BASH script alphabetical conditional operator query.

lupusarcanus · 03-14-2010, 12:19 PM

Hello All.

I got a simple question, but Google has not assisted me in this query.

I am using "if" to force a word under the condition that the first letter of that word must be a letter of the alphabet, regardless of capitalization, using the " "" != "" " syntax.

Like so:

Code:

if [[ "$interface" != "WHAT DO I PUT HERE?" ]] ; then
   echo "Invalid input"
   exit 1
fi

Thank you in advance.

troop · 03-14-2010, 12:38 PM

Code:

#!/bin/bash

test="gtw3 tw"
if [[ "$test" =~ ^[A-Za-z] ]]; then
  echo "ok"
else
   echo "Invalid input"
   exit 1
fi

lupusarcanus · 03-14-2010, 12:53 PM

Quote:

Originally Posted by troop

Code:

#!/bin/bash

test="gtw3 tw"
if [[ "$test" =~ ^[A-Za-z] ]]; then
  echo "ok"
else
   echo "Invalid input"
   exit 1
fi

That did work. However, for sake of code cleanliness, I was hoping for the syntax to say: If the variable does not begin with a letter of the alphabet (regardless of capitalization), then display an error message and exit with status code 1. Is this possible?

The reason is because if I go with your code, this is how it would look:

Code:

if [[ "$interface" =~ ^[A-Za-z] ]]; then
	echo "" >/dev/null
else
   echo "
	ERROR 7: No usable interface label detected.	
"
   exit 1
fi

Thank you for your reply. You gave much more information than my weak Googling skills gave me.

penguiniator · 03-14-2010, 01:14 PM

Code:

if ! [[ "$interface" =~ ^[A-Za-z] ]]; then
    error message ...
else
    other code
fi

Valery Reznic · 03-15-2010, 03:04 AM

Code:

case "$interface" in
   [A-Za-z]*)
       Do something
   ;;

   *)
    error message ...
   ;;

It's not bash specific and works for other shells too

konsolebox · 03-15-2010, 03:14 AM

alternatives

Code:

[[ ${interface:0:1} != [A-Za-z] ]]
[[ ! ${interface:0:1} = [[:alpha:]] ]]
[[ ! $interface =~ ^[A-Za-z] ]]
[[ $interface != [[:alpha:]]* ]]

where you can have

Code:

if [[ ... ]]; then
	echo "ERROR 7: No usable interface label detected."
	exit 1
fi

...