tee: Usage, Options & Common CI Errors

tee splits a stream: it passes data through while also saving a copy to a file.

tee lets you log output and keep it flowing down the pipe. In CI it is the trick for writing root-owned files (sudo tee) and the reason a pipeline can hide a failing command exit code.

What it does

tee reads stdin and writes it to both stdout and one or more files. It is used to capture build logs while still displaying them, and to write files that need elevated permissions.

Common usage

Terminal

make build 2>&1 | tee build.log
echo "deb ..." | sudo tee /etc/apt/sources.list.d/x.list
some_cmd | tee -a run.log          # append
echo "127.0.0.1 host" | sudo tee -a /etc/hosts
cmd | tee out.txt | grep ERROR

Options

Flag	What it does
-a / --append	Append to files instead of overwriting
-i / --ignore-interrupts	Ignore SIGINT
(sudo) tee	Write a file as root from a non-root pipe

Common errors in CI

The big gotcha: cmd | tee log.txt exits with tee's status, not cmd's - a failing build can look green because tee succeeded. Use set -o pipefail so the pipeline reflects cmd's failure (or check ${PIPESTATUS[0]} in bash). "tee: /etc/...: Permission denied" means you piped into a privileged path without sudo tee. Appending without -a truncates the file each run.

Related guides

cat: Usage, Options & Common CI Errorscat concatenates and prints files and writes here-docs. Reference for -n, -A, heredocs, and the "No such file… set -o pipefail: Usage & Common CI Gotchasset -o pipefail makes a pipeline fail if any stage fails, not just the last. Reference for its tee/head inter… head: Usage, Options & Common CI Errorshead prints the first lines or bytes of input. Reference for -n, -c, negative counts, and the SIGPIPE/exit-14… tail: Usage, Options & Common CI Errorstail prints the last lines of input or follows a growing file. Reference for -n, -f, +N, and the "-f never ex…