Advanced Bash−Scripting Guide

Chapter 11. Internal Commands and Builtins
A builtin is a command contained within the Bash tool set, literally built in. This is either for performance
reasons −− builtins execute faster than external commands, which usually require forking off a separate
process −− or because a particular builtin needs direct access to the shell internals.
When a command or the shell itself initiates (or spawns) a new subprocess to carry out a task, this is called
forking. This new process is the "child", and the process that forked it off is the "parent". While the child
process is doing its work, the parent process is still executing.
Generally, a Bash builtin does not fork a subprocess when it executes within a script. An external system
command or filter in a script usually will fork a subprocess.
A builtin may be a synonym to a system command of the same name, but Bash reimplements it internally. For
example, the Bash echo command is not the same as /bin/echo, although their behavior is almost
identical.
#!/bin/bash
echo "This line uses the \"echo\" builtin."
/bin/echo "This line uses the /bin/echo system command."
A keyword is a reserved word, token or operator. Keywords have a special meaning to the shell, and indeed
are the building blocks of the shell's syntax. As examples, "for", "while", "do", and "!" are keywords. Similar
to a builtin, a keyword is hard−coded into Bash, but unlike a builtin, a keyword is not by itself a command,
but part of a larger command structure. [24]
I/O
echo
prints (to stdout) an expression or variable (see Example 4−1).
echo Hello
echo $a
An echo requires the −e option to print escaped characters. See Example 5−2.
Normally, each echo command prints a terminal newline, but the −n option suppresses this.
An echo can be used to feed a sequence of commands down a pipe.
if echo "$VAR" | grep −q txt # if [[ $VAR = *txt* ]]
then
echo "$VAR contains the substring sequence \"txt\""
fi
An echo, in combination with command substitution can set a variable.
Chapter 11. Internal Commands and Builtins 124