Delete all lines beginning with a # from a file

Question

All of the lines with comments in a file begin with #. How can I delete all of the lines (and only those lines) which begin with #? Other lines containing #, but not at the beginning of the line should be ignored.

Does it have to work with the common convention that `#blah \blah` counts as a single "logical line" because the backslash escapes the newline? — sarnold, Nov 21 '11 at 01:18
@sarnold: apart from `make`, which utilities use the 'backslash splices lines before ending a comment'? The shells (bash and ksh tested) don't. C and C++ do handle newline splicing before other processing of preprocessor directives, but they're directives rather than comments. — Jonathan Leffler, Nov 21 '11 at 02:16
@Jonathan: Awesome. I had assumed that the common `\` escaping would also work on comments. But wow I was wrong. I haven't been able to find another example yet... :) Thanks! — sarnold, Nov 21 '11 at 02:31

score 395 · Accepted Answer · edited May 02 '20 at 15:48

395

This can be done with a sed one-liner:

sed '/^#/d'

This says, "find all lines that start with # and delete them, leaving everything else."

edited May 02 '20 at 15:48

Vlad

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answered Nov 21 '11 at 01:20

Raymond Hettinger

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shorter version: `sed /^#/d` – kev Mar 11 '12 at 14:46
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For linux noobs like me: `sed '/^#/ d' < inputFile.txt > outputFile.txt` – Neil McGuigan Jul 28 '14 at 19:12
How can i keep the lines which start with # and delete every other lines i need to remove every contents except which start with # – Babin Lonston Oct 02 '14 at 16:22
64

Shortest version: `sed -i '/^#/d' filepath`. – lesderid Oct 11 '14 at 18:50
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And `sed -i '' '/^#/d' filepath` on Mac ([because the -i suffix is mandatory](http://stackoverflow.com/a/16746032/125246)) – paulcm Aug 05 '16 at 10:44
What if I need to replace only the first line starting with a string? – Viesturs Aug 07 '18 at 12:44
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@Viesturs Try this: ``awk '/^#/ && !first { first=1 ; next } { print $0}' `` – Raymond Hettinger Aug 10 '18 at 06:39
@NeilMcGuigan better make sure `inputFile.txt` is NOT the same as `outputFile.txt` – Larpon Oct 22 '18 at 21:58
To apply this to all files matching a shell pattern, use something like this (replace *.txt as needed): `for e in *.txt ; do sed -i '/^#/d' $e ; done` – Per Lundberg Aug 21 '20 at 19:00
and how do I specify a file? – Black Jul 21 '21 at 13:21

score 68 · Answer 2 · answered Nov 21 '11 at 04:10

I'm a little surprised nobody has suggested the most obvious solution:

grep -v '^#' filename

This solves the problem as stated.

But note that a common convention is for everything from a # to the end of a line to be treated as a comment:

sed 's/#.*$//' filename

though that treats, for example, a # character within a string literal as the beginning of a comment (which may or may not be relevant for your case) (and it leaves empty lines).

A line starting with arbitrary whitespace followed by # might also be treated as a comment:

grep -v '^ *#' filename

if whitespace is only spaces, or

grep -v '^[  ]#' filename

where the two spaces are actually a space followed by a literal tab character (type "control-v tab").

For all these commands, omit the filename argument to read from standard input (e.g., as part of a pipe).

I have added a [new answer](http://stackoverflow.com/a/19671392/832230) which builds upon this answer. — Asclepius, Oct 30 '13 at 16:19
I had troubles using grep in this way on Windows. The solution is to replace ' by ", e.g. `grep -v "^#" filename` — Serg, Oct 12 '14 at 15:51

score 22 · Answer 3 · answered Nov 21 '11 at 03:14

22

The opposite of Raymond's solution:

sed -n '/^#/!p'

"don't print anything, except for lines that DON'T start with #"

answered Nov 21 '11 at 03:14

ata

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ok and what's the advantage here? – lacostenycoder Apr 14 '22 at 13:50
@lacostenycoder It's about the same as any other option presented here, which is enough merit to mention it. – ata Apr 14 '22 at 19:37

rubo77 · Answer 4 · 2019-10-08T10:48:27.723

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you can directly edit your file with

sed -i '/^#/ d'

If you want also delete comment lines that start with some whitespace use

sed -i '/^\s*#/ d'

Usually, you want to keep the first line of your script, if it is a sha-bang, so sed should not delete lines starting with #!. also it should delete lines, that just contain only a hash but no text. put it all together:

sed -i '/^\s*\(#[^!].*\|#$\)/d'

To be conform with all sed variants you need to add a backup extension to the -i option:

sed -i.bak '/^\s*#/ d' $file
rm -Rf $file.bak

edited Oct 08 '19 at 10:48

answered Jun 05 '17 at 20:42

rubo77

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plus 1 for the one which space – deepaksingh pawar Feb 05 '19 at 12:44

jaypal singh · Answer 5 · 2011-11-21T03:02:22.403

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You can use the following for an awk solution -

awk '/^#/ {sub(/#.*/,"");getline;}1' inputfile

edited Nov 21 '11 at 03:02

answered Nov 21 '11 at 02:45

jaypal singh

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score 6 · Answer 6 · edited May 23 '17 at 12:03

6

This answer builds upon the earlier answer by Keith.

egrep -v "^[[:blank:]]*#" should filter out comment lines.

egrep -v "^[[:blank:]]*(#|$)" should filter out both comments and empty lines, as is frequently useful.

For information about [:blank:] and other character classes, refer to https://en.wikipedia.org/wiki/Regular_expression#Character_classes.

edited May 23 '17 at 12:03

Community

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answered Oct 29 '13 at 23:57

Asclepius

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Assuming your `egrep` supports that syntax; older versions might not. – Keith Thompson Oct 30 '13 at 17:45

score 0 · Answer 7 · answered Oct 05 '21 at 17:20

0

You also might want to remove empty lines as well

sed -E '/(^$|^#)/d' inputfile

answered Oct 05 '21 at 17:20

Ahmed Khaled

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score -3 · Answer 8 · edited Sep 28 '17 at 11:35

-3

Here is it with a loop for all files with some extension:

ll -ltr *.filename_extension > list.lst

for i in $(cat list.lst | awk '{ print $8 }') # validate if it is the 8 column on ls 
do
    echo $i
    sed -i '/^#/d' $i
done

edited Sep 28 '17 at 11:35

filmor

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answered Sep 28 '17 at 11:00

Bruno Damas

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Delete all lines beginning with a # from a file

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