Linux Add A Prefix String To Beginning Of Each L

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# If you want to edit the file in-place
sed -i -e 's/^/prefix/' file

# If you want to create a new file
sed -e 's/^/prefix/' file > file.new

If prefix contains /, you can use any other character not in prefix, or escape the /, so the sed command becomes

's#^#/opt/workdir#'
# or
's/^/\/opt\/workdir/'

edited Oct 25, 2018 at 22:35

PhysicalChemist

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answered Jan 20, 2010 at 6:38

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Alok Singhal

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@benjamin, I had already upvoted your answer, however, I prefer sed for lightweight tasks such as this. If “prefix” is known, it’s very easy to pick a character not from “prefix”.

– Alok Singhal

Commented Jan 20, 2010 at 6:56
7

Don’t forget you can also use sed in a pipeline, e.g. foo | sed -e 's/^/x /' | bar.

– Mattie

Commented Mar 13, 2014 at 18:11
1

@Dataman cool. Another way would be sed -e '2,$s/^/prefix/'.

– Alok Singhal

Commented Oct 21, 2014 at 13:57
1

@BinChen escape the / like \/ (in single-quoted strings) or \\/ (in double-quoted strings)

– user6516765

Commented Mar 28, 2017 at 5:23
11

Use sed -e 's/$/postfix/' file if you want to add string to the end of each line.

– Brian

Commented Jun 29, 2017 at 7:54

Show 16 more comments

159

awk '$0="prefix"$0' file > new_file

In awk the default action is '{print $0}' (i.e. print the whole line), so the above is equivalent to:

awk '{print "prefix"$0}' file > new_file

With Perl (in place replacement):

perl -pi 's/^/prefix/' file

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answered Jan 20, 2010 at 6:41

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Vijay

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With a pipe/stream or variable: prtinf "$VARIABLE\n" | awk '$0="prefix"$0'

– ThorSummoner

Commented Feb 25, 2015 at 18:20
5

With a large file (12 G), awk reports awk: out of memory in readrec 1 source line number 1, but the solution with sed completes successfully.

– jrm

Commented Jul 6, 2017 at 16:04
1

This is the best answer, with AWK it Worked right off the bat without having to annoyingly deal with escaping regex special characters

– Maximo Migliari

Commented Jan 6, 2022 at 21:50
With “normal files” split into lines awk shouldn’t run out of memory…

– rogerdpack

Commented Feb 2, 2022 at 19:07

You can use Vim in Ex mode:

ex -sc '%s/^/prefix/|x' file

% select all lines
s replace
x save and close

edited Nov 5, 2016 at 14:30

answered Sep 30, 2012 at 16:18

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Zombo

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For me, I just open the file in vim and type :%s/^/prefix/, since this strategy ends up being useful in many situations

– Frank Bryce

Commented Feb 2, 2017 at 16:35

If your prefix is a bit complicated, just put it in a variable:

prefix=path/to/file/

Then, you pass that variable and let awk deal with it:

awk -v prefix="$prefix" '{print prefix $0}' input_file.txt

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answered Feb 17, 2015 at 21:35

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Melka

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Here is a oneliner solution using the ts command from moreutils

$ cat file | ts prefix

And how it’s derived step by step:

# Step 1. create the file

$ cat file
line1
line2
line3

# Step 2. add prefix to the beginning of each line

$ cat file | ts prefix
prefix line1
prefix line2
prefix line3

Note that the prefix will be space separated from the content

edited Jan 23 at 0:12

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Ulysse BN

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answered Apr 19, 2019 at 9:41

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btwiuse

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‘ts’ is not installed by default on many Linux distros. Also, downvoting because the trailing “tr -d ‘ ‘” in this answer will remove all spaces from the lines, not just the space that was added by ‘ts’

– Tim Bird

Commented May 13, 2020 at 17:21

If you have Perl:

perl -pe 's/^/PREFIX/' input.file

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answered Dec 11, 2013 at 20:08

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Majid Azimi

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Using & (the whole part of the input that was matched by the pattern”):

cat in.txt | sed -e "s/.*/prefix&/" > out.txt

OR using back references:

cat in.txt | sed -e "s/\(.*\)/prefix\1/" > out.txt

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answered Apr 5, 2020 at 1:15

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Ark25

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Using the shell:

#!/bin/bash
prefix="something"
file="file"
while read -r line
do
 echo "${prefix}$line"
done <$file > newfile
mv newfile $file

edited Dec 21, 2023 at 17:56

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answered Jan 20, 2010 at 7:05

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ghostdog74

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While I don’t think pierr had this concern, I needed a solution that would not delay output from the live “tail” of a file, since I wanted to monitor several alert logs simultaneously, prefixing each line with the name of its respective log.

