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Overview

The which command checks if one or more commands are recognized Nash built-in commands. It reports whether each command is available and identifies it as a “nash builtin” if found.

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Syntax

which COMMAND [COMMAND...]

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Description

The which command verifies whether specified command names are recognized Nash built-in commands. Unlike the standard Unix which that searches for executables in PATH, Nash’s which only checks against its list of built-in commands since Nash is a sandboxed shell that doesn’t execute system binaries.

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Options

The which command does not accept any flags or options. It only accepts command names as arguments.

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Arguments

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COMMAND
string
required
One or more command names to check. At least one command name must be provided.

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Exit Status

All specified commands are found:
which ls cd pwd
# Output:
# ls (nash builtin)
# cd (nash builtin)
# pwd (nash builtin)
# Exit code: 0

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Examples

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Check Single Command

which ls
# Output: ls (nash builtin)

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Check Multiple Commands

which cat grep sed
# Output:
# cat (nash builtin)
# grep (nash builtin)
# sed (nash builtin)

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Check Non-Existent Command

which python
# Output: python: not found

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Mixed Results

which echo docker ls
# Output:
# echo (nash builtin)
# docker: not found
# ls (nash builtin)

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Check if Command Exists (Script)

if which jq > /dev/null; then
  echo "jq is available"
else
  echo "jq is not available"
fi
# Output: jq is available

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Silent Check

which grep > /dev/null && echo "Found"
# Output: Found

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Available Nash Built-in Commands

The following commands are recognized by which: 
which cat cp mv rm touch mkdir
# Output:
# cat (nash builtin)
# cp (nash builtin)
# mv (nash builtin)
# rm (nash builtin)
# touch (nash builtin)
# mkdir (nash builtin)

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Practical Use Cases

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Pre-flight Checks

# Verify required commands before running script
REQUIRED="jq grep sed"
for cmd in $REQUIRED; do
  if ! which $cmd > /dev/null; then
    echo "Error: Required command '$cmd' not found"
    exit 1
  fi
done
echo "All required commands available"

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Feature Detection

# Check if advanced features are available
if which jq > /dev/null; then
  echo "JSON processing available"
  cat data.json | jq .
else
  echo "JSON processing not available"
  cat data.json
fi

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Command Availability Report

# Generate report of available tools
echo "Checking available commands..."
echo ""

echo "Data Processing:"
which jq grep sed awk | grep builtin

echo ""
echo "File Management:"
which ls cp mv rm find | grep builtin

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Conditional Logic

# Use different commands based on availability
if which tree > /dev/null; then
  tree -L 2
else
  ls -R
fi

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Debugging Scripts

# Debug which commands a script depends on
echo "Script dependencies:"
which cat grep sed jq echo test

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Documentation Generation

# List all available commands
echo "Available Nash commands:"
which cat cd clear cp cut echo env export false file find grep head \
     help history jq ls mkdir mv pwd rm sed sort stat tail test touch \
     tree true uniq unset wc which | grep builtin | cut -d' ' -f1

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Checking Test Command

The test command has two names: 
which test
# Output: test (nash builtin)

which [
# Output: [ (nash builtin)
Both refer to the same command.

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Error Handling

# Missing argument
which
# Output: which: missing argument
# Exit code: 1

# Multiple commands, some not found
which ls docker python grep
# Output:
# ls (nash builtin)
# docker: not found
# python: not found
# grep (nash builtin)
# Exit code: 1 (because some were not found)

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Comparison with Unix which

Nash’s which behaves differently from standard Unix which:Unix which:
  • Searches for executables in PATH
  • Returns full path to executable
  • Can find system binaries, scripts, etc.
Nash which:
  • Only checks Nash built-in commands
  • Returns “(nash builtin)” for found commands
  • Cannot find system binaries (Nash is sandboxed)
# Unix which
which python
# Output: /usr/bin/python

# Nash which
which python
# Output: python: not found

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Notes

  • At least one command name must be provided
  • Command names are case-sensitive
  • Exit code is 0 only if all commands are found
  • Exit code is 1 if any command is not found
  • Nash only recognizes its built-in commands
  • System commands like git, python, node, etc. will always return “not found”

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Full List of Recognized Commands

All 28+ Nash built-in commands:
  • [ - Test command (alternate syntax)
  • cat - Concatenate and display files
  • cd - Change directory
  • clear - Clear terminal screen
  • cp - Copy files
  • cut - Cut fields from lines
  • echo - Print text
  • env - List environment variables
  • export - Set environment variables
  • false - Return false (exit 1)
  • file - Detect file type
  • find - Search for files
  • grep - Filter lines by pattern
  • head - Show first lines
  • help - Show help information
  • history - Show command history
  • jq - Process JSON data
  • ls - List directory contents
  • mkdir - Create directories
  • mv - Move or rename files
  • pwd - Print working directory
  • rm - Remove files/directories
  • sed - Stream editor
  • sort - Sort lines
  • stat - File status information
  • tail - Show last lines
  • test - Evaluate expressions
  • touch - Create empty files
  • tree - Display directory tree
  • true - Return true (exit 0)
  • uniq - Filter duplicate lines
  • unset - Unset environment variables
  • wc - Count lines/words/bytes
  • which - Show command availability
  • help - Display comprehensive help for all commands
  • test - Test if conditions are true
  • history - Show previously executed commands

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