which

Overview

The which command locates executable files in the system PATH. It shows the full path of commands that would be executed when typed in the shell.

Syntax

which [options] command...

Common Options

Option Description
-a Show all matches in PATH
-s Silent mode (exit status only)
--version Show version
--help Show help

Key Use Cases

  1. Find executable locations
  2. Verify command availability
  3. Check PATH configuration
  4. Troubleshoot command issues
  5. Script validation

Examples with Explanations

Example 1: Find Command Location

which python

Shows the path to the python executable

Example 2: Multiple Commands

which python java gcc

Shows paths for multiple commands

Example 3: All Matches

which -a python

Shows all python executables in PATH

Understanding Output

  • Returns full path if found
  • No output if command not found
  • Exit status 0 if found, 1 if not found
  • Searches PATH directories in order

Common Usage Patterns

  1. Check if command exists:

    which git > /dev/null && echo "Git is installed"

  2. Find all versions:

    which -a python

  3. Script validation:

    command -v python || echo "Python not found"

PATH Environment

The which command searches directories in the PATH environment variable:

echo $PATH

PATH order matters: - First match is returned - Earlier directories take precedence - Use -a to see all matches

Performance Analysis

  • Very fast operation
  • No filesystem scanning
  • Only searches PATH directories
  • Minimal resource usage
  • Efficient for scripting

Additional Resources

Best Practices

  1. Use in scripts to check dependencies
  2. Combine with conditional statements
  3. Use command -v for portability
  4. Check exit status for automation
  5. Use -a to see all available versions

Alternative Commands

  1. command -v (POSIX standard):

    command -v python

  2. type command:

    type python

  3. hash for cached locations:

    hash python

Scripting Examples

  1. Dependency check:

    for cmd in git python make; do
    which "$cmd" > /dev/null || echo "$cmd not found"
done

  2. Version selection:

    PYTHON=$(which python3 || which python)

  3. Conditional execution:

    which docker > /dev/null && docker --version

Troubleshooting

  1. Command not found in PATH
  2. Permission issues
  3. Symlink resolution
  4. Shell built-ins not shown
  5. Alias interference

Shell Built-ins

Note: which doesn’t find shell built-ins like: - cd - echo (in some shells) - pwd - history

Use type to identify built-ins:

type cd

Integration Examples

  1. With if statements:

    if which node > /dev/null; then
    node --version
fi

  2. With variables:

    EDITOR=$(which vim || which nano || which vi)

  3. Error handling:

    which python3 || { echo "Python3 required"; exit 1; }