tee

Write stdin to file and stdout

$ echo "text" | tee [file]

$ echo "text" | tee -a [file]

$ echo "text" | tee [file1] [file2]

$ [command] | tee [output.log]

$ echo "text" | sudo tee [/etc/file]

$ [command] | tee [file] | [another_command]

$ [command] | tee [file] > /dev/null

tee [options] [file...]

tee reads from standard input and writes to both standard output and one or more files simultaneously. Named after the T-shaped pipe fitting in plumbing, it "splits" the data stream.
Common uses include logging command output while still viewing it, saving intermediate pipeline results for debugging, and writing to files requiring elevated privileges via sudo.
Without -a, tee overwrites existing files. With multiple files, the same content is written to all of them.
Tee continues the pipeline—output goes to both files and stdout, allowing further processing with additional pipe stages.

-a, --append

Append to files instead of overwriting

Ignore SIGINT (interrupt signal)

Diagnose errors writing to non-pipes

Set behavior on write error (warn, warn-nopipe, exit, exit-nopipe)

Display help and exit

Display version and exit

For writing to protected files with sudo, use sudo tee rather than sudo echo > because redirection happens in the current shell before sudo.
In pipelines with set -o pipefail, tee failures can go unnoticed. Use --output-error=exit for strict error handling.
When combined with commands that buffer output, you may not see immediate results. Use stdbuf or command-specific unbuffering options if needed.

cat(1), script(1), pv(1), sponge(1)