tee
Write a stream to a file and pass it on
tee reads standard input, writes an unmodified copy to every file you name, and passes the
same bytes along to standard output. It is named after a T-junction in plumbing, which is
exactly what it does to a pipeline.
That covers two jobs that look unrelated. The first is keeping a copy of something you are
watching go past, without running the command twice: make | tee build.log puts the build on
screen and on disk at once.
The second is the one people arrive here for. sudo echo something > /etc/somefile does not
work, and the reason surprises everyone: sudo applies to echo, but the redirect is
performed by your shell, which is still you. sudo tee moves the writing into the command that
was elevated, which is why every guide tells you to pipe into it.
Sample files used on this page
Every example below was run against these files. Recreate them to follow along.
wordlist.txt the same wordlist the sort and uniq pages use - duplicates and mixed case
banana
apple
Cherry
apple
date
banana
apple
Elderberry
cherry
access.log standard combined log format, for the examples that watch a pipeline go past
203.0.113.5 - - [13/Aug/2026:09:12:01] "GET /index.html HTTP/1.1" 200 512
203.0.113.5 - - [13/Aug/2026:09:12:03] "GET /style.css HTTP/1.1" 200 231
198.51.100.7 - - [13/Aug/2026:09:14:22] "GET /index.html HTTP/1.1" 200 512
198.51.100.7 - - [13/Aug/2026:09:14:25] "GET /missing.html HTTP/1.1" 404 162
203.0.113.5 - - [13/Aug/2026:09:15:47] "GET /index.html HTTP/1.1" 200 512
192.0.2.44 - - [13/Aug/2026:09:16:03] "POST /login HTTP/1.1" 302 0
192.0.2.44 - - [13/Aug/2026:09:16:04] "GET /dashboard HTTP/1.1" 200 4021
198.51.100.7 - - [13/Aug/2026:09:18:51] "GET /index.html HTTP/1.1" 200 512
203.0.113.5 - - [13/Aug/2026:09:19:10] "GET /api/status HTTP/1.1" 500 89
192.0.2.44 - - [13/Aug/2026:09:20:33] "GET /dashboard HTTP/1.1" 200 4021
Save a copy without losing the stream
> sends output to a file instead of the screen. tee sends it to a file as well as the screen, and on to the next command in the pipeline if there is one.
Write output to a file and still see it
sort wordlist.txt | tee sorted.txtThe sorted list appears on screen exactly as it would without tee, and sorted.txt now holds the same bytes. Compare sort wordlist.txt > sorted.txt, which shows you nothing.
Show output
Cherry
Elderberry
apple
apple
apple
banana
banana
cherry
date
Keep the file, skip the screen
sort -u wordlist.txt | tee unique.txt > /dev/null && cat unique.txtRedirecting tee's own output to /dev/null throws away the copy that would have gone to the terminal. Pointless on its own - that is just > - but it is the standard shape for sudo tee, further down.
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Cherry
Elderberry
apple
banana
cherry
date
Put tee in the middle of a pipeline
sort wordlist.txt | tee sorted.txt | wc -lwc receives the full stream unchanged while sorted.txt captures it in passing. This is what tee is for: a tap on a pipe, not a fitting on the end of one.
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9
Prove the tap and the pipeline agree
sort -u wordlist.txt | tee unique.txt | wc -l && wc -l unique.txtThe count downstream and the count in the file agree. tee copies bytes and changes nothing, which is what makes it safe to drop into a pipeline you are debugging.
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6
6 unique.txt
Write the same stream to several files at once
head -3 wordlist.txt | tee copy-a.txt copy-b.txtEvery filename argument gets its own complete copy. There is no flag for this - extra files are just extra arguments.
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banana
apple
Cherry
Confirm every copy is identical
head -3 wordlist.txt | tee copy-a.txt copy-b.txt > /dev/null && tail -n +1 copy-a.txt copy-b.txttail -n +1 on several files prints each with a header, which is a quick way to show two files side by side without running diff.
