Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
Commands
cd /hpme # Oops! Mistyped '/home'.
pwd # /home
# The shell corrected the misspelling.
true
false
A command that returns a successful (zero) exit status, but does nothing else.
# Endless loop
while true # alias for ":"
do
operation−1
operation−2
...
operation−n
# Need a way to break out of loop.
done
A command that returns an unsuccessful exit status, but does nothing else.
# Null loop
while false
do
# The following code will not execute.
operation−1
operation−2
...
operation−n
# Nothing happens!
done
type [cmd]
Similar to the which external command, type cmd gives the full pathname to "cmd". Unlike which,
type is a Bash builtin. The useful −a option to type identifies keywords and builtins, and also
locates system commands with identical names.
bash$ type '['
[ is a shell builtin
bash$ type −a '['
[ is a shell builtin
[ is /usr/bin/[
hash [cmds]
Record the path name of specified commands (in the shell hash table), so the shell or script will not
need to search the $PATH on subsequent calls to those commands. When hash is called with no
arguments, it simply lists the commands that have been hashed. The −r option resets the hash table.
help
help COMMAND looks up a short usage summary of the shell builtin COMMAND. This is the
counterpart to whatis, but for builtins.
bash$ help exit
exit: exit [n]
Exit the shell with a status of N. If N is omitted, the exit status
is that of the last command executed.
Chapter 11. Internal Commands and Builtins 143