Shell

A running collection of interactive shell tricks worth knowing.

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End of Options (--)

-- tells a command to stop parsing options — everything after it is a literal operand, even if it starts with -. Place it after the command’s own flags, before the operands:

rm -- -f          # removes a file literally named "-f"
rm -rf -- -f       # real options first, then --, then the operand
touch -- --help    # creates a file named "--help" instead of printing help

Most useful when a filename or argument you don’t control could start with - and get mistaken for a flag.

Process Substitution

Feed a command’s output to another command as if it were a file, without writing a temp file yourself.

diff <(cmd1) <(cmd2)

Fish has no <() syntax — use psub instead, which pipes into a temp file under the hood:

diff (cmd1 | psub) (cmd2 | psub)

Time-Limit a Command (timeout)

Kill a command if it runs longer than a given duration — useful for flaky network calls or scripts that might hang.

timeout 5s curl https://example.com
timeout -k 2s 5s some-command   # send SIGTERM at 5s, SIGKILL 2s later if still alive
timeout --signal=INT 10s cmd    # send a specific signal instead of the default SIGTERM

Exit code 124 means the command was killed by the timeout, not that it failed on its own.

For a command you’re not sure will terminate, timeout replaces the background-and-kill dance (cmd & ...; kill %1) — it cleans up after itself and the exit code tells you what happened:

timeout 30s python main.py; test $status -eq 124; and echo "timed out"   # fish
timeout 30s python main.py; [ $? -eq 124 ] && echo "timed out"            # bash/zsh
timeout --foreground 30s <command>   # needed when the command reads from the TTY interactively

macOS ships no timeout; install GNU coreutils (brew install coreutils), which provides gtimeout.