hash

View cached command locations for the current shell

$ hash

$ hash -r

$ hash -d [command]

$ hash -t [command]

$ hash --help

hash [-lrtd] [-p filename] [-v command ...]

-d
    Forget the remembered location of each command. If command is not specified, forget all remembered locations.

-l
    Display the remembered locations in a format that can be reused as input. Typically used for recreating the hash table.

-p filename
    Associate command with filename. Useful when you have a command with the same name in different locations and want to explicitly use a certain one.

-r
    Forget all remembered locations. The shell will search $PATH for each command again upon next execution.

-t
    Write the remembered location of each name to standard output. If a name is not found, nothing is displayed.
Useful for script development.

-v
    Print the location of each command that would be executed.

command ...
    Commands to hash. If no arguments are supplied, the command displays the contents of the hash table.

The hash command in Linux is a built-in shell command used to remember the full pathnames of executable files. When you enter a command, the shell has to search your $PATH environment variable to find the executable. This search process can take time, especially if the executable is located in a directory that is low in the $PATH list. The hash command stores the full path of frequently used commands in an internal hash table. Subsequent calls to these commands will bypass the path search, leading to faster execution.

You can use hash to view the contents of the hash table, add new commands to it manually, or clear the table. It's a useful tool for optimizing shell performance, especially in scripts or interactive sessions where certain commands are executed repeatedly.

The hash command only affects the current shell environment. When a new shell is started, the hash table is empty. The -p option may not work as intended if the specified filename is not an absolute path.

Returns 0 unless a command is not found or an invalid option is supplied.

hash ls: Add the ls command to the hash table.
hash -r: Clear the hash table.
hash: Display the contents of the hash table.

The hash command has been a standard part of Unix-like operating systems for many years. Its purpose has always been to improve command lookup performance within the shell. Initially, its behavior and available options varied slightly between different shell implementations (e.g., Bourne shell, C shell, Korn shell). Over time, POSIX standards helped to define a more consistent interface, though subtle differences may still exist in specific shell implementations.

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