[SOLVED] grep command with regular expression

amanbijpuria · 01-13-2016, 07:51 AM

I have a file name abc.txt which contain lines

lightdm:x:112:118:Light DisplayyDiManager:/var/lib/lightdm:/bin/false
dnsmasq:x:104:655:dnsmasq,,,:/var/lib/misc:/bin/false

when i do grep -i 'display*' abc.txt it is giving me output i.e 1st line
but when i do grep -i 'display+' abc.txt it gives nothing but it should give .i dont know why. please let me know if anyone knows. thanks in advance

TB0ne · 01-13-2016, 08:22 AM

Quote:

Originally Posted by amanbijpuria

I have a file name abc.txt which contain lines

lightdm:x:112:118:Light DisplayyDiManager:/var/lib/lightdm:/bin/false
dnsmasq:x:104:655:dnsmasq,,,:/var/lib/misc:/bin/false

when i do grep -i 'display*' abc.txt it is giving me output i.e 1st line but when i do grep -i 'display*' abc.txt it gives nothing but it should give .i dont know why. please let me know if anyone knows. thanks in advance

Your post makes no sense...you posted the same command twice, saying the first time you run it it WORKS, then it DOESN'T...they both can't be true. Also, in the context you're using it, you don't need the wildcard, since grep will find the word display, regardless of what's before or after it.

amanbijpuria · 01-13-2016, 10:05 AM

first of all sorry for writing same code again ,i have corrected it.So instead of grep -i 'display*' abc.txt in 2nd line it will be grep -i 'display+' abc.txt .

rknichols · 01-13-2016, 10:58 AM

In basic regular expressions the "+" character is not special, so you are looking for that character to appear literally in the line. To use its special meaning, you would have to preceed it with a backslash:

Code:

grep -i 'display\+' abc.txt

But again, why are you doing that? That looks for the string "displa" followed by one or more repetitions of the character "y". Just plain "display" would also match that.

amanbijpuria · 01-13-2016, 11:08 AM

Hi Rknicholes,
Thankyou for your reply .I have just given an example where my file data is like that. Now my question here grep -i 'display*' abc.txt is working without any backslash but in case of grep -i 'display\+' abc.txt i have to use backslash .why?

grail · 01-13-2016, 11:17 AM

You need to pay closer attention. The asterisk will work in a basic regular expression for grep but the plus is considered an extended expression so you either need to escape it to let grep
know it is special or use the -E switch to use extended expressions.

As pointed out, neither seems really relevant, so your actual examples must need this as the current one it seems superfluous.

amanbijpuria · 01-13-2016, 11:24 AM

Hi Grail,
Thanks a lot for your reply Grail.I think this information is very useful .