What is the difference between $@ and $*?

Both expand to all positional parameters. Unquoted they behave the same, but quoted they differ: "$@" expands to each argument as a separate quoted word, preserving spaces, while "$*" joins them into one word separated by the first character of IFS. In loops always use "$@".

Why use $BASHPID instead of $$?

$$ holds the PID of the main shell and does not change inside a subshell, so it is stable but can be misleading. $BASHPID gives the PID of the current subshell process, which differs inside parentheses or a pipeline. Use $BASHPID when you need the actual current process.

How do I get the exit status of a piped command?

$? only reports the exit status of the last command in a pipeline. The array PIPESTATUS holds the status of every command in the most recent pipeline, so ${PIPESTATUS[0]} is the first command's status. Capture it immediately because the next command overwrites it.

Does $? need to be saved right away?

Yes. $? is updated after every command, including the ones in your error-handling logic. If you need both an exit code and to run other commands, assign it first, as in status=$? and then branch on $status.

$_ expands to the last argument of the previous command, and at shell startup to the absolute path of the script or shell being run. It is handy interactively, for example mkdir build && cd $_ to enter the directory you just created.

Bash Special Variables Reference

Bash exposes a set of special variables the shell maintains for you — exit status, process IDs, positional parameters, and more. Misreading them, or forgetting that $? is overwritten immediately, causes subtle scripting bugs. This tool is a searchable reference of every special and automatic variable with its meaning, scope, and a usage example.

How it works

These variables fall into a few families:

Status & control — $? (last exit code), $- (current shell flags), PIPESTATUS (per-command status of the last pipeline).
Process IDs — $$ (shell PID), $BASHPID (current process PID), $! (PID of the last background job), $PPID (parent PID).
Positional parameters — $0 (script name), $1…$9, ${10}, $# (count), $@ and $* (all args), $_ (last arg of previous command).
Shell internals — $IFS (field separator), $RANDOM, $SECONDS, $LINENO, $FUNCNAME, $BASH_SOURCE.

Worked example

#!/usr/bin/env bash
process() {
  grep -q "$1" "$2"          # may succeed or fail
  local status=$?            # capture immediately
  if (( status != 0 )); then
    echo "not found (exit $status)" >&2
  fi
}

for arg in "$@"; do          # quoted "$@" keeps each arg intact
  process "$arg" config.txt
done

false | true
echo "${PIPESTATUS[0]}"      # 1 — the failing 'false', not the pipeline's 0

Notes

Always quote "$@" in loops and when forwarding arguments; unquoted $@ re-splits on whitespace and breaks filenames with spaces.
$? is clobbered by the next command — including the test in an if. Save it to a local variable the moment you need it.
$$ stays constant across subshells (it is the main shell’s PID); reach for $BASHPID when you genuinely need the current subshell’s PID.
$RANDOM returns a value 0–32767 and is not cryptographically secure; use /dev/urandom for security-sensitive randomness.

Bash Special Variables Reference

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How it works

Worked example

Notes