Sed

In-place editing and backups:

# In-place
sed -i 's/a/b/' config.txt

# Backup
sed -i.bak        's/a/b/' c.txt
sed -i.bak        's/a/b/' c1.txt c2.txt  # Backup two files
sed -i'bkp.*'     's/a/b/' c.txt          # Use prefix
sed -i'backups/*' 's/a/b/' c1.txt c2.txt  # Move backups to existing directory

Search and replace with s/address/replacment/flags:

printf 'one two three' |sed 's/t/T/'   # one Two three
printf 'one two three' |sed 's/t/T/g'  # one Two Three

Map pattern space from source to destination with y/src/dst:

printf 'one two three four five' |sed 'y/aeiouy/AEIOUY/'  # OnE twO thrEE fOUr fIvE

Conditional execution with /address/flags or /address/flags! for inverted:

$ printf '1,2,3,4\na,b,c,d\n' | sed '/2/ s/,/-/g'
1-2-3-4
a,b,c,d

$ printf '1,2,3,4\na,b,c,d\n' | sed '/2/! s/,/-/g'
1,2,3,4
a-b-c-d

Drop lines:

$ # same as: grep -v 'at'
$ printf 'sea\neat\ndrop\n' | sed '/at/d'
sea
drop

$ # same as: grep 'at'
$ printf 'sea\neat\ndrop\n' | sed '/at/!d'
eat

$ # same as: cat file
$ sed '!d' file

Quit:

$ # Exit after matching line
$ sed '/match/q' file
$ # Exit before matching line
$ sed '/match/Q' file

$ # To get lines after match, use tac
$ tac file | sed '/match/q' | tac

$ # Provide exit status
$ sed '/match/q255'

Notice: for multiple files, sed will quit after the match, even if there files left to be processed. Otherwise, use -s flag