We have two options here: * `sed` is a powerful **stream** editor although it also features a handy in-place/`-i` flag that may modify the source file. Unfortunately this is a non-standard FreeBSD extension and therefore results in unportable code.

ex.sh

# Replace the shell executing the script
$ ex - '+g/^#!/s,bash,zsh,' -cwq install.sh

# Comment all non-commentary lines
$ ex - '+g/^[[:space:]]*[^#]/s/^/#/' -cwq test.sh

sed.sh

# Replace the shell executing the script
$ sed -i -e 's,bash,zsh,' install.sh

# sed can also create a backup file if an extension is provided to the -i flag
$ sed -i.bak -e 's/private/public/' MyClass.java
$ ls MyClass.*
Myclass.java  MyClass.java.bak