which

Updated

September 4, 2026

Overview

which locates executables by searching the PATH environment variable and printing the path of the first match. It is a traditional convenience for interactive use. For scripts, prefer command -v (POSIX) or type -P (bash) — they integrate better with shell functions, aliases, and builtins.

Syntax

which [options] command...

Implementations differ (GNU which, Debian shell script which, BusyBox). Options are not fully portable.

Common Options (GNU / common)

Option Description
-a Show all matches on PATH, not just the first
-s Silent; exit status only (some versions)
--version / --help Version / help (GNU)

Key Use Cases

  1. Interactive “where is this binary?”
  2. See which of several installs comes first (-a)
  3. Quick sanity check after modifying PATH
  4. Compare with shell resolution (type)

Examples with Explanations

Basic

which python3
which ls
which docker kubectl

All matches

which -a python3
which -a java

Shows shadowing (e.g. /usr/local/bin before /usr/bin).

Prefer command -v in scripts

# good for scripts
if command -v jq >/dev/null 2>&1; then
  jq --version
fi

# bash: PATH only, ignore functions/aliases
if type -P jq >/dev/null; then
  echo "jq binary found"
fi

Compare resolution tools

type python3
type -a python3
command -v python3
which python3
which -a python3
Tool Sees aliases Sees functions Sees builtins PATH binaries
type yes yes yes yes
command -v yes* yes* yes yes
which often no no no yes

*shell-dependent; still better than which for “what will run”.

Debug PATH order

echo "$PATH" | tr ':' '\n' | nl
which -a node

Silent existence (when -s exists)

which -s docker && echo present
# portable:
command -v docker >/dev/null && echo present

Notes / Pitfalls

  • Debian/Ubuntu historically shipped which as a shell script with different flags; don’t rely on GNU-only options in portable scripts.
  • which may not know about shell aliases/functions you use interactively — misleading for “what runs when I type this”.
  • Hashed commands in bash (hash -r to clear) affect speed of lookup, not usually which’s external search.
  • Relative PATH entries (. or empty) are a security footgun — which will show what they resolve to.
  • In login vs non-login shells, PATH differs; results differ.

2026-relevant notes

  • Version managers (asdf, nvm, pyenv, mise) put shims early on PATH — which -a is useful to see real binaries behind shims.
  • Containers often have a minimal PATH; command -v in entrypoints is the standard check.
  • Prefer documenting command -v in runbooks and automation.

Additional Resources

  • man which (if present), help type, help command