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echo -e "a\nb\nc\nd\ne\nf\ng\nh" | sed '3,5a test'

This will match lines 3, 4 and 5.

But I am trying to match only lines 3 and 5 (not 4). And append 'test' after them.

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3 Answers 3

up vote 7 down vote accepted
echo ... | sed -e '3a test' -e '5a test'

If the operation is more complex than in this case then you can use a structure like this:

sed 'b pattern; : action; a \
b end; : pattern; 3b action; 5b action; : end'

I.e. you put all commands you need between b pattern; and b end;.

And you add all your patterns (line numbers or whatever) after : pattern;.

What happens is this:

  1. The first command jumps over the action part (maybe it's easier to read if the patterns are at the beginning and b end; directly before the action part).

  2. If a pattern matches then the execution jumps to the action part. After the action part execution jumps to the end.

I try to tidy this up:

sed '3b action; 5b action; b end; : action; a \
: end'

In a single line would be like:

sed "3b idAction; 5b idAction; b; : idAction; a test"

Portably, you need to write it:

sed '
   3b action
   5b action
   : action

(b without label branches to the end, so you don't need an explicit end label, ; is valid a character in a label in standard sed implementations).

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Wow, sed subroutines. Cool –  glenn jackman Feb 28 at 17:29
Wow indeed, could you expand on those? They look really useful! –  terdon Feb 28 at 17:32
the one with 'pattern' and 'action' seems very good as I do not need to repeat 'test' (or 'lalala'), got to still better understand it tho thx! :) –  Aquarius Power Feb 28 at 17:35
@terdon What could I? –  Hauke Laging Feb 28 at 17:39
I meant that the second one looks great but is hard to understand and if you have the time, I'd appreciate an explanation. –  terdon Feb 28 at 17:42

With sed (see @HaukeLaging's answer)

With awk:

$ echo -e "a\nb\nc\nd\ne\nf\ng\nh" | awk 'NR==3 || NR==5{$0=$0"\ntest"}1;'

With perl:

$ echo -e "a\nb\nc\nd\ne\nf\ng\nh" | perl -pe '$_ .="test\n" if $. == 3 || $. == 5'
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Can golf your perl a bit, using the "smart match" operator: perl -lnE 'say if $. ~~ @{[3,5]}' –  glenn jackman Feb 28 at 17:28
@glennjackman ah, yes indeed, thanks. I have to admit that I'm a bit wary of ~~, don't understand it as well as I should. Also, both say and ~~ need perl v >= 5.10 right? –  terdon Feb 28 at 17:29
I need to append 'test' after that lines, I see now it would have been better to have clarified that from the beginning. I just edited the OP. –  Aquarius Power Feb 28 at 17:32
@AquariusPower, perl -pE 'say "test" if $. ~~ @{[4,6]}' -- this prepends the line: perl's -p does an implicit print at the end of the given program. –  glenn jackman Feb 28 at 17:36
@AquariusPower also see updated answer. –  terdon Feb 28 at 17:38

As @HaukeLaging aleady said, this command does what you want:

sed -e'3a test' -e'5a test'

Now, this can become rather cumbersome to type if you want to match, e.g., 20 lines.

In these cases, assuming your shell supports brace expansion, you can use this command instead:

sed -e{3,5}'a test'

(Note that the braces and the comma must remain unquoted.)

As a result, the shell will pass the arguments to -e3a test and -e5a test to sed, which is exactly what the first command does.

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