2

So, a while ago I saw this snippet for extracting text between two "markers":

# Usage: extract file "opening marker" "closing marker"
    while IFS=$'\n' read -r line; do
        [[ "$extract" && "$line" != "$3" ]] &&
            printf '%s\n' "$line"

        [[ "$line" == "$2" ]] && extract=1
        [[ "$line" == "$3" ]] && extract=
    done < "$1"

(Here i just took the liberty to remove it from the function and put it in a file called extract) Now, it does work fine on "most" pair of markers. But i noticed it doesn't always work:

Following the original snippet's example, using N repeated char (using "#" instead of "`" because of formatting error on SO):

###sh
test
###

works when doing extract file '###sh' '###' but if we use the following marker:

###
test
###

and do extract file '###' '###', then it doesn't work?

Though i can see that the condition in the script does evaluate correctly (the extract variable being equal to 1 when using set -x).

What's wrong here?

PS: By saying "It doesn't work", I do mean that it doesn't print anything in the instance when it doesn't work, of course.

The two example output above shouldn't contain the markers (just the texts extracted between two markers)...

I prefer a bash/shell solution if possible.

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17
  • 1
    Alternative: sed -n "/$2/,/$3/p" $1, although a bit more work is required if the markers contain quotes or slashes. Commented May 8, 2021 at 23:25
  • I prefer if the solution is in bash/shell if possible :) (or at least to know why the above doesn't work as expected). I appreciate this alternative though, thanks @berndbausch Commented May 8, 2021 at 23:26
  • 2
    It fails when the markers are the same because if either "$line" == "$2" or "$line" == "$3" is true, then the other is necessarily true also? Commented May 8, 2021 at 23:32
  • 1
    It can't work if the two markers are identical. extract is set to 1, then to the empty string. My sed solution is not much better, I am afraid. Commented May 8, 2021 at 23:32
  • 1
    Always paste your script into https://shellcheck.net, a syntax checker, or install shellcheck locally. Make using shellcheck part of your development process.. Commented May 8, 2021 at 23:46

2 Answers 2

Reset to default
4

As stated by others in comment to your question, your script does not work because when the start condition [[ "$line" == "$2" ]] is met, extract is set to 1, but on the next line the end condition [[ "$line" == "$3" ]] is also met, which reset extract to the empty string.

Here is your script fixed:

# Usage: extract file "opening marker" "closing marker"
while IFS=$'\n' read -r line; do
    if [ "$extract" ]; then
        if [[ "$line" == "$3" ]]; then
             extract=
        else
            printf '%s\n' "$line"
        fi
    elif [[ "$line" == "$2" ]]; then
        extract=1
    fi
done < "$1"

And, in case you need this, at @Freddy's suggestion, here is a slightly modified version that requires that the end marker be present for the text to be printed:

# Usage: extract file "opening marker" "closing marker"
while IFS=$'\n' read -r line; do
    if [ "$extract" ]; then
        if [[ "$line" == "$3" ]]; then
            printf '%s\n' "${lines[@]}"
            lines=() extract=
        else
            lines+=( "$line" )
        fi
    elif [[ "$line" == "$2" ]]; then
        extract=1
    fi
done < "$1"

(lines are accumulated in the lines array and are only printed when the end marker is met)

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2
  • 1
    This also prints the extracted text if the end marker is missing in the input file. Commented May 9, 2021 at 0:50
  • 2
    @Freddy Agreed. OP didn't ask to fix that, but I did it anyway. Thanks for the suggestion. Commented May 9, 2021 at 1:14
0

Add a toggling logic to the extract variable whenever $2 is seen. Thanks to xhiene for pointing it out.!

[[ $line == $2 ]] && case $extract in '') extract=1;; *) extract=; esac

And remove the $3 dependency on extract variable now.

HTH.

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4
  • 2
    If the two markers are the same, when will you hit the [[ "$line" == "$3" ]] line, then? Commented May 9, 2021 at 0:13
  • Yes that is correct, it will never enter there. Toggling extract is the way to go in that case. Commented May 9, 2021 at 0:16
  • Usually the OP uses the same script with different markers. Now he asks us to fix it when the markers are the same. By toggling like you are doing, the script is now broken when the markers are different. Commented May 9, 2021 at 0:28
  • There's another flaw (not yours). The input only needs the start marker to print the text. Commented May 9, 2021 at 0:29

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