command

What it does

command is a POSIX shell builtin (commonly in bash, dash, ksh, zsh). It helps you answer:

• “Does this command exist?”
• “What will my shell execute when I type this?”
• “How do I bypass an alias or function?”

In scripts, command -v is the most reliable, portable way to test for tools.

How it works (mechanical)

When you type a name, the shell resolves it in this general order (varies slightly by shell):

• aliases
• shell functions
• shell builtins
• PATH search (external programs)

command asks the shell to perform its normal resolution logic and report the result — and optionally to execute the resolved external command while skipping alias/function overrides.

10 Practical Examples

# 1) Check if a command exists (exit 0 if found, non-zero if not)
command -v mtr >/dev/null 2>&1

# 2) Print what it resolves to (path, alias, function, builtin)
command -v mtr

# 3) Verbose description (bash: more descriptive than -v)
command -V ls

# 4) In a script: fail fast if missing
if ! command -v mtr >/dev/null 2>&1; then
  echo "mtr not found. Exiting."
  exit 1
fi

# 5) Bypass an alias/function and run the external program (common pattern)
command ls

# 6) Prefer default PATH search (ignore shell PATH modifications; shell-dependent)
command -p ls

# 7) Show the PATH-resolved executable for a tool (portable)
MTR="$(command -v mtr)"
echo "$MTR"

# 8) Diagnose “why did /usr/bin/command -v fail?” (because command is usually a builtin)
type command
# output: "command is a shell builtin"

# 9) Compare with which (which is external and can be inconsistent)
which mtr
command -v mtr

# 10) Avoid alias surprises in scripts (example)
# If someone aliased rm='rm -i', this still runs rm normally:
command rm -rf /tmp/testdir

Notes & Gotchas

• Usually not /usr/bin/command: on many systems command is only a shell builtin. So /usr/bin/command -v fails because that file often doesn’t exist.
• Use in scripts: prefer command -v over which for portability.
• -V output varies: the descriptive format differs between shells.
• -p behavior varies: support differs by shell; don’t rely on it unless you know your shell.

Historical Context

command is part of POSIX shell behavior and has been present for decades. It exists specifically because scripts need a predictable way to test for tools and to bypass interactive shell customizations (aliases/functions) that can break automation.

Related Commands

• type — show how a name is interpreted (builtin, function, alias, file)
• which — locate executables (external; can differ from shell resolution)
• hash — manage the shell’s command lookup cache
• whereis — locate binaries/source/man pages (system database approach)