How BASH Shell Command Search Sequence Works

This article was organically contributed by monk.

HASH tables and PATH is not the first method locating your program / executable files. Your program can be a shell function or builtin command or an alias. Here is the complete sequence adopted by BASH shell to execute your command:

1. Before a command is executed REDIRECTION is done. Then following sequence used by SHELL
2. ALIASES
3. Parameter expansion, command substitution, arithmetic expansion, and quote removal before being assigned to the variable
4. Shell FUNCTION
5. BUILTIN command
6. HASH tables
7. PATH variable
8. If everything fails, you see command not found error message.

Examples

Try out following simple example to clear your idea about SHELL sequence

a) Create your own bin directory and add to a PATH:
$ mkdir ~/bin
$ export PATH=$PATH:~/bin

b) Create function called hello:
$ function hello() { echo "Hello from function()" ; }

c) Create alias called hello:
$ alias hello='echo Hello from alias'

d) Create script called hello in /home/you/bin directory (which is added to your PATH)
$ echo 'echo Hello from script' > ~/bin/hello
$ chmod +x ~/bin/hello

e) Test it...
Now you have three different commands with same name (i.e. hello). Which one will execute first? Type hello command and find out if above sequences works or not:
$ type hello
$ hello
Output:

Hello from alias

f) Remove alias and run hello again:
$ unalias hello
$ hello
Output:

Hello from function()

g) Remove function hello and run hello again:
$ unset hello
Output:

Hello for script

h) Remove script hello and run hello again:
$ rm ~/bin/hello
$ hello
Output:

bash: /home/monk/bin/hello: No such file or directory

You just removed the hello script but Bash shell still looking hello program at /home/monk/bin/hello location. What is going on here?
Answer is pretty simple - HASH table. (shell is looking for a cached PATH entry for hello script). To verify this just type hash command to see HASH table:
$ hash
Output:

hits    command
1    /bin/rm
1    /bin/cat
2    /usr/bin/print
3    /home/monk/bin/hello
1    /usr/bin/man

Your SHELL remembered path settings name in HASH table. There is an easy solution, just clear hash table and see the effect again:
$ hash -r
$ hello
Output:

bash: hello: command not found

I hope this basic explanation will help to all newbie(s) out there. As a result next time you run a command remember shell goes through quite a complicated sequence of operations to process your request.

Continue reading the second part of "How Linux or UNIX Understand which program to run" series (this is part I).

• PART I : How Linux or UNIX Understand which program to run
• PART II : An example: How shell Understand which program to run

Updated for accuracy.

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