Bash: How to read file line by line

IFS=$'\n'

for line in $(cat file.txt)
do
       echo $line
done


OR

while read line
do
        echo $line
done < file

[While loop is the easiest method but it obliterates the formatting of lines including spaces and tabs.]

Revert last submitted changelist from Perforce branch

Let's say Perforce trunk has changes c1,c2,c3. c3 is HOL. How do we revert the changes done through c3??

p4 sync @c2

p4 diff -se ...@c3 | p4 -x - edit
p4 sync @c3
p4 resolve -ay

How to enable warnings for Perl execution

1. Enable warnings from command line: perl -w

2. Through Shebang line: #!/usr/bin/perl -w

However this doesn't work in case of #!/usr/bin/env perl -w because env tries to treat "perl -w" as single argument.

3. use warnings - However this has lexical implications and only applies to current code

4. BEGIN{
$^W = 1
}

BEGIN block is executed during compilation and hence this sets warnings switch to true and applies to all the scripts that get executed/imported during this execution.