The which, type, and readlink linux commands

The which command identifies the locations of commands. It searches the system PATH for executables.

$ which ls
/usr/bin/ls

The -a option prints all matching locations.

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

To inspect the PATH, use tr to split it into separate lines for improved 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

Here are the fundamentals of navigating manual pages:

Press q to exit.
Press h for help.
Press /pattern to search forward.
Press n to repeat search forward.
Press N to repeat search backward.
Use arrow keys, Space, Home, End, PgUp, PgDn, etc.
Many Vim motions are supported.

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 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

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")"

As an aside, use update-alternatives to change alternatives.

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

The which, type, and readlink linux commands ​

The which, type, and readlink linux commands