Add a prefix string to beginning of each line

Question

I have a file as below   line1 line2 line3   And I want to get   prefixline1 prefixline2 prefixline3   I could write a Ruby script  but it is better if I do not need to   prefix will contain    It is a path   opt workdir  for example

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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  gt  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

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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  lt  lt 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

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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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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  quot rock quot  in it  sed -i -e  s    rock     amp    file name

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SETLOCAL ENABLEDELAYEDEXPANSION  YourPrefix blabla  YourPath C  path  for  f  quot tokens   quot    a in   YourPath  longfile csv      do  echo  YourPrefix   a   gt  gt   YourPath  Archive output csv

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Using the shell      bin bash prefix  something  file  file  while read -r line do  echo    prefix  line  done  lt  file  gt  newfile mv newfile  file

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Here is a hightly readable oneliner solution using the ts command from moreutils    cat file   ts prefix   tr -d       And how it s derived step by step     Step 0  create the file    cat file line1 line2 line3     Step 1  add prefix to the beginning of each line    cat file   ts prefix prefix line1 prefix line2 prefix line3     Step 2  remove spaces in the middle    cat file   ts prefix   tr -d     prefixline1 prefixline2 prefixline3

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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  lt enter gt    to end monitoring   PGRP     for LOGFILE in widget framas dweezil do       tail -f  LOGFILE 2 gt  amp 1       nl -s  M  LOGFILE  gt        amp  sleep 1 done  read KILLEM  kill -- -  PGRP

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awk   0  prefix  0  file  gt  new file   With Perl in place replacement    perl -pi  s   prefix   file

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Using ed   ed infile  lt  lt  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  lt  lt  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

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You can use Vim in Ex mode   ex -sc   s   prefix  x  file      select all lines s replace x save and close

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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  gt  tmp  amp  amp  mv tmp foo txt

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Using  amp   the whole part of the input that was matched by the pattern       cat in txt   sed -e  s    prefix amp     gt  out txt   OR using back references   cat in txt   sed -e  s        prefix 1    gt  out txt

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If you have Perl   perl -pe  s   PREFIX   input file

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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

[linux] Add a prefix string to beginning of each line

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