Illustrated UNIX System V/BSD: Module 71 ksh/csh/sh

Vi Editing Mode

To edit commands saved in your history file simply press the Escape key. You are now in vi command mode. If you want to make a correction to the previous command simply press k. Now move around on the line using vi commands. Make changes to the line then press Return. You don't have to press Esc before pressing Return.

For further information about editing the history file using vi commands refer to Module 50.

The vi editor is described in Module 151.

INTERNAL COMMANDS

There are two types of UNIX commands: internal and external commands. Internal commands are part of the ksh. They are executed within the shell itself. No new process has to be spawned, no code has to be read from disk and loaded into memory, and no PATH searching is performed. Therefore, internal commands are executed much faster than external commands.

Except for the following two internal commands, all internal commands have been placed in a module of their own name or in the Programming Constructs module.

The dot command. The dot command executes a shell script in the current environment. The shell script is read just as if you had typed all the lines at your terminal, without a new child process being started. Usually, a shell script is executed in a subshell (a child process is started) and changes to the environment do not affect the current shell. The dot command circumvents this restriction. Therefore, any environmental changes made in the shell script executed by the dot command will be reflected in the current shell environment.

The colon command. The colon command is the same as the true command. It does nothing. It always returns a 0 exit status. It is often used in while loops or if statements to fill a required command position. For example,

                       while :
                       do
                       echo "Stuck in an infinite loop."
                       done

is just one of many ways to use the colon command.


C Shell
The C shell uses the source command to perform the dot (.) command function used by the ksh.

COMMAND EXECUTION

The procedures that are followed by the shell to execute a command are described in Module 23.

DIAGNOSTICS AND BUGS

Users often have the problem of the shell appearing to be hung or not responding. There are several situations to be aware of and solutions to each.

The first situation is the Ctrl-S and Ctrl-Q dilemma. If you type input on the keyboard and there is no response, first press Ctrl-Q to resume output to your terminal. If this does not work, the system may be extremely slow, in which case you must be patient.

Another possibility is the shell or last program you executed is hung. Try pressing Delete two or three times. If you are in applications, you might try pressing Esc or Ctrl-D depending on the application's required interrupt or attention key.

RELATED COMMANDS

Refer to the following modules.


Module	Module Name	Module	Module Name

4	alias	104	popd
9	bg	106	print
14	cd	107	printenv
23	Commands	108	Programming Constructs
38	echo	110	pushd
41	env	111	pwd
42	eval	112	read
44	exec	113	Regular Expressions
45	exit	118	set
47	export	119	setenv
49	false	120	shift
50	fc	121	sleep
51	fg	125	stop
54	Filenames	129	suspend
59	getopts	136	time
64	history	137	times
67	I/O and Redirection	141	trap
68	jobs	142	true
70	kill	145	typeset
73	let	146	ulimit
74	line	147	umask
92	newgrp	152	wait
94	nice	156	whence
96	nohup	157	whereis
99	pack	158	which

RELATED FILES

The following list describes the files the ksh command uses.

standard input	Reads from the terminal keyboard or the output of a pipe.
standard output	Writes to the terminal screen or the input of a pipe.
standard error	Writes errors to the terminal screen or the input of a pipe.
/etc/profile	Executes the commands stored in this file during login to perform common system login procedures.
/etc/alias	Executes the commands stored in this file during login to set up common aliases.
$HOME/.profile	Executes the commands stored in this file during login to set up user-defined environment.
$ENV	Executed each time a new ksh is spawned.
/tmp/ksh* /tmp/sh*	Used for the history file, some implementations use sh* instead of ksh*.
/dev/null	The system trash can, used to dump unwanted output.
/etc/passwd	Used to find home directories for ~name substitutions.

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