Edit text in a scriptable manner.

# To replace the first occurrence of a string in a file, and print the result:
sed 's/<find>/<replace>/' <filename>

# To replace all occurrences of an extended regular expression in a file:
sed -E 's/<regex>/<replace>/g' <filename>

# To replace all occurrences of a string in a file, overwriting the file (i.e. in-place):
sed -i '' 's/<find>/<replace>/g' <filename>

# To replace only on lines matching the line pattern:
sed '/<line_pattern>/s/<find>/<replace>/' <filename>

# To print only text between n-th line till the next empty line:
sed -n '<line_number>,/^$/p' <filename>

# To apply multiple find-replace expressions to a file:
sed -e 's/<find>/<replace>/' -e 's/<find>/<replace>/' <filename>

# To replace separator / by any other character not used in the find or replace patterns, e.g., #:
sed 's#<find>#<replace>#' <filename>