What is the purpose of the hash command?

hash is a bash built-in command. The hash table is a feature of bash that prevents it from having to search $PATH every time you type a command by caching the results in memory. The table gets cleared on events that obviously invalidate the results (such as modifying $PATH)

The hash command is just how you interact with that system (for whichever reason you feel you need to).

Some use cases:

• Like you saw it prints out how many times you hit which commands if you type it with no arguments. This might tell you which commands you use most often.

• You can also use it to remember executables in non-standard locations.

Example:

[root@policyServer ~]# hash -p /lol-wut/whoami whoami
[root@policyServer ~]# whoami
Not what you're thinking
[root@policyServer ~]# which whoami
/usr/bin/whoami
[root@policyServer ~]# /usr/bin/whoami
root
[root@policyServer ~]#

Which might be useful if you just have a single executable in a directory outside of $PATH that you want to run by just type the name instead of including everything in that directory (which would be the effect if you added it to $PATH).

An alias can usually do this as well, though and since you're modifying the current shell's behavior, it isn't mapped in programs you kick off. A symlink to the lone executable is probably the preferable option here. hash is one way of doing it.

• You can use it to un-remember file paths. This is useful if a new executable pops up in an earlier PATH directory or gets mv'd to somewhere else and you want to force bash to go out and find it again instead of the last place it remembers finding it.

Example:

[root@policyServer ~]# hash
hits    command
   1    /bin/ls
[root@policyServer ~]# cp /bin/ls /lol-wut
[root@policyServer ~]# hash
hits    command
   1    /bin/cp
   1    /bin/ls
[root@policyServer ~]# hash -d ls
[root@policyServer ~]# ls
default.ldif  newDIT.ldif  notes.txt  users.ldif
[root@policyServer ~]# hash
hits    command
   1    /bin/cp
   1    /lol-wut/ls
[root@policyServer ~]#

The cp command caused a new version of the ls executable to show up earlier in my $PATH but didn't trigger a purge of the hash table. I used hash -d to selectively purge the entry for ls from the hash table. Bash was then forced to look through $PATH again and when it did, it found it in the newer location (earlier in $PATH than it was running before).

You can selectively invoke this "find new location of executable from $PATH" behavior, though:

[root@policyServer ~]# hash
hits    command
   1    /bin/ls
[root@policyServer ~]# hash ls
[root@policyServer ~]# hash
hits    command
   0    /lol-wut/ls
[root@policyServer ~]#

You'd mostly just want to do this if you wanted something out of the hash table and weren't 100% that you could logout and then back in successfully, or you wanted to preserve some modifications you've made to your shell.

To get rid of stale mappings, you can also do hash -r (or export PATH=$PATH) which effectively just purges bash's entire hash table.

There are lots of little situations like that. I don't know if I'd call it one of the "most useful" commands but it does have some use cases.

Here's the classic usage, simplified:

# My PATH contains /home/rici/bin as well as the Usual Suspects:
# (the real one has lots more)
$ echo $PATH
/home/rici/bin:/usr/local/bin:/usr/bin:/bin

# I've installed a program called hello in /usr/local/bin
$ $ cat /usr/local/bin/hello
#!/bin/bash

echo Hello, world. I live at $0

# The program works.
$ hello
Hello, world. I live at /usr/local/bin/hello

# Now I want to create a better hello, just for me. I put it in
# my own bin directory, and according to my PATH, it should come first.
$ cp /usr/local/bin/hello ~/bin/hello

# So now I will try running it
$ hello
Hello, world. I live at /usr/local/bin/hello

# WTF? Oh, forgot to run hash.
# Tell bash to update where to look for hello
$ hash hello
$ hello
Hello, world. I live at /home/rici/bin/hello

# Ah, all is well.

Here's a useful use of hash:

hash php 2> /dev/null || hash -p /usr/local/foobar/php/bin/php php 2> /dev/null

It means: if php isn't in the PATH, then use

/usr/local/foobar/php/bin/