Insert text in specific lines of a file

awk 'NR==1{print "Exp1"}NR==5{print "Exp2"}1' file

1 at the end means {print}.

sed '1i\
Exp1
5i\
Exp2
' file

If you want to extend it to more numbers, awk makes that easy:

awk 'NR%4==1{print "Exp"++i}1' file

With different numbers, we'll just say sed isn't the right tool for the job. It's possible, but not pretty.

---

As an academic curiosity, here it is in sed:

sed '1{ 
    h
    s/^.*$/Exp0/
    x   
}
x
s/^4//
s/^3/4/
s/^2/3/
s/^1/2/
/^E/ { 
    s/$/|/
    :x
    s/0|/1/
    s/1|/2/
    s/2|/3/
    s/3|/4/
    s/4|/5/
    s/5|/6/
    s/6|/7/
    s/7|/8/
    s/8|/9/
    s/9|/|0/
    tx
    s/^Exp|/Exp1/
    p
    s/^/1/
}
x' file

If the number of data points (I'm assuming) in each of your experiments (again, I'm assuming) is always 4, you can use this Perl one-liner:

perl -ple '$. % 4 == 1 and print "Exp", ++$i' your_file

How it works

• The -p switch tells Perl to loop over the given file(s) line by line (by default) and print each line after executing whatever code you supply.
• The -l switch tells Perl to automatically append a newline (by default) to each print statement.
• The -e switch informs Perl that whatever comes after it is code to execute (the code that will be executed before each line of the file is printed).
• The special variable $. holds the line number currently being processed. If that line number is congruent to 1 modulo 4 ($. % 4 = 1) it means that we need to insert Exp # before it.
• So we test $. % 4 == 1 and if it's true, we print Exp ++$i\n (where \n was added by the -l switch). This works because an undefined variable is given a value of zero by Perl when it's first used as an integer.

To make it work with multiple files

• The $. variable will not reset its value if you're processing multiple files at once. Obviously, this is not what you want. You can work around this by explicitly writing out the implicit loop created by -p in the above one-liner so you can manipulate the continue block.

  perl -le '
      while(<>){
          $. % 4 == 1 and print "Exp ",++$i
      } continue {
          print;             # Print the current line
          if(eof){           # If the next line is the end-of-file
              close ARGV ;   # Close the current filehandle to reset $.
              $i = 0     ;   # Reset the experiment counter if you need
          }
      }
  ' list_of_your_files
  

Notes

• Both of the above solutions will print the modified file(s) to standard output. To emulate an edit in place (i.e. edit the files themselves), add a -i switch:

  perl -pi -le '$. % 4 == 1 and print "Exp", ++$i' your_file
  

  • Proceed similarly for the other solution.

  • Make sure you don't put another switch directly after -i as it interprets anything after it as the extension to use in order to create a backup file.

  • I recommend you test without -i first and then add it if the results are satisfactory.

You don't need external languages, a few standard commands can do the trick. For example, to insert something at line 5 of origin.txt you could do:

tmpfile=`mktemp`
head -n4 origin.txt > $tmpfile
echo "I'm line 5 now" >> $tmpfile
tail -n +5 origin.txt >> $tmpfile
mv $tmpfile origin.txt