Unfortunately, sed, cut, etc. introduced too much buffering and kept me from seeing the most current lines. Steven Penny’s suggestion to use the -s option of nl was intriguing, and testing proved that it did not introduce the unwanted buffering that concerned me.

There were a couple of problems with using nl, though, related to the desire to strip out the unwanted line numbers (even if you don’t care about the aesthetics of it, there may be cases where using the extra columns would be undesirable). First, using “cut” to strip out the numbers re-introduces the buffering problem, so it wrecks the solution. Second, using “-w1” doesn’t help, since this does NOT restrict the line number to a single column - it just gets wider as more digits are needed.

It isn’t pretty if you want to capture this elsewhere, but since that’s exactly what I didn’t need to do (everything was being written to log files already, I just wanted to watch several at once in real time), the best way to lose the line numbers and have only my prefix was to start the -s string with a carriage return (CR or ^M or Ctrl-M). So for example:

#!/bin/ksh

# Monitor the widget, framas, and dweezil
# log files until the operator hits <enter>
# to end monitoring.

PGRP=$$

for LOGFILE in widget framas dweezil
do
(
    tail -f $LOGFILE 2>&1 |
    nl -s"^M${LOGFILE}>  "
) &
sleep 1
done

read KILLEM

kill -- -${PGRP}

edited Dec 21, 2023 at 17:56

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answered Mar 7, 2014 at 21:44

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ScriptGuy

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3

use the -u option to sed to avoid the buffering.

– Bryan Larsen

Commented Mar 7, 2014 at 21:48
2

Buffering can be turned off with unbuffer/stdbuf, see unix.stackexchange.com/q/25372/6205

– myroslav

Commented Jun 4, 2015 at 19:15

Using ed:

ed infile <<'EOE'
,s/^/prefix/
wq
EOE

This substitutes, for each line (,), the beginning of the line (^) with prefix. wq saves and exits.

If the replacement string contains a slash, we can use a different delimiter for s instead:

ed infile <<'EOE'
,s#^#/opt/workdir/#
wq
EOE

I’ve quoted the here-doc delimiter EOE (“end of ed”) to prevent parameter expansion. In this example, it would work unquoted as well, but it’s good practice to prevent surprises if you ever have a $ in your ed script.

answered Jun 28, 2018 at 14:32

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Benjamin W.

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Here’s a wrapped up example using the sed approach from this answer:

$ cat /path/to/some/file | prefix_lines "WOW: "

WOW: some text
WOW: another line
WOW: more text

prefix_lines

function show_help()
{
  IT=$(CAT <<EOF
    Usage: PREFIX {FILE}

    e.g.

    cat /path/to/file | prefix_lines "WOW: "

      WOW: some text
      WOW: another line
      WOW: more text
  )
  echo "$IT"
  exit
}

# Require a prefix
if [ -z "$1" ]
then
  show_help
fi

# Check if input is from stdin or a file
FILE=$2
if [ -z "$2" ]
then
  # If no stdin exists
  if [ -t 0 ]; then
    show_help
  fi
  FILE=/dev/stdin
fi

# Now prefix the output
PREFIX=$1
sed -e "s/^/$PREFIX/" $FILE

edited Dec 21, 2023 at 17:57

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answered Mar 16, 2016 at 13:24

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Brad Parks

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2

This will not work if PREFIX contains any characters special to sed like a slash.

