8.10. Shell Invocation Options

When the shell is started using the sh command, it can take options to modify its behavior. See Table 8.7.

Table 8.7. Shell Invocation Options

Option	Meaning
“i	Shell is in the interactive mode. QUIT and INTERRUPT are ignored.
“s	Commands are read from standard input and output is sent to standard error.
“c string	Commands are read from string.

8.10.1 The set Command and Options

The set command can be used to turn shell options on and off, as well as for handling command-line arguments. To turn an option on, the dash ( “ ) is prepended to the option; to turn an option off, the plus sign ( + ) is prepended to the option. See Table 8.8 for a list of set options.

Example 8.67.

 1   $ set -f 2   $ echo *     * 3   $ echo ??     ?? 4   $ set +f 

Table 8.8. The set Command Options

Option	Meaning
“a	Marks variables that have been modified or exported
“e	Exits the program if a command returns a nonzero status
“f	Disables globbing (filename expansion)
“h	Locates and remembers function commands as functions when they are defined, not just when they are executed
“k	Places all keyword arguments in the environment for a command, not just those that precede the command name
“n	Reads commands but does not execute them; used for debugging
“t	Exits after reading and executing one command
“u	Treats unset variables as an error when performing substitution
“v	Prints shell input lines as they are read; used for debugging
“x	Prints commands and their arguments as they are being executed; used for debugging
“ “	Does not change any of the flags

EXPLANATION

1. The f option is turned on; filename expansion is disabled.

2. The asterisk is not expanded.

3. The question marks are not expanded.

4. The f is turned off; filename expansion is enabled.

8.10.2 Shell Built-In Commands

The shell has a number of commands that are built into its source code. Because the commands are built-in, the shell doesn't have to locate them on disk, making execution much faster. The built-in commands are listed in Table 8.9.

Table 8.9. Built-In Commands

Command	What It Does
:	Do-nothing command; returns exit status 0
. file	The dot command reads and executes command from file
break [n]	See "The break Command" on page 361
cd	Change directory
continue [n]	See "The continue Command" on page 362
echo [ args ]	Echo arguments
eval command	Shell scans the command line twice before execution
exec command	Runs command in place of this shell
exit [ n ]	Exit the shell with status n
export [ var ]	Make var known to subshells
hash	Controls the internal hash table for quicker searches for commands
kill [ “signal process ]	Sends the signal to the PID number or job number of the process; see /usr/include/sys/signal.h for a list of signals
getopts	Used in shell scripts to parse command line and check for legal options
login [ username ]	Sign onto the system
newgrp [ arg ]	Logs a user into a new group by changing the real group and effective group ID
pwd	Print present working directory
read [ var ]	Read line from standard input into variable var
readonly [ var ]	Make variable var read-only; cannot be reset
return [ n ]	Return from a function where n is the exit value given to the return
set	See Table 8.8
shift [ n ]	Shift positional parameters to the left n times
stop pid	Halt execution of the process number PID
suspend	Stops execution of the current shell (but not if a login shell)
times	Print accumulated user and system times for processes run from this shell
trap [ arg ] [ n ]	When shell receives signal n ( 0, 1, 2, or 15 ), execute arg
type [ command ]	Prints the type of command; for example, pwd has a built-in shell, in ksh , an alias for the command whence “v
umask [ octal digits ]	User file creation mode mask for owner, group, and others
unset [ name ]	Unset value of variable or function
wait [ pid#n ]	Wait for background process with PID number n and report termination status
ulimit [ options size ]	Set maximum limits on processes
umask [ mask ]	Without argument, print out file creation mask for permissions
wait [ pid#n ]	Wait for background process with PID number n and report termination status