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Why does my tail command -n+NUM not work?

I want to understand the following command in Linux:

# tail -n+454 /path/to/a/file | head -n 6

I expect that tail -n+454 /path/to/a/file prints the lines, starting at line 454 and the following 5 lines.

The | sends that output tohead as an input. Then only the first 10 lines are taken.

Finally, -n 6 defines that only the first 6 lines are printed to the screen.

Did I translate the command correctly?

Now I have the following problem: Let's assume I have a file and the following line in it:

# Step #6: Configure output plugins

I want to print the 5 lines immediately before that line (including that line).

First I checked, what line number my line in question has:

nl /path/to/a/file | grep output

enter image description here

The line number is 459.

I want the 5 lines preceding line 459 as well as line 459 itself (that is, line 454 to 459).

The command tail -n+454 /path/to/a/file | head -n 6 gives me the following output:

enter image description here

...and this is line 380 to 384:

enter image description here

I expected to get lines 454 to 459. What did I not understand? Is my command not correct?

Solution

  • The mistake I made was that I displayed only the non-empty lines in the file, which was wrong.

    It's better to use...

    nl -ba [FILE]

    to number all lines in the file. Then look up the lines of interest and use the head and tail commands (with piping) to get the final results.

    Example:

    tail -n +539 [FILE] | tail -n 6
tail -n +539 [FILE] | head -n -212
head -n 544 [FILE] | tail -n 6
head -n 544 [FILE] | tail -n +539

    All commands lead to the same result.

    Another mistake I made was the syntax. There should be a space between -n and +NUM.

    By the way, the line numbers in my OP are wrong, because I used the wrong numbering line command. The line I refer to is 544 not 459.