Sed in Unix/Linux

Sed has several commands, but most people only learn the substitute command: s. The substitute command changes all occurrences of the regular expression into a new value. A simple example is changing "day" in the "old" file to "night" in the "new" file:

sed s/day/night/ new

Or another way (for Unix beginners),

sed s/day/night/ old >new

and for those who want to test this:

echo day sed s/day/night/

This will output "night".

I didn't put quotes around the argument because this example didn't need them. If you read my earlier tutorial, you would understand why it doesn't need quotes. However, I recommend you do use quotes. If you have meta-characters in the command, quotes are necessary. And if you aren't sure, it's a good habit, and I will henceforth quote future examples to emphasize the "best practice." Using the strong (single quote) character, that would be:

sed 's/day/night/' new

I must emphasize the the sed editor changes exactly what you tell it to. So if you executed

echo Sunday sed 's/day/night/' new

This would output the word "Sunnight" bacause sed found the string "day" in the input.

There are four parts to this substitute command:

s Substitute command
/../../ Delimiter
day Regular Expression Pattern Search Pattern
night Replacement string

The search pattern is on the left hand side and the replacement string is on the right hand side.