Bash check file for pattern

MTK358 · 02-12-2011, 07:41 PM

How do I write an if statement in Bash so that it executes only if the first line in a file matches or doesn't match a regex?

crts · 02-12-2011, 07:53 PM

Do you mean something like this

Code:

if [[ "$(head -n1 file)" =~ RegEx ]];then
...
else
...
fi

Notice, that the RegEx is not enclosed in braces, quotes or whatsoever.
Alternatively,

Code:

read var < file
if [[ "$var" =~ RegEx ]];then
...
else
...
fi

MTK358 · 02-12-2011, 08:00 PM

Is it OK if the regex contains whitespace, are quotes still not needed?

crts · 02-12-2011, 08:11 PM

Quote:

Originally Posted by MTK358

Is it OK if the regex contains whitespace, are quotes still not needed?

Nope, but you have to escape the spaces. Bash does "funny" things with your RegEx if you enclose it in quotes. Go ahead and see for yourself what happens:

Code:

set -x
if [[ $(read ...) =~ "some space .*" ]]; ...

Notice, how '.*' gets mangled. Maybe there is a shopt or setting to control this behavior. Not sure.

Nominal Animal · 02-13-2011, 12:57 AM

If you use sh or older bash versions without [[ support, you can always use sed:

Code:

if sed -ne '1 { /pattern/ q0 } ; q1' file ; then
    echo file contains pattern on the first line
fi

When invoked like this, sed only reads the first line of file.

Hope this helps,

Nominal Animal