tee

Additional Notes

tee reads from standard input and writes the same data to standard output and to one or more files. It is the "T" junction of a pipeline: it lets you both display a command's output and save it at the same time.

A common trick is sudo tee FILE > /dev/null to write to a root-owned file from a normal user's pipeline, since > redirection happens before sudo applies.

Syntax

command | tee [options] [file ...]

Parameters

• options: Flags that change how tee behaves.
• file: One or more files to write the input to.

Common Options

• -a, --append: Append to files instead of overwriting them.
• -i, --ignore-interrupts: Ignore interrupt signals so a Ctrl-c does not stop the write.
• --output-error: Control behavior on write errors.
• -p: Diagnose errors writing to non-pipes.

Examples

ls -l | tee listing.txt

Print the listing and save a copy to listing.txt.

make 2>&1 | tee build.log

Save a build log while still watching output.

echo "config line" | sudo tee -a /etc/app.conf > /dev/null

Append a line to a root-owned file from a normal user's shell.

cat data.csv | tee part1.csv part2.csv

Write the same stream to two files and also show it.

./monitor.sh | tee -a run.log

Keep appending live output to a log file.

Practical Notes

• Without -a, tee overwrites the target file; use -a to keep history.
• tee > /dev/null discards the screen copy and keeps only the file write.
• sudo tee is the standard way to redirect into privileged files from a pipeline.
• Useful in long-running pipelines where you want both a live view and a saved record.