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Cheat sheets

tee cheat sheet

Save a command's output to a file while still passing it on: logging long jobs, writing root-owned files with sudo, and splitting a stream to several places.

2 min read

tee copies stdin to stdout and to one or more files at the same time, like a T-joint in a pipe.

Basics

bash
make 2>&1 | tee build.log               # watch and save
make 2>&1 | tee -a build.log            # append instead of overwriting
cmd | tee out1.txt out2.txt             # several files at once
cmd | tee /dev/tty | wc -l              # show on screen, count in the pipe
FlagWhat it does
-aAppend to the files
-iIgnore Ctrl-C (SIGINT)
-pKeep going when writing to a pipe fails (GNU)

Write as root

bash
# fails: the redirect runs as you, not as root
sudo echo "vm.swappiness=10" > /etc/sysctl.d/99-swap.conf

# works: tee runs as root
echo "vm.swappiness=10" | sudo tee /etc/sysctl.d/99-swap.conf > /dev/null
echo "192.0.2.5 db.internal.example.com" | sudo tee -a /etc/hosts > /dev/null

# a whole file
sudo tee /etc/apt/sources.list.d/example.sources > /dev/null <<'EOF'
Types: deb
URIs: https://repo.example.com/apt
Suites: stable
Components: main
Signed-By: /etc/apt/keyrings/example.gpg
EOF

The > /dev/null keeps tee from echoing the file contents to your terminal.

Log a long job

bash
./migrate.sh 2>&1 | tee "migrate-$(date +%F-%H%M).log"
rsync -av /data/ /backup/ | tee -a /var/log/rsync.log | tail -n 5

Split a stream

bash
# save the raw log and a filtered copy in one pass
tail -F app.log | tee raw.log | grep --line-buffered ERROR > errors.log

# checksum while downloading
curl -fsSL https://example.com/app.tar.gz | tee app.tar.gz | sha256sum

# send to two commands (bash process substitution)
cat access.log | tee >(grep ' 500 ' > 5xx.log) >(wc -l > count.txt) > /dev/null

Exit codes

bash
make 2>&1 | tee build.log
echo $?                                 # tee's status, not make's

set -o pipefail                         # the pipe fails if make fails
make 2>&1 | tee build.log || echo "build failed"

make 2>&1 | tee build.log
echo "${PIPESTATUS[0]}"                 # make's own status (bash), read it right after the pipe

Keep in mind

  • tee file truncates the file first, like >. Use -a for logs you keep.
  • Output may arrive in chunks rather than line by line when the command before tee buffers. Use stdbuf -oL cmd | tee log or the program’s own line-buffer flag.

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