How can I check if a program exists from a Bash script?

To check if a program exists in a Bash script, use the command -v built-in command, which is POSIX-compliant and reliable. Here’s how to implement it:

Check if a Program Exists

if command -v <program_name> &> /dev/null; then
    echo "<program_name> exists"
else
    echo "Error: <program_name> is not installed" >&2
    exit 1
fi

Example (check for curl):

if command -v curl &> /dev/null; then
    echo "curl is installed"
else
    echo "curl is not installed" >&2
    exit 1
fi

Explanation

1. command -v:

• Returns the path to the executable if it exists in $PATH.
• Works for built-in commands, aliases, and external programs.
• Exits with code 0 (success) if the program exists, 1 (failure) otherwise.

1. &> /dev/null:

• Silences both stdout and stderr to avoid unwanted output.

1. exit 1:

• Terminates the script with an error code if the program is missing.

Why Avoid which or type?

• which:
• Not POSIX-compliant and may behave inconsistently across systems.
• Only checks for external executables (ignores built-ins).
• type:
• Bash-specific (not guaranteed in all shells).
• Output parsing is error-prone.

Check for Executability

To ensure the program is executable, add a secondary check:

program_path=$(command -v <program_name>)
if [[ -x "$program_path" ]]; then
    echo "<program_name> is executable"
fi

Examples in Action

1. Check for git:

if command -v git &> /dev/null; then
    echo "Git is installed at $(command -v git)"
else
    echo "Git is required but not installed." >&2
    exit 1
fi

2. Check for docker and docker-compose:

for cmd in docker docker-compose; do
    if ! command -v "$cmd" &> /dev/null; then
        echo "Error: $cmd is not installed" >&2
        exit 1
    fi
done

Key Takeaways

• Use command -v for portable and reliable checks.
• Redirect output to /dev/null to suppress messages.
• Always handle missing dependencies gracefully in scripts.