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built-in commands

summary

    This subchapter looks at built-in commands in a UNIX (and Linux) BASH shell.

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built-in commands

    This subchapter looks at built-in commands in a UNIX (and Linux) BASH shell.

tools

    Most of the commands in the UNIX or Linux shell are actually programs. If you look at the usr/bin directory, you will see the actual programs. Most of these programs were written in the C programming language.

    There is a common core of tools/commands that will be available on almost every UNIX or Linux machine, but exactly how many and which commands/tools are available varies widely.

    The good news is that if a command or tool is missing from your system, you can go out and get the source code and recompile it for your local computer.

built-in

    Many of the shells have special built-in commands. These are not separate programs, but are part of the code for the shell itself.

    One example would be the shell command cd that you just saw in the previous quick tour chapter.

    There are some built-in commands are only available in selected shells and these can make your scripts shell-dependent.

    Some examples of built-in commands include the history command in the C shell, and the export command in the Bourne shell. The cd command is built-in in both bash and csh.

    echo is an example of a command that is built into both bash and csh, but also exists externally as a utility.

overriding built-in commands

    You can override any built-in commands by giving the full path name to an external command or utility. If bash finds a slash character ( ) anywhere in a command, the shell will not run the built-in command, even if the last component of the specified command matches the name of a builtin command.

    As an example, using the command echo will run the version of the command that is built into bash, while specifying /bin/echo or ./echo will ignore the built-in comamnd and instead run the designated utility.

    Overriding can be used to run alternative versions of commands or to extend the built-in command to add additional features.

determining builtin or external

    You can use the type command to determine if a particular command is a built-in command or an external utility. If the command is an external utility, you will also be told the path to the external command.

    $ type echo
    echo is a shell builtin
    $ type mkdir
    mkdir is /bin/mkdir
    $

    You can use the which command to locate a program in your path.

    $ which echo
    /bin/echo
    $

    You can use the whereis command to locate a program in your path.

    $ whereis echo
    /bin/echo
    $

    In csh and tcsh you can use the where command to locate a program in your path.

    % where echo
    /bin/echo
    %

problems

    If something bad happens to your computer, if the shell is still loaded in memory and running, any of the built-in commands will still work correctly, even if the entire file system (including all hard drives) disappears or becomes unavailable for any reason.

built in command chart

    The following chart shows the built-in commands and external utilities for bash and csh for Mac OS X. This wil be similar for Linux and other UNIXes.

    External commands marked No** under the External column do exist externally, but are implemented using the built-in command.

Command	External	csh	bash
!	No	No	Yes
%	No	Yes	No
.	No	No	Yes
:	No	Yes	Yes
{	No	No	Yes
}	No	No	Yes
alias	No**	Yes	Yes
alloc	No	Yes	No
bg	No**	Yes	Yes
bind	No	No	Yes
bindkey	No	Yes	No
break	No	Yes	Yes
breaksw	No	Yes	No
builtin	No	No	Yes
builtins	No	Yes	No
case	No	Yes	Yes
cd	No**	Yes	Yes
chdir	No	Yes	Yes
command	No**	No	Yes
complete	No	Yes	No
continue	No	Yes	Yes
default	No	Yes	No
dirs	No	Yes	No
do	No	No	Yes
done	No	No	Yes
echo	Yes	Yes	Yes
echotc	No	Yes	No
elif	No	No	Yes
else	No	Yes	Yes
end	No	Yes	No
endif	No	Yes	No
endsw	No	Yes	No
esac	No	No	Yes
eval	No	Yes	Yes
exec	No	Yes	Yes
exit	No	Yes	Yes
export	No	No	Yes
false	Yes	No	Yes
fc	No**	No	Yes
fg	No**	Yes	Yes
filetest	No	Yes	No
fi	No	No	Yes
for	No	No	Yes
foreach	No	Yes	No
getopts	No**	No	Yes
glob	No	Yes	No
goto	No	Yes	No
hash	No	No	Yes
hashstat	No	Yes	No
history	No	Yes	No
hup	No	Yes	No
if	No	Yes	Yes
jobid	No	No	Yes
jobs	No**	Yes	Yes
kill	Yes	Yes	No
limit	No	Yes	No
local	No	No	Yes
log	No	Yes	No
login	Yes	Yes	No
logout	No	Yes	No
ls-F	No	Yes	No
nice	Yes	Yes	No
nohup	Yes	Yes	No
notify	No	Yes	No
onintr	No	Yes	No
popd	No	Yes	No
printenv	Yes	Yes	No
pushd	No	Yes	No
pwd	Yes	No	Yes
read	No**	No	Yes
readonly	No	No	Yes
rehash	No	Yes	No
repeat	No	Yes	No
return	No	No	Yes
sched	No	Yes	No
set	No	Yes	Yes
setenv	No	Yes	No
settc	No	Yes	No
setty	No	Yes	No
setvar	No	No	Yes
shift	No	Yes	Yes
source	No	Yes	No
stop	No	Yes	No
suspend	No	Yes	No
switch	No	Yes	No
telltc	No	Yes	No
test	Yes	No	Yes
then	No	No	Yes
time	Yes	Yes	No
times	No	No	Yes
trap	No	No	Yes
true	Yes	No	Yes
type	No	No	Yes
ulimit	No	No	Yes
umask	No**	Yes	Yes
unalias	No**	Yes	Yes
uncomplete	No	Yes	No
unhash	No	Yes	No
unlimit	No	Yes	No
unset	No	Yes	Yes
unsetenv	No	Yes	No
until	No	No	Yes
wait	No**	Yes	Yes
where	No	Yes	No
which	Yes	Yes	No
while	No	Yes	Yes

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