Compare first characters of a string in UNIX shell

loopoo · 12-11-2006, 09:14 AM

Hello all!

I want to verify if the first 2 characters of a string are "+-".
Can you please help me with that? How can I compare if the varaible (let's say P1 begins with "+-" )

Thanks in advance,
Chris

mjgould · 12-11-2006, 09:40 AM

What is it that you are looking to output from comparing the values? For example are you looking to output the number of lines in a file that starts with "+-". Knowing what type of information you are looking to gather will help me understand what type of expression you are looking for.

~Mike

unSpawn · 12-11-2006, 09:46 AM

declare var="+-something"
Variable substring check in Bash (pure) you can use

Code:

[ "${var:0:2}" = "+-" ] && echo OK

If you want / need compatibility use for instance

Code:

case $var in +-*) echo OK;; esac

matthewg42 · 12-11-2006, 09:47 AM

Code:

if echo "$P1" |grep -q '^+-'; then 
    echo "P1 begins with +-"
else 
    echo "P1 does not being with +-"
fi

matthewg42 · 12-11-2006, 09:49 AM

unspawn's solution is better - no external program invoked. I feel foolish now for using grep. *blush*

On the bright side, this is my 1000'th post! Post party!

loopoo · 12-11-2006, 10:13 AM

Thank you all for your help.

I prefered unspawn solution.
I had to change from sh to bash...because of the string splitting

Once again, thanx!

loopoo · 12-11-2006, 10:51 AM

I need one more small thing: before this check, i want to verify first if the string variable has at least 2 characters (without doing this check i get the following error: "[: =: unary operator expected")

Thanks,
Chris

matthewg42 · 12-11-2006, 11:17 AM

Quote:

Originally Posted by loopoo

I need one more small thing: before this check, i want to verify first if the string variable has at least 2 characters (without doing this check i get the following error: "[: =: unary operator expected")

Thanks,
Chris

Make sure you quote the test variable, i.e. do this:

Code:

[ "${var:0:2}" = "+-" ] && echo OK

not this:

Code:

[ ${var:0:2} = "+-" ] && echo OK

loopoo · 12-11-2006, 11:35 AM

Thanks for your suggestion....

i switched meanwhile to on another solution using "case" for tests.

Chris