The which, type, and readlink Linux commands

The which command locates executable files by searching the system PATH.

$ which ls
/usr/bin/ls

Use the -a option to display all matching locations.

$ which -a ls
/usr/bin/ls
/bin/ls

To examine the PATH variable, use tr to split it into separate lines for better readability:

$ echo $PATH
/usr/local/bin:/usr/bin:/bin

$ echo $PATH | tr ':' '\n'
/usr/local/bin
/usr/bin
/bin

Built-in commands, such as cd, have no location; they are built into the shell. The which command prints nothing for built-in commands and there is no manual page. The shell's manual page contains information about the built-in commands.

$ which cd

$ man cd
No manual entry for cd

$ echo $SHELL
/bin/bash

$ man bash

Navigating Manual Pages

• q: Exit
• h: Display help
• /pattern: Search forward
• n: Repeat search forward
• N: Repeat search backward
• Use arrow keys, Space, Home, End, PgUp, PgDn
• Supports Vim motions

The type Command

The type command can determine if a command is a built-in, its location, the alias expansion for aliases, and the complete function for functions.

$ type cd
cd is a shell built-in

$ type which
which is /usr/bin/which

$ type ls
ls is aliased to 'ls --color=auto'

$ type greet
greet is a function
greet ()
{
    echo "Hello, world."
}

The readlink Command

The which and type commands don't resolve symbolic links.

Use the readlink command to resolve symbolic links, and use the -f option to resolve them recursively. Since the readlink command expects a path, use the which command to locate the path first.

$ which x-terminal-emulator
/usr/bin/x-terminal-emulator

$ readlink `which x-terminal-emulator`
/etc/alternatives/x-terminal-emulator

$ readlink -f `which x-terminal-emulator`
/usr/bin/snap

Handling Newlines and Options

The readlink command outputs a trailing newline by default, which can be an issue in a scripting environment. Use the -n option to omit the newline.

RESOLVED_PATH="$(readlink -nf /usr/bin/x-terminal-emulator)"

You may want to use --, the standard end-of-options signal, to prevent the path argument from being interpreted as an option. For example, without --, the argument -version would be interpreted as an option, not a path.

RESOLVED_PATH="$(readlink -nf -- "$1")"

Managing Alternatives

To modify symbolic link alternatives, use update-alternatives:

$ sudo update-alternatives --config x-terminal-emulator