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==> copy-a.txt <==
banana
apple
Cherry
==> copy-b.txt <==
banana
apple
Cherry
Use tee with no filenames at all
tee < wordlist.txt | head -3With no file arguments tee is simply cat: stdin to stdout, nothing captured. Occasionally useful when a script builds the argument list and it comes out empty.
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banana
apple
Cherry
Overwriting and appending
tee overwrites by default
printf 'old\n' > notes.txt && printf 'new\n' | tee notes.txt && cat notes.txtLike >, tee truncates the file before writing. The output appears twice here because tee printed it once and cat read it back.
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new
new
Append instead, with -a
printf 'first run\n' | tee build.log && printf 'second run\n' | tee -a build.log && cat build.log-a (--append) is tee's equivalent of >>. The two tee calls print as they go, then cat shows the accumulated file.
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first run
second run
first run
second run
-a creates the file if it is not there yet
printf 'a\nb\n' | tee -a fresh.log && cat fresh.logNo need to touch the file first. -a only means "do not truncate", not "the file must already exist".
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a
b
a
b
Writing files your shell cannot
The reason most people meet tee. A redirect is performed by the shell, and the shell is still you even when the command after sudo is not.
The problem: a redirect into a file you do not own
echo 'vm.swappiness=10' > /etc/sysctl.d/99-swappiness.confFails with Permission denied as an ordinary user. Verified in the sandbox: nothing is written and the shell reports the failure before echo ever runs.
The trap: sudo does not fix it
sudo echo 'vm.swappiness=10' > /etc/sysctl.d/99-swappiness.confExactly the same Permission denied, which is the surprising part. sudo elevates echo, but the > is carried out by your shell before sudo is involved at all - and your shell has no more right to write /etc than it did a moment ago. Verified in the sandbox: the file is still not created.
The fix: pipe into sudo tee
echo 'vm.swappiness=10' | sudo tee /etc/sysctl.d/99-swappiness.confNow the process doing the writing is the one that was elevated. tee echoes what it wrote, which is why you see the line twice in a terminal - once from tee, and it is the only copy.
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vm.swappiness=10
Silence the echo with > /dev/null
echo 'vm.swappiness=10' | sudo tee /etc/sysctl.d/99-swappiness.conf > /dev/null && cat /etc/sysctl.d/99-swappiness.confThe idiomatic form. Without > /dev/null every line you write is repeated back at you, which is noise on a multi-line file. The cat here just proves the write landed.
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vm.swappiness=10
Write a whole config file from a heredoc
sudo tee /etc/sysctl.d/99-swappiness.conf > /dev/null <<'EOF'
# Reduce swap pressure on a database host.
vm.swappiness=10
vm.vfs_cache_pressure=50
EOF
cat /etc/sysctl.d/99-swappiness.confThe pattern for creating a root-owned config file in one step. Quoting the delimiter as 'EOF' stops the shell expanding $ and backticks inside the block, which matters for anything containing a variable you meant literally.
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# Reduce swap pressure on a database host.
vm.swappiness=10
vm.vfs_cache_pressure=50
Check who ends up owning the file
echo 'vm.swappiness=10' | sudo tee /etc/sysctl.d/99-swappiness.conf > /dev/null && stat -c '%U %G %a %s' /etc/sysctl.d/99-swappiness.confOwned by root, mode 644, because tee created it as the elevated user. stat -c is used here rather than ls -l because it prints no timestamp - see chmod for what the mode means and chown for changing the two names in front of it.
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root root 644 17
Append to a root-owned file
echo 'vm.vfs_cache_pressure=50' | sudo tee -a /etc/sysctl.d/99-swappiness.conf > /dev/null && cat /etc/sysctl.d/99-swappiness.confsudo tee -a is the elevated >>. The same trap applies: sudo echo x >> /etc/file fails for exactly the reason the plain redirect did.
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vm.vfs_cache_pressure=50
Watching a pipeline go past
Dropping tee between two stages is the cheapest way to find out which stage broke, because it captures the intermediate result without changing it.
