Seven Unexpected Bash Variables

Continuing the series of notes on lesser known functions bash, I'll show you seven variables you might not know about.

1) PROMPT_COMMAND

You may already know how to manipulate the prompt to show various useful information, but not everyone knows that you can run a shell command every time the prompt is shown.

In fact, many complex prompt manipulators use this variable to execute commands to collect the information that is displayed in the prompt.

Try running this in a new shell and see what happens to the session:

$ PROMPT_COMMAND='echo -n "writing the prompt at " && date'

2) HISTTIMEFORMAT

If you run history in the console, you will get a list of commands previously executed under your account.

$ HISTTIMEFORMAT='I ran this at: %d/%m/%y %T '

Once this variable is set, the new entries record the time along with the command, so the output will look like this:

1871 I ran this at: 01/05/19 13:38:07 cat /etc/resolv.conf 1872 I ran this at: 01/05/19 13:38:19 curl bbc.co.uk 1873 I ran this at : 01/05/19 13:38:41 sudo vi /etc/resolv.conf 1874 I ran this at: 01/05/19 13:39:18 curl -vvv bbc.co.uk 1876 I ran this at: 01 /05/19 13:39:25 sudo su -

Formatting matches characters from man date.

3) CDPATH

To save time on the command line, you can use this variable to change directories as easily as you call commands.

Like PATH, variable CDPATH is a colon-separated list of paths. When you run the command cd with a relative path (i.e. no leading slash), by default the shell looks in your local folder for matching names. CDPATH will look in the paths you gave for the directory you want to change to.

If you install CDPATH in this way:

$ CDPATH=/:/lib

and then enter:

$ cd /home
$ cd tmp

then you will always end up in /tmp no matter where you are.

However, be careful, because if you do not specify a local (.) folder, then you will not be able to create any other folder tmp and navigate to it as usual:

$ cd /home
$ mkdir tmp
$ cd tmp
$ pwd
/tmp

Whoops!

This is similar to the confusion I felt when I realized that the local folder was not included in the more familiar variable PATH... but you must do it in the PATH variable, because you can be tricked into running a fake command from some downloaded code.

Mine is set by starting point:

CDPATH=.:/space:/etc:/var/lib:/usr/share:/opt

4) SHLVL

Have you ever wondered typing exit will log you out of the current bash shell into another "parent" shell, or just close the console window completely?

This variable keeps track of how deep you are nested in the bash shell. If you create a new terminal, then it is set to 1:

$ echo $SHLVL
1

Then, if another shell process is started, the number increases:

$ bash
$ echo $SHLVL
2

This can be very useful in scripts where you are not sure whether to exit or not, or keep track of where you are by nesting.

5) LINENO

The variable is also useful for analyzing the current state and debugging. LINENO, which reports the number of commands executed in the session so far:

$ bash
$ echo $LINENO
1
$ echo $LINENO
2

This is most commonly used when debugging scripts. By inserting lines like echo DEBUG:$LINENO, you can quickly determine where in the script you are (or not).

6) REPLY

If, like me, you usually write code like this:

$ read input
echo do something with $input

it may come as a surprise that you don't have to worry about creating a variable at all:

$ read
echo do something with $REPLY

This does the same.

7) TMOUT

In order not to stay on production servers for too long for security purposes, or accidentally run something dangerous in the wrong terminal, setting this variable acts as a safeguard.

If nothing is entered within the set number of seconds, the shell is exited.

That is, it is an alternative sleep 1 && exit:

$ TMOUT=1

Source: habr.com