Bash: Print usage statement & exit; otherwise continue using Bash shorthand operators

stefanlasiewski · 01-20-2006, 12:51 PM

I'm trying to write a Bash script using some shorthand Bash operators.

I want a short script to test for a commandline argument.
- If $1 is not present, then print a Usage statement and exit from the function.
- If there is a value, then continue with the rest of the function.

Code:

function testIt {
  # If no arguments, print usage statement & exit from function
  [[ -n "$1" ]] || ( echo "Usage: testIt foo"; exit 0 ; )
  # Else, continue and print the arguments
  echo "FOO=$1"
}

Here's the problem. If I run the command with no arguments, the 'echo "FOO=$1"' is still printed.

Code:

$ testIt
Usage: testIt foo
FOO=

I know this is simple, but I just can't wrap my head around it this morning. It's been a long week.

Any help is appreciated.

-= Stefan

dogpatch · 01-20-2006, 01:19 PM

Try this:

Code:

#!/bin/sh
if test "$1" == "" ; then
	echo $'\a'Supply 1 parameter, e.g:
	echo $0 BAR
	exit
fi
echo FOO=$1

gilead · 01-20-2006, 02:27 PM

I think the reason you were still getting the "FOO=" output is that ( echo "Usage: testIt foo"; exit 0 ; ) runs in a subshell because of the brackets. It exits from the subshell at your exit statement and continues with the script. dogpatch's example executes in one level...

stefanlasiewski · 01-20-2006, 08:20 PM

Thanks Dogpatch. The 'if ; then; fi' certainly works.

I was just trying to figure out how to do this with the shorthand operators like [[ TEST ]] || and && .

Since I write these sorts of scripts often, I was hoping to find a one-liner to do this. Plus, it'd slick.

It *should* work at some level, right?

Hrm, I'll have to work on this more.

jlliagre · 01-20-2006, 08:55 PM

Here is one way to do it:

Code:

..
[[ -n "$1" ]] || { echo "Usage: testIt foo"; exit 0 ; }
..

gnashley · 01-21-2006, 12:07 PM

[[ -n "$1" ]] && echo "FOO=$1" || echo "Usage: testIt foo"

jlliagre · 01-21-2006, 01:21 PM

Quote:

Originally Posted by gnashley

[[ -n "$1" ]] && echo "FOO=$1" || echo "Usage: testIt foo"

You are missing the exit requirement.

stefanlasiewski · 01-24-2006, 03:56 PM

Quote:

Originally Posted by jlliagre

Here is one way to do it:

Code:

..
[[ -n "$1" ]] || { echo "Usage: testIt foo"; exit 0 ; }
..

This statement will exit from the current shell. And actually, this is probably the preferred behavior within a shell script-- the script spawns a subshell (e.g. #!/bin/bash ), and the script & subshell will exit if it encounters an error.

However, if I put this in function within my current interactive shell, my current shell will exit.

I'm not saying that your solution is wrong

It is just an interesting thing to notice.

jlliagre · 01-25-2006, 01:12 AM

Quote:

Originally Posted by stefanlasiewski

This statement will exit from the current shell. And actually, this is probably the preferred behavior within a shell script-- the script spawns a subshell (e.g. #!/bin/bash ), and the script & subshell will exit if it encounters an error.

However, if I put this in function within my current interactive shell, my current shell will exit.

I'm not saying that your solution is wrong

It is just an interesting thing to notice.

If you want only the function to be exited, here's a way:

Code:

..
[[ -n "$1" ]] || { echo "Usage: testIt foo"; return ; }
..

stefanlasiewski · 02-07-2006, 05:20 PM

Thank you jlliagre. Both of your solutions work.

Thanks!