Count matches and keep them
grep ' 200 ' access.log | tee matched.log | wc -lThe count comes back on screen while matched.log holds the lines themselves, so you can look at them without re-running grep.
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7
Capture a full ranking while showing only the top of it
sort -k9 -rn access.log | tee ranked.log | head -2head closes the pipe after two lines, but tee has already written every line it saw to the file. The full ranking is on disk, the interesting part is on screen.
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192.0.2.44 - - [13/Aug/2026:09:20:33] "GET /dashboard HTTP/1.1" 200 4021
192.0.2.44 - - [13/Aug/2026:09:16:04] "GET /dashboard HTTP/1.1" 200 4021
See the data and the summary at the same time
sort -u wordlist.txt | tee /dev/stderr | wc -lWriting to /dev/stderr puts the stream on your terminal while stdout carries on to wc. Nothing is captured to disk, which is what you want when you only need to look once.
Show output
Cherry
Elderberry
apple
banana
cherry
date
6
Fan a stream into another command, not a file
sort -u wordlist.txt | tee >(wc -l > count.txt) > /dev/null>(...) is bash process substitution: it looks like a filename to tee, but it is a pipe into the command inside. Handy for running two analyses over one stream. The two branches finish independently, so read count.txt in a later step rather than immediately.
Exit status, errors, and one way to lose your file
tee hides the exit status of the command before it
false | tee out.txt; echo "exit $?"$? is tee's status, not false's, because the shell reports the last command in a pipeline. A pipeline that failed halfway therefore looks like a success.
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exit 0
Get the real status with pipefail
set -o pipefail; false | tee out.txt; echo "exit $?"set -o pipefail makes a pipeline report the first non-zero status in it. Worth setting in any script that pipes through tee and then checks whether the work succeeded.
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exit 1
Or read the status of a specific stage
false | tee out.txt; echo "pipestatus ${PIPESTATUS[0]}"PIPESTATUS is a bash array holding one status per pipeline stage. [0] is the command before the first pipe. See exit codes and error handling for the wider story.
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pipestatus 1
tee will not create a missing directory
echo hi | tee nodir/file.txtThe error goes to stderr and tee still writes the stream to stdout, then exits 1. It creates files, never directories - mkdir -p first if the path might not exist.
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tee: nodir/file.txt: No such file or directory
hi
What tee does when a write fails
echo hi | tee /dev/full; echo "exit $?"/dev/full is a device that is always out of space, which is how you test this without filling a disk. tee reports the failing file by name and exits non-zero; --output-error tunes whether it also gives up on the remaining ones.
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hi
tee: /dev/full: No space left on device
exit 1
Never tee into the file you are reading
sort -n big.txt | tee big.txt | wc -ltee truncates its output file as it starts, while the command on the left is still reading the same file, so the input is destroyed underneath it. Measured in the sandbox on a 200,000-line file: this reported 0 lines and left the file empty, and the cat version of the same mistake kept 44,176 lines on one run - the amount that survives is a race, which is why it can look harmless on a small file. Write to a temporary file and mv it into place instead.
Discard the copy entirely
echo hi | tee /dev/nullWriting to /dev/null keeps the stream flowing to stdout and captures nothing. Occasionally the right answer when a script's filename variable is legitimately empty.
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hi
Chain tee more than once
seq 1 5 | tee a.txt | tee b.txt | tail -2Each tee taps the stream and passes it on, so both files get the whole thing. One tee a.txt b.txt is the clearer way to write this, but chained taps turn up in pipelines that were built up a stage at a time.
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4
5
Ignore Ctrl-C while writing
printf 'deploy\n' | tee -i deploy.log && cat deploy.log-i (--ignore-interrupts) makes tee ignore SIGINT, so a Ctrl-C aimed at a long-running command upstream does not also kill the thing recording its output. The log survives the interrupt with everything written up to that point.
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deploy
deploy