The commands you can use in a Bash script fall into three major categories:
- Shell builtin commands: Included in Bash itself. These commands don't correspond to executable program files on your system; they are implemented in the bash binary itself. Examples are echo, type, and source.
- Runtime commands: Defined in the shell at runtime, and written in the Bash language. These can be aliases or functions. They don't have executable program files of their own on disk either, and are defined at runtime during a Bash session, often by reading startup files. Examples vary between systems and users.
- System commands: Invoke executable program files on your filesystem. These are the only kinds of commands that can also be run outside of Bash. Examples are grep, ping, and rm.
The executable programs called by system commands may be written in any...