– josch

Commented Jan 3, 2017 at 11:57
Good point… If you find you use slash alot, you could use a different delimiter with the sed part, as detailed here, which would allow you to use it in searches. Other special sed chars can be put in by escaping with a slash, e.g prefix_lines \*

– Brad Parks

Commented Jan 3, 2017 at 13:19

You can also achieve this using the backreference technique
```
sed -i.bak 's/$.*$/prefix\1/' foo.txt
```

You can also use with awk like this

awk '{print "prefix"$0}' foo.txt > tmp && mv tmp foo.txt

edited Dec 21, 2023 at 17:58

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answered Apr 11, 2020 at 8:44

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k_vishwanath

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Using Pythonise (pz):

pz '"preix"+s' <filename

answered Mar 22, 2021 at 9:56

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Hans Ginzel

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You can do it using AWK

echo example| awk '{print "prefix"$0}'

awk '{print "prefix"$0}' file.txt > output.txt

For suffix: awk '{print $0"suffix"}'

For prefix and suffix: awk '{print "prefix"$0"suffix"}'

edited Dec 21, 2023 at 17:59

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answered Jan 11, 2022 at 8:57

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Rauf

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Simple solution using a for loop on the command line with bash:

for i in $(cat yourfile.txt); do echo "prefix$i"; done

Save the output to a file:

for i in $(cat yourfile.txt); do echo "prefix$i"; done > yourfilewithprefixes.txt

edited Dec 21, 2023 at 17:58

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answered Apr 18, 2021 at 15:19

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Michael

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If you need to prepend a text at the beginning of each line that has a certain string, try following. In the following example, I am adding # at the beginning of each line that has the word “rock” in it.

sed -i -e 's/^.*rock.*/#&/' file_name

edited Dec 21, 2023 at 17:59

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answered Dec 10, 2020 at 19:31

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mandroid

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For people on BSD/OSX systems there’s utility called lam, short for laminate. lam -s prefix file will do what you want. I use it in pipelines, eg:

find -type f -exec lam -s "{}: " "{}" \; | fzf

…which will find all files, exec lam on each of them, giving each file a prefix of its own filename. (And pump the output to fzf for searching.)

edited Dec 15, 2019 at 18:15

answered Apr 19, 2019 at 1:36

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Ray

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You’re right, it appears this is a BSD only command. POSIX replaced it with paste, but paste doesn’t have the feature of adding a full separator string. I’ll update my answer.

– Ray

Commented Dec 15, 2019 at 18:14

Another alternative (sponge is optional):

$ cat file | xargs -I{} echo 'prefix-{}-suffix' | sponge file

Example:

$ seq 1 10 | xargs -I{} echo 'prefix-{}-suffix'
prefix-1-suffix
prefix-2-suffix
prefix-3-suffix
prefix-4-suffix
prefix-5-suffix
prefix-6-suffix
prefix-7-suffix
prefix-8-suffix
prefix-9-suffix
prefix-10-suffix

answered Jul 5 at 10:20

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FarK

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-1

SETLOCAL ENABLEDELAYEDEXPANSION

YourPrefix=blabla

YourPath=C:\path

for /f "tokens=*" %%a in (!YourPath!\longfile.csv)     do (echo !YourPrefix!%%a) >> !YourPath!\Archive\output.csv

edited Dec 21, 2023 at 18:00

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answered Nov 10, 2020 at 15:15

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Please add some explanation to your answer such that others can learn from it

– Nico Haase

Commented Nov 10, 2020 at 16:15
1

Please don’t post only code as an answer, but also provide an explanation of what your code does and how it solves the problem of the question. Answers with an explanation are usually more helpful and of better quality, and are more likely to attract upvotes.

– Yagiz Degirmenci

Commented Nov 10, 2020 at 17:03
If you were looking for an answer in a knowledgebase, trying to solve an issue, do you think this answer would be helpful? Not so much.

– Trenton McKinney

Commented Nov 10, 2020 at 23:33
Thank you for your interest in contributing to the Stack Overflow community. This question already has quite a few answers—including one that has been extensively validated by the community. Are you certain your approach hasn’t been given previously? If so, it would be useful to explain how your approach is different, under what circumstances your approach might be preferred, and/or why you think the previous answers aren’t sufficient. Can you kindly edit your answer to offer an explanation?

– Jeremy Caney

Commented Dec 22, 2023 at 0:08