Hash Commands in Bash Scripting

In this blog, we will learn about the Hash Commands in Bash Scripting. We will see all the options that are present in Linux. Now, let’s get started with the blog.

Introduction to Hash Commands in Bash

A hash is a built-in command of the bash shell on UNIX-like operating systems that is used to list a hash table of recently run commands. It is utilized for manual adjustments, resets, and viewing of the bash path hash. Every time we request to see them, it displays the locations of recently run apps. Each command name’s full pathname is provided.

In other words, the shell begins looking for any command that is executed without specifying its path among the folders that are mentioned in the path variable. The position of the command is stored in a hash table when it is received by Bash so that it can remember where it is. Bash then stops looking for the command again and starts looking in the table to find its placement. It speeds up the execution of the command.

The shell won’t be able to locate the command if it is moved after registering its location in a table. To obtain the command data in this instance, a thorough search of all the directories in the path is carried out.

The built-in ‘hash’ command is responsible for maintaining the hash table. Without any switches, the hash lists the previously used commands, their locations, and the number of times they have been executed during the session.

Syntax:

hash [-l] [-r] [-p pathname] [-d] [-t] [command_name . . .]

The entire pathname of each “command_name” can be found and remembered using the syntax mentioned above. It displays information about previously used commands and their locations if there are no arguments.

A) Exit Status

The hash command returns ‘0’ for success. A value other than zero means that the “command_name” is not found or an invalid option is given.

B) Listing Bash hash Table

The “hash" command in Linux is used to display or clear the command hash table. The hash table is a data structure used by the shell to quickly look up the paths of executable files (commands) that have been run recently in the current session.

This helps improve the speed of command execution by avoiding the need to search the entire “PATH" every time a command is run.

┌──(kali㉿kali)-[~/Downloads/Bash/Conditional_Statement]
└─$ hash
1password2john=/usr/bin/1password2john
2to3-2.7=/usr/bin/2to3-2.7
7z=/usr/bin/7z
7z2john=/usr/bin/7z2john
7za=/usr/bin/7za
7zr=/usr/bin/7zr

Here, you can see the hash table of my last used commands in the system.

Note: If a new session is opened where no command has been executed, then there will be no hash table for that session.

C) Clearing the hash Table

To clear the entire command hash table, you can use the -r option:

┌──(kali㉿kali)-[~/Downloads/Bash/Conditional_Statement]
└─$ hash -r
                                                                                                                                                                                                                                           
┌──(kali㉿kali)-[~/Downloads/Bash/Conditional_Statement]
└─$ hash

From the above command, this will clear all entries from the hash table, causing the shell to rebuild the table the next time a command is executed.

D) Adding a Command Path and Name to the Bash Hash Table

The hash table can be expanded with items that can be used repeatedly in the shell. The hash table only lives in the shell’s current active session, it should be kept in mind. When a new shell is opened, bash will build a new hash table based on the commands that were run in that shell.

Bash starts building a hash table as soon as we start executing the first command. We can use the -p option, the path, and then the name to add a command to the hash table.

┌──(kali㉿kali)-[~/Downloads/Bash/Conditional_Statement]
└─$ hash -p /usr/local/bin ls

E) Delete Specific commands from the hash table

We also have the option to delete or forget a remembered location of commands in hash bash. We can simply use the -d option followed by the name to perform this task:

$ hash -d bash

Here, we have deleted /home/bash.sh from the hash table, which was mapped with the name ‘bash‘.

Conclusion

Other options we can see in the hash command are:

Options	Description
-d	Forget the remembered locations of command_name.
-l	Display the information that can be used again as an input for another program.
-p	Use pathname as the full path of command_name.
-r	Display the information that can be used again as input for another program.
-t	Print the remembered location of each command_name. If multiple command_names are given there, precede each location with corresponding command_name
command_name	Each command_name specified is searched for in the path environment variable, and if found, is added to the list of remembered commands.

All Bash Hash Options Command

Note: This blog is mainly referenced from Javatpoint Bash Hash Commands.

FAQ

What are hash commands in Linux?

How to clear the hash table in Linux?

hash -r

Hash Commands in Bash Scripting | Bash hash Commands

Introduction to Hash Commands in Bash