Option -l of exec shell command

Question

Could you please clarify on the use of -l option of exec shell command. I didn't notice any difference when I ran exec ls | cat and exec -l ls | cat.

Answer 1

The -l option of exec adds a - at the beginning of the name of your command. For example:

exec -l diff | head

-diff: missing operand after '-diff'
-diff: Try '-diff --help' for more information.

Note the - everywhere before diff.

The point of all this? If you have a - before a command to start a shell it will act as a login shell. From man bash:

A login shell is one whose first character of argument zero is a -, or one started with the --login option.

Now, man exec states that:

If the -l option is supplied, the shell places a dash at the beginning of the zeroth argument passed to command. This is what login(1) does.

So exec -l bash will run bash as a login shell. To test this, we can use the fact that a login bash executes the file ~/.bash_profile, so:

$ cat ~/.bash_profile 
#!/bin/sh

printf "I am a login shell!\n"

If I start a login bash, the command printf "I am a login shell!\n" will be executed. Now to test with exec:

$ exec bash
$

Nothing is displayed, we are on a non-login shell.

$ exec -l bash
I am a login shell!
$

Here we have a login shell.