hash

What does hash do?

When you type a command into your terminal, the shell embarks on a search to locate the corresponding executable file. This involves checking directories specified in your PATH environment variable, a process that can be time-consuming, especially with lengthy PATHs or frequently used commands.

The hash command cleverly addresses this performance bottleneck by creating a cache of command-to-path mappings. Once an executable’s location is found, hash stores this information. Subsequent calls to the same command will directly retrieve the path from the cache, bypassing the lengthy search. This results in faster command execution.

Basic Usage of hash

The most straightforward use of hash is to simply cache the location of a command:

hash my_script.sh

Assuming my_script.sh is an executable script within a directory listed in your PATH, this command will locate and cache its path.

You can also hash multiple commands simultaneously:

hash ls grep find

This caches the locations of ls, grep, and find.

Listing Cached Commands

To see what commands are currently hashed, use the hash -t option:

hash -t

This will output a list of hashed commands along with their respective paths.

You can also check if a specific command is hashed:

hash -t my_script.sh

This will only output information if my_script.sh is already hashed. If not, it will produce no output.

Removing Cached Entries

Sometimes, you might need to remove a command from the hash table. This is typically necessary when you’ve moved or renamed an executable, or if you want to force the shell to re-search for the command. This is done using the -d option:

hash -d my_script.sh

This removes my_script.sh from the hash cache. The next time you run my_script.sh, the shell will perform a full path search.

To clear the entire hash table, use:

hash -r

This will remove all cached entries, forcing the shell to re-search for all commands upon their next execution.

hash and Shell Startup

It’s important to note that the hash table is typically cleared when you start a new shell session. However, some shells might persist the cache across sessions. The behavior depends on your specific shell configuration.

Example Scenario: Optimizing a Frequently Used Script

Let’s say you have a frequently used script, backup_data.sh, located in /home/user/scripts/. To optimize its execution speed:

1. Add the script directory to your PATH: This ensures the shell can find the script. You’d typically add this to your shell’s configuration file (e.g., ~/.bashrc, ~/.zshrc).

   export PATH="$PATH:/home/user/scripts"

2. Hash the script: After adding the directory to your PATH and opening a new terminal or sourcing your configuration file, hash the script:

   hash backup_data.sh

Now, each subsequent invocation of backup_data.sh will be faster.

Advanced Usage with -p

The -p option allows you to hash a command even if it’s not in your PATH. This is useful for commands in specific directories you want to run frequently:

hash -p /usr/local/bin/my_special_tool

This will hash /usr/local/bin/my_special_tool, regardless of whether /usr/local/bin is in your PATH. However, you’ll still need to specify the full path when running the command.