sed insert # at the beginning of a line

makyo · 02-06-2008, 10:12 AM

Hi.

Quote:

Originally Posted by jschiwal

I did triple check my results with a sample example, but I'll take your word for it. You could use the quit command if you are extracting information from a file.

Code:

jschiwal@hpamd64:~> cat test
line 1
line 2
line 3
line 4
line 5
line 6
line 7
line 8

jschiwal@hpamd64:~> sed -i '/4/s/4/four/;/4/q' test
jschiwal@hpamd64:~> cat test
line 1
line 2
line 3
line four
line 5
line 6
line 7
line 8

Code:

sed --version
GNU sed version 4.1.5

Your sed replaces the string that is searched for in the sed quit sub-command "/4/q". However, because that was replaced, the line for the quit never matched, and all lines were copied.

If we add a line with two "4" strings, it will show the truncation. If we do the substitution at the beginning of line as we did earlier, it will also truncate. Here's the former case:

Code:

#!/usr/bin/env sh

# @(#) user1    Demonstrate truncation with sed.

#  ____
# /
# |   Infrastructure BEGIN

echo
set -o nounset

debug=":"
debug="echo"

## The shebang using "env" line is designed for portability. For
#  higher security, use:
#
#  #!/bin/sh -

## Use local command version for the commands in this demonstration.

set +o nounset
echo "(Versions displayed with local utility \"version\")"
version >/dev/null 2>&1 && version =o $(_eat $0 $1) sed
set -o nounset

echo

FILE=${1-user-data1}
cat >$FILE <<'EOF'
line 1
line 2
line 3
line 4
line 5
line 6
line 7
line 8
line 9 two fours to change 4 4
line 10
EOF

echo " Input file $FILE:"
cat $FILE
echo
echo "( $( wc $FILE ) )"

# |   Infrastructure END
# \
#  ---

echo
echo " Results from processing:"
# jschiwal@hpamd64:~> sed -i '/4/s/4/four/;/4/q'
sed -i '/4/s/4/four /;/4/q' $FILE
cat $FILE

echo " --- line above is last line in $FILE"

exit 0

Producing:

Code:

% ./user1

(Versions displayed with local utility "version")
Linux 2.6.11-x1
GNU bash, version 2.05b.0(1)-release (i386-pc-linux-gnu)
GNU sed version 4.1.2

 Input file user-data1:
line 1
line 2
line 3
line 4
line 5
line 6
line 7
line 8
line 9 two fours to change 4 4
line 10

( 10 26 95 user-data1 )

 Results from processing:
line 1
line 2
line 3
line four
line 5
line 6
line 7
line 8
line 9 two fours to change four  4
 --- line above is last line in user-data1

cheers, makyo

ubungu · 12-17-2012, 03:07 AM

Quote:

Originally Posted by Tinkster

Depends on your preferences, the task at hand and your knowledge
of the file to edit in question.

I reckon that if I *know* that line 19375 needs to be
commented my sed

Code:

sed -i '19375 s/^/#/' file

is going to be done quicker than someones pico (nano, whatever)

Cheers,
Tink

Thank you very much, this is the way for insert string beginning of line :d

David the H. · 12-19-2012, 08:21 AM

Please don't re-open old threads unless you have something substantial to add to that discussion, such as a correction or updated information. If you have a new question of your own, start a new thread and refer back to the old one if necessary. Thanks.

Now to supply my own substantial addition to the thread

, these days I like to use ed for many things like this. It has a more flexible addressing system because it operates on the whole file at once.

Code:

printf '%s\n' '19375 s/^/# /' 'w' | ed -s file.txt
printf '%s\n' 'g/pattern/ s/^/# /' 'w' | ed -s file.txt

How to use ed:
http://wiki.bash-hackers.org/howto/edit-ed
http://snap.nlc.dcccd.edu/learn/nlc/ed.html
(also read the info page)