How to Find Lines Matching a Pattern and Delete Them?

In addition to the very good grep and sed answers you've received, here are some other tools that can do the same thing:

• A few Perl ways:

  perl -ne '/^HERE IT IS/ || print' file > newfile
  perl -ne 'print if !/^HERE IT IS/' file > newfile
  perl -ne 'print unless /^HERE IT IS/' file > newfile
  

  You can add the -i switch to any of the examples to edit the file in place:

  perl -i.bak -ne '/^HERE IT IS/ || print' file        
  

• (g)awk

  awk '!/^HERE IT IS/' file > newfile
  

  Newer versions (4.1.1 and later) of GNU awk (the default awk on Linux) can also edit the file in place:

  gawk -i inplace  '!/^HERE IT IS/' file
  

• Shell (bash, zsh, ksh, probably others). This is kind of silly though, it can be done but other tools are better.

  while IFS= read -r line; do 
    [[ $line =~ ^"HERE IT IS" ]] || printf "%s\n" "$line"
  done < file > newfile
  

I would use grep to filter them out. For example :

grep -v "^HERE IT IS" infile > outfile

Then move outfile back to infile.

sed is definitely the way to go.

This slight modification of the command @heemayl gave you will delete the line whether the same case is used in the pattern or not, due to the I in the pattern reference.

sed -i '/HERE IT IS/Id' <file>

If you had several files in a directory that you wanted to do this on, you could combine it with find like so.

find . -maxdepth 1 -type f -exec sed -i.bak '/HERE IT IS/Id' {} +

The maxdepth option means this won't recurse into directories.

Another python option:

#!/usr/bin/env python3
[print(l, end = "") for l in open(f).readlines() if not l.startswith("HERE IT IS")]

Where f is the path to the file, between quotes.

Grep

grep -P '^(?!HERE IT IS)' file

(?!HERE IT IS) negative lookahead assertion which makes the regex engine to match all the line starting boundary (which is usually matched by ^) only if it's not followed by the string HERE IT IS

python

#!/usr/bin/python3
import sys
fil = sys.argv[1]
with open(fil) as f:
    for line in f:
        if not line.startswith('HERE IT IS'):
            print(line, end="")

Save the script in a file, say script.py and then run it through the below command on the terminal.

python3 script.py infile

You can use Vim in Ex mode:

ex -sc 'g/^HERE IT IS/d' -cx file

1. g global search

2. d delete

3. x save and close