Temporarily declare a variable in Bash

Question

To declare a variable in Bash, say in a Bash script-file (that doesn't include Bash function), I do for example x=y and when I finish using it inside that script-file I do unset x.

Is there a way (without using a function), to unset the variable after say 5 minutes, both in one line? A plausible approach might be x=y && echo "unset x" | at now + 5 minutes.

In this particular case I run the script-file directly in the terminal by copy-pasting its content from GitHub to terminal. This falls under sourcing I assume".

---

Given I use GitHub, an alternative might be executing a raw version of the script-file directly from GitHub with bash in a separate shell as follows, but I don't like that way because it can't be user/repo/branch/path/file-agnostic:

wget -O - https://raw.githubusercontent.com/<username>/<repo>/<branch>/<path>/<file> | bash

Answer 1

You can't use at jobs because they run in a different context, and can't affect the current shell.

But we can do something similar. This code will trigger an alarm signal, which we can catch and perform action on

#!/bin/bash

x=100

trap 'unset x' SIGALRM

mypid=$$

( /bin/sleep 3 ; kill -ALRM $mypid) &

for a in 1 2 3 4 5 6
do
  echo Now x=$x
  sleep 1
done

This example is only 3 seconds long to demonstrate the solution; you can pick your delay as you need.

In action:

Now x=100
Now x=100
Now x=100
Now x=
Now x=
Now x=

You can easily make it one line with ;...

Answer 2

Sure:

trap 'unset x; trap - USR1' USR1; { sleep 5m; kill -USR1 $$; } &

This sets a trap on the USR1 signal, then (cheating with a semicolon to put it on one line) groups together the sleep and kill commands into a background job. When that job completes, it will send the USR1 signal to the current shell, which will execute the trap. The trap unsets x and then clears the trap.

No functions!

Answer 3

With your revised question, based around the command line and not a script, your simplest solution is to use a subshell to do your work.

eg

$ bash
$ x=100
$ echo $x
100
$ exit
$ echo $x

$

All the variables you set between running bash and exiting that shell will be forgotten.

This is very close to how the wget ... | bash part works as well, except you can cut'n'paste/type commands.

Answer 4

Actually the simplest way might be to just use a compound command with a variable declaration at the start.

If you declare a variable at the start of a command line, the variable is temporarily defined only for the scope of that command.

madumlao@lezard:~$ x=foo
madumlao@lezard:~$ x=y bash -c 'echo $x'
y
madumlao@lezard:~$ echo $x
foo

As you can see, the line with a variable declaration only sets the variable in the environment for that line only.

Note that the following will not work:

madumlao@lezard:~$ x=foo
madumlao@lezard:~$ x=y echo $x

madumlao@lezard:~$ echo $x
foo

The $x in the second line is parsed during command-line parsing, before the value is actually assigned, so it is unexpectedly blank!