exit status, as in if grep -q pattern file; and </dev/null gives a command an empty stdin so it cannot wait for the keyboard.
The streams, all in one place
| Syntax | stdin (0) | stdout (1) | stderr (2) |
|---|---|---|---|
cmd | keyboard | screen | screen |
cmd < f | file f | screen | screen |
cmd > f | keyboard | file f | screen |
cmd 2> f | keyboard | screen | file f |
cmd > f 2>&1 | keyboard | file f | file f |
cmd \| other | keyboard | other's stdin | screen |
cmd \| tee f | keyboard | file f and screen | screen |
cmd 2>/dev/null | keyboard | screen | discarded |
Putting it together
A realistic line uses several of these at once. find / -name '*.conf' 2>/dev/null | sort | tee confs.txt | wc -l searches the whole disk, hides the permission errors, sorts the paths, keeps them in a file and prints how many there were. Read it left to right, one bar at a time, asking what text flows through, and nothing in it is mysterious. That habit, more than any single command, is what this module was about.
One detail specific to this VM: there are no /dev/stdout or /dev/stderr files, which some scripts use as targets. Use >&2 to write to stderr and plain stdout for stdout; both are portable and work everywhere.
Commands in this lesson
| Command | What it does |
|---|---|
cmd | tee file | Save stdout to file and still show it. |
cmd | tee -a file | Same, appending to file. |
cmd | tee f1 f2 | Write the output to several files at once. |
cmd > /dev/null | Discard normal output. |
cmd 2> /dev/null | Discard error messages. |
cmd > /dev/null 2>&1 | Discard everything; keep only the exit status. |
cmd < /dev/null | Run cmd with an empty stdin. |
Quiz
What does `sort data.txt | tee sorted.txt` do?
- Writes the sorted lines to sorted.txt and also prints them
- Writes them to sorted.txt silently
- Prints sorted.txt and data.txt side by side
Which command hides both output and errors, keeping only the exit status?
- `cmd > /dev/null`
- `cmd 2> /dev/null`
- `cmd > /dev/null 2>&1`
What happens when a program reads from /dev/null?
- It blocks waiting for data
- It gets end-of-file immediately
- It reads zeros forever
How do you add a command's output to the end of log.txt while still watching it?
- `cmd >> log.txt`
- `cmd | tee -a log.txt`
- `cmd | tee log.txt`
Practice
Sort /root/lab/l30/data.txt and, in one command, save the sorted lines in /root/lab/l30/sorted.txt while they also appear on the screen.
/root/lab/l30/run.log contains `run 1`. Using echo and tee, append the line `run 2` to it so that the line is also shown on screen.
/root/lab/l30/noisy is a program that prints `ok: 3 files processed` on stdout and `warning: cache is cold` on stderr. Run it so that the normal output is saved in /root/lab/l30/ok.txt and the warning is discarded.
Open this lesson in the app to do the tasks in a real Linux machine and have them checked.