Check regexp condition inside while loop (shell)

Question

I have problem with running my simple shell script, where I am using while loop to read text file and trying to check condition with matching regexp:

#!/bin/sh
regex="^sometext"


cat input.txt | 
{
  while read string
    do if [[ $string ~= $regex ]]; then
            echo "$string" | awk '{print $1}' >> output.txt
       else
            echo "$string" >> outfile.txt
       fi
    done
}

but I recieve only errors like

[[: not found

could you please advise me?

Answer 1

[[ expr ]] is a bash-ism; sh doesn't have it.

Use #!/bin/bash if you want to use that syntax.

---

That said, why use a shell script?

Awk can already do everything you're doing here - you just need to take advantage of Awk's built-in regex matching and the ability to have multiple actions per line:

Your original script can be reduced to this command:

awk '/^sometext/ {print $1; next} {print}' input.txt >> output.txt

If the pattern matches, Awk will run the first {}-block, which does a print $1 and then forces Awk to move to the next line. Otherwise, it'll continue on to the second {}-block, which just prints the line.

Answer 2

Because you're using shebang for /bin/sh and [[ is only available in BASH.

Change shebang to /bin/bash

btw your script can be totally handled in simple awk and you don't need to use awk inside the while loop here.