What is the purpose of the special parameter "_" (single underscore) in environment?

I'm trying to understand how the environment variable _ can be used. Below is an example of using it:

$ echo $_

$ echo $_
echo

$ ls non-existant-filename
ls: cannot access 'non-existant-filename': No such file or directory

$ echo $_
non-existant-filename

• First it returns nothing
• Second it returns the last command used
• Last it returns the last parameter used

This might be a handy variable for bash scripts but only if it's function is fully understood.

Some useful applications of _

I found some useful applications of _.

_ contains the last filename you can recycle

In this example _ is used to keep the last filename which you can reuse in subsequent commands without retying it.

$ ll ~/python/scroll1.py
-rwxrwxrwx 1 rick rick 2384 Dec 27 09:15 /home/rick/python/scroll1.py*

$ $_
# The python program ~/python/scroll1.py is executed

$ cat $_
#!/usr/bin/env python
# -*- coding: utf-8 -*-
   (... SNIP ... remaining contents of ~/python/scroll1.py appears on screen)

• First command uses ll to list a python script filename. The filename is saved to _ for future use.
• Second command $_ runs the python script.
• Third command cat $_ lists the contents of the python script.

So the $_ variable/parameter can save some typing.

_ contains the last program run

Here's an example of differences between env and printenv updating the _ variable/parameter:

$ env > env.txt

$ printenv > printenv.txt

$ diff env.txt printenv.txt
66c66
< _=/usr/bin/env
---
> _=/usr/bin/printenv

Because a parameter wasn't passed to either command, the _ isn't updated with the last used parameter as in the previous example but, it is updated with the last command used.

Also noticed how _ is updated before the commands env and printenv are executed because _ it appears in the output.

Solution 1:

It's not an "Environment Variable". It's a special Bash parameter that is handled specially by Bash.

From the Bash Beginners Guide which explains this pretty well for Bash:

3.2.5. Special parameters

The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed.

...

$_: The underscore variable is set at shell startup and contains the absolute file name of the shell or script being executed as passed in the argument list. Subsequently, it expands to the last argument to the previous command, after expansion. It is also set to the full pathname of each command executed and placed in the environment exported to that command. When checking mail, this parameter holds the name of the mail file.

^{(Format of the quote adjusted for Ask Ubuntu, but contains all the information)}

They also include a nice example of how $_ expands:

franky ~> grep dictionary /usr/share/dict/words
dictionary

franky ~> echo $_
/usr/share/dict/words