5 replies to this topic

[agnl666]

Hi Everyone.

I looked around and have yet to find a solution so I was wondering. In bash how would I compare whether an argument is equal to some string.

Currently I have :

if [ $1 -eq 'someString' ]

This doesn't seem to work so I was wondering what might?

Thanks!

[dargueta]

if [[ $1 == 'someString' ]]; then
    #blah
fi

You can also do != and, if you want to get fancy, use =~ with a regular expression on the right-hand-side.

[DarkLordofthePenguins]

String comparison operators in bash are the same as the numeric comparison operators in other languages.

if [[ "$str1" == "$str2" ]]

then

    # do stuff

elif [[ "$str1" < "$st2" ]]

then

    # do stuff

else

    # do stuff

fi

bash is an odd language because technically all variables are implemented as strings. I don't know why strings use the numeric comparison symbols though.

[dargueta]

Numbers use flags like -eq -gt -ge, etc. I think part of the reason why they're implemented as strings is because [ is actually its own program, not a true operation. The return value is what the if statement looks at.

If used on the command line, passing < | > or & would cause the shell to treat them as pipes or whatever, which would screw up the entire operation. With the [[ ]] construct, which isn't a program, you're allowed to use < and >. Because of the alphabetical sorting nature, it will also work for positive integers, but once you stick a negative sign in front of it, it breaks.

[[ "3" > "4" ]] --> false
[[ "3" < "4" ]] --> true
[B] [[ "3" > "-4" ]] --> false[/B]
[[ "-3" < "4" ]] --> true

This is why we use those flags. Otherwise you get something like Perl, where you'd have to force a string comparison on strings that represent valid integers.

Edited by dargueta, 08 November 2011 - 10:24 AM.
Formatting

[agnl666]

Hey, I just wanted to say thanks you. I never knew that Bash stored everything as strings but this makes more sense.

[dargueta]

No problem. :)