Strip leading/trailing whitespace

Trim leading and trailing whitespace from each line

sed 's/^[[:space:]]*//;s/[[:space:]]*$//' file.txt

More sed commands

Address range substitution

sed '5,15s/DEBUG/INFO/g' app.log

Apply substitution only to lines 5 through 15
Extract value from key=value file

sed -n 's/^DB_HOST=//p' .env

Print the value of DB_HOST from a .env file
Substitute first match per line

sed 's/foo/bar/' file.txt

Replace the first occurrence of 'foo' with 'bar' per line
Substitute all matches (global)

sed 's/foo/bar/g' file.txt

Replace every occurrence per line with the /g flag
In-place edit

sed -i 's/old/new/g' config.cfg

Modify file in-place (no backup); on macOS add empty string: -i ''