Looping through a file and removing certain lines

Question

I want to loop through a file and remove certain lines. Example file:

test.txt

a
b
c
d

What I have:

FILE=/tmp/test.txt
while read FILE
do
  # if the line starts with b delete it otherwise leave it there
  ?
done

Answer 1

This is a job for sed - the UNIX stream editor:

(sed '/^b/d' yourfile > yourfile~ && mv yourfile~ yourfile) || rm yourfile~

The command will delete all lines which begin with a b and writes the remaing lines to a backup file. If this succeed the backup file will be renamed to the original file if it fails the backup file will be deleted.

Answer 2

you could do it with a grep oneliner :

grep -v "^b" test.txt > newfile