How to replace line in a file after htting a particular line in bash

Question

So I have a simple use case, I have a file like

line_1
anything
anything
anything
aaa
line_2
line_3
anything
anything
anything
aaa
anything
anything
line_4
aaa

Now I have to replace the first aaa which comes after line_3.

Output-

line_1
anything
anything
anything
aaa
line_2
line_3
anything
anything
anything
YES REPLACE THIS
anything
anything
line_4
aaa

I was able to replace anything on the very next line using-

sed -i '/line_3/{n; s/aaa/YES REPLACE THIS/}' file.txt

But aaa is not of the next line, I do not know after how many lines it occurs, any solution using sed, awk or anything bash related will work, any help will be appreciated.

score 4 · Answer 1 · answered Jul 17 '22 at 09:13

4

Using awk

awk '/line_3/{f=1} f&&/aaa/{$0="YES REPLACE THIS";f=0} 1' file

answered Jul 17 '22 at 09:13

M. Nejat Aydin

9,597
1
7
17

score 3 · Accepted Answer · answered Jul 17 '22 at 09:09

Set a flag once it sees line_3, unset it once it finds and replaces the pattern

perl -wpe'if ($on) { $on = 0 if s/aaa/XXX/ }; $on = 1 if /^line_3$/' file

This only prints the resulting lines to screen. If you want ot change the file ("in-place") add -i switch (perl -i -wpe'...'), or better -i.bak to keep a backup, too.

score 2 · Answer 3 · answered Jul 17 '22 at 09:16

2

Using sed

$ sed '/line_3/,/line_4/{0,/^aaa/s/^aaa/YES REPLACE THIS/}' input_file
line_1
anything
anything
anything
aaa
line_2
line_3
anything
anything
anything
YES REPLACE THIS
anything
anything
line_4
aaa

answered Jul 17 '22 at 09:16

HatLess

10,622
5
14
32

1

*first `aaa` after `line_3`* I would write: `/line_3/,/aaa/{ ...` instead! – F. Hauri - Give Up GitHub Jul 17 '22 at 16:56
@F.Hauri-GiveUpGitHub I simply did not think of that. Don't know why. Thanks – HatLess Jul 18 '22 at 10:47
Something simplier: `/line_3/,/^aaa$/{/^aaa$/s/^.*$/YES REPLACE THIS/}` – F. Hauri - Give Up GitHub Jul 18 '22 at 10:51
And for fun: `sed '/line_3/,/^aaa$/{//{/line_3/!s/^.*$/YES REPLACE THIS/}}'` will do the job too! – F. Hauri - Give Up GitHub Jul 18 '22 at 10:54

Daweo · Answer 4 · 2022-07-18T07:12:21.057

2

Using just replacement in perl, let file.txt content be

line_1
anything
anything
anything
aaa
line_2
line_3
anything
anything
anything
aaa
anything
anything
line_4
aaa

then

perl -p -0777 -e 's/(line_3.*?)aaa\n/$1YES REPLACE THIS\n/s' file.txt

gives output

line_1
anything
anything
anything
aaa
line_2
line_3
anything
anything
anything
YES REPLACE THIS
anything
anything
line_4
aaa

Explanation: -p and -e means work like sed, -0777 means shove whole file as single line. s/ - substitute, /s - make . also match newline character, I match line_3 and non-greedily any characters before aaa\n and replace all of that with what I matched before aaa\n denoted by ( and ) followed by YES REPLACE THIS\n.

(tested in perl 5.30.0)

edited Jul 18 '22 at 07:12

answered Jul 17 '22 at 13:29

Daweo

31,313
3
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25

Avoid needless capture with `\K`: `s/line_3.*?\Kaaa\n//s` – ikegami Jul 17 '22 at 16:52
And handle all such instances in the whole file, with `/g` (added, so `/sg`) – zdim Jul 17 '22 at 22:01
1

@zdim fixed, now `YES REPLACE THIS` is place where first `aaa` after `line3` is – Daweo Jul 18 '22 at 07:13

ikegami · Answer 5 · 2022-07-17T19:07:52.600

2

This was proposed (by the currently-accepted answer):

perl -pe'$f = 1 if /^line_3$/; if ($f) { $f = 0 if s/^aaa$/YES REPLACE THIS/ };'

It simplifies to this:

perl -pe'$f ||= /^line_3$/; $f &&= !s/^aaa$/YES REPLACE THIS/;'

But the flip-flop operator does a lot of that work for us!

perl -pe'/^line_3$/ .. s/^aaa$/YES REPLACE THIS/'

See Specifying file to process to Perl one-liner.

edited Jul 17 '22 at 19:07

answered Jul 17 '22 at 16:50

ikegami

367,544
15
269
518

score 1 · Answer 6 · answered Jul 17 '22 at 13:44

1

ed is good for automated editing of files:

ed -s file.txt <<<EOF
/line_3/;/aaa/,.c
YES REPLACE THIS
.
w
EOF

Finds the first line matching line_3, then changes the first line after that matching aaa, and finally writes the modified file back out.

answered Jul 17 '22 at 13:44

Shawn

47,241
3
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How to replace line in a file after htting a particular line in bash

6 Answers6