Use sed command to apply a condition to check if a pattern has been matched

Solution 1:

grep does that job well enough.

$ echo "::=BEGIN" > testfile1.txt

$ grep "::=BEGIN" -q testfile1.txt && echo "FOUND"
FOUND

$ grep "::=BEGIN" -q testfile1.txt && echo "FOUND" || echo "NOTFOUND"
FOUND

$ grep "whever" -q testfile1.txt && echo "FOUND" || echo "NOTFOUND"
NOTFOUND

What the code does is simply running a quiet search within subshell. && , or logical AND operand, checks if the first command succeeded, and if it did, then it runs echo "FOUND" . || checks if that echo has been run, i.e., if we found anything. Second echo runs only if the first command failed.

Here's an awk version:

$ awk '{i="NOTFOUND";if($0~/::=BEGIN/) i="FOUNDIT";exit } END { print i}' testfile1.txt
FOUNDIT

$ awk '{i="NOTFOUND";if($0~/whatevs/) i="FOUNDIT";exit } END { print i}' testfile1.txt
NOTFOUND

Basic idea here is to set i to NOTFOUND, and if we find the string - change it to FOUNDIT . At the end after the first set of has finished processing file, we will print i, and it will tell us if we found something or not.

Edit: In the comment you mentioned that you want to place the result into a variable. Shell already provides a variable that reports exit status of the previous command, $0. Exit status 0 means success, everything else - fail. For instance,

$ grep "::=BEGIN" -q testfile1.txt  
$ echo $?
0

If you want to use exit status in a variable, you can do it like so : MYVAR=$(echo $?). Remember , $? report exit status of previous command. So you must use this right after the grep command.

If you want to still use my earlier awk and grep one-liners, you can use same trick MYVAR=$() and place whatever command you want between the braces. The output will get assigned to variable MYVAR.

Solution 2:

With a GNU sed (as is installed by default on any Ubuntu system):

{ sed -n '/::=BEGIN/Q 1'; i=$?; } <infile

Some facts about this answer:

1. sed Quits input just as soon as the very first match is found anywhere in the file - which means it does not waste any more time continuing to read the infile.

2. sed returns in its exit code the numeric value 0 or 1 depending on whether it couldn't/could find the addressed regexp.

3. $i is set to sed's return upon exit.

4. The { grouping ; } is used to robustly handle any possible redirection error - $i is not set in the case the shell cannot open the file at all, thus avoiding false positives.

This probably shouldn't be preferred to grep -q, though, which can basically also do all of the above (if in reverse), do it POSIXly, and usually faster.

The already given grep -q answer is very good, but, a little terser:

grep -q pattern ./infile; i=$((!$?))