UNIX Shell

Services for UNIX Version 1.0 includes a Korn shell. The shell is a command language interpreter that acts as the interface to the UNIX operating system. The shell interprets commands, calls the appropriate program, and returns standard output. Many shells also provide a high-level programming language that can be used to achieve complex tasks by combining basic utilities and functions provided by the operating system.

Korn shell, developed by David Korn at AT&T, combines many of the desirable features of the C and Bourne shells. Bourne shell, developed at AT&T by Steven Bourne, was the first UNIX shell. Bourne shell provides a powerful programming language. C shell, another UNIX shell, provides a number of features not available with the Bourne shell, such as command aliases, a command history mechanism, and job control of command processing.

Table 11.7 Shell Feature Summary

Other shells are available for the UNIX operating system. Bash (Bourne Again shell) is an extension of Bourne shell that incorporates features of both the Korn and C shells and is generally used with Linux. Tcsh is an extended version of C shell that includes command completion, a command-line editor, and enhanced history manipulation.

Using the Korn Shell

The implementation of the Korn shell included with Services for UNIX differs from the standard UNIX Korn shell in the following ways:

• Semicolons used instead of colons to separate entries in the PATH variable.
• Current directory in PATH is referred to as ;; or ;.; instead of period (.).
• Startup file is called profile.ksh instead of .profile.
• Startup file for system-wide environment variables is called /etc/profile.ksh instead of /etc/profile.
• History file, which stores the command history of a user, is called sh_histo file instead of sh_history.
• Partial job control enables running of jobs in the background using the ampersand on the command line.

If your system administrator sets up the Korn shell as your default shell in Telnet Server, it is the shell you log into when accessing a Services for UNIX server via Telnet. If you want to use Korn shell without logging into it, you can access it using the sh command (ksh in the standard UNIX Korn shell).

Environment Variables

A variable consists of a name and its assigned value. You can define variables and use them in shell scripts. Other variables, called shell variables, are set by the shell. A variable name can contain letters, numbers (but not as the first character), and the underscore. The equal sign with no spaces on either side is used to assign a value to the variable. Once a variable is defined, you must use the export command to make the value of the variable available to other processes.

The Korn shell runs the profile.ksh file when you login. The profile.ksh file is used to set user-specific environment variables and terminal modes. (The system administrator can also use /etc/profile.ksh to set variables system-wide for all user accounts on the system.) Some of the variables used in .profile include PATH, HOME, VISUAL, EDITOR, SHELL, HISTSIZE, HISTFILE, PS1, PS2, CDPATH.

Table 11.8 lists many of the environment variables used by the Services for UNIX Korn shell. For a complete list of the shell variables supported by the Services for UNIX Korn shell, consult the Services for UNIX online help for sh.

Table 11.8 Korn Shell Environment Variables

Metacharacters

Korn shell recognizes a special meaning for certain characters. When a regular expression contains a metacharacter, the Korn shell interprets the character as shown in Table 11.9.

Table 11.9 Korn Shell Metacharacters

Shell Commands

When you enter a command at the shell prompt, the shell evaluates the command, makes substitutions for variables and aliases, and then runs the command.

The basic structure of a command:

command-name argument1 argument2 >file-name

Commands can take options, which modify the action of a command. For example, ls lists the contents of a directory, but does not include the hidden (.) files. Use ls -a to also see the hidden files.

The shell processes the command after you press ENTER. Commands can also be separated by semicolons and entered on a single line; the commands on the line are not processed, however, until after you press ENTER.

When the shell runs a command, it starts a process. Each process has a process ID (PID), which is used to access the process. Processes can be run in the foreground or the background and can also be suspended or cancelled. Parent processes forked child processes, which are assigned their own PIDs.

A command receives standard input from the terminal and sends standard output and standard error to the terminal.

It is possible to redirect the standard input from the terminal to a file:

command-name < file-name

You can also redirect the standard output from the terminal to a file:

command-name > file-name

You can append it to an existing file:

command-name >> file-name

In addition, you can redirect the standard error to a file:

command-name 1>file-name1 2>file-name2

The standard output is sent to file-name1 and the standard error is sent to file-name2.

Pipes can be used to connect the standard output of one command to the standard input of another command:

command-name | command-name >file-name

The Services for UNIX Korn shell is a programmable shell that supports the following structured commands. For a complete list of supported shell commands, see the Services for UNIX online help for sh.

Table 11.10 Shell Programming Services for UNIX Korn Shell

The Services for UNIX Korn shell has built-in commands. Built-in commands are run by the shell's own process. The built-in commands available with the Services for UNIX Korn shell are listed below. For details about each command, consult Services for UNIX online help.

Table 11.11 Services for UNIX Korn Shell Built-In Commands

Command Aliases

You can assign an alias, which is a name, usually easy to remember, that the shell translates to another name or string, for a command, including command-line options. The shell substitutes the command and options for the alias you enter. Creating an alias at the command line makes the alias available in the current shell environment. To make the alias part of the work environment, add a line to the shell start-up file (.kshrc) that defines the alias and exports it:

alias newname=´command -option´; export newname

The command alias -x exports the alias to the child process only.

To remove an alias, use unalias followed by the alias name:

unalias newname

The Services for UNIX Korn shell provides a set of predefined aliases. For more information, see the Services for UNIX online help for alias.

Command History

The Services for UNIX Korn shell features a history file, which contains a list of a defined number of executed commands. These commands can be accessed for editing and persist in the file between login sessions.

You can set the number of commands saved in the history file using the HISTSIZE variable:

HISTSIZE=number; export HISTSIZE

If you do not define this variable, UNIX saves a system-defined number of commands.

You can define the name and location of the history file using the HISTFILE variable:

HISTFILE=file-name; export HISTFILE

If you do not define this variable, your history file is named .sh_histo and stored in your home directory.

Command Line Editing

You can edit the commands in the history file, using built-in Korn shell editors such as vi or emacs, or the built-in fc command, or the complete vi editor. You can use this feature to correct mistakes or to reuse work you have completed.

To define vi as your default editor:

set -o vi

– Or –

VISUAL=/sfu/shell/vi; export VISUAL

The built-in editor provided with the Korn shell provide a subset of the full functionality available with the UNIX vi editor. You can access the vi editor to edit a command by entering the command, pressing ENTER, and then typing vi. This will allow you to edit a multiline command.

Arithmetic Evaluation

The Services for UNIX Korn shell has a built-in arithmetic expression feature. It supports logical and arithmetic operators. The syntax for arithmetic operators is $((<arithmetic expression>)) or $(<arithmetic expression>). The Korn shell replaces the arithmetic expression with its value, beginning with the innermost nested expression. Table 11.12 lists the operators.

Table 11.12 Arithmetic and Logical Operators

Shell Scripts

A shell script is a file containing a series of commands that together perform a function. You can access a Korn shell script from the command line if you are running the Korn shell and have permission to execute the script by typing the file name. You can also run the shell script if Korn shell is not running by entering the following command:

sh file-name

Windows NT does not support execution of a script invoked from the command line only by file name; under UNIX, scripts may be executed in this manner if the path and file name of the shell are specified on the first line of the script, like the following:

#!/bin/sh

Each file or file name extension must be associated with a program. In particular, .sh or .ksh can be associated with Korn shell.

Job Control

You can use job control to run a command in the foreground or the background or temporarily suspend it. In addition, you can see a list of the commands currently running.

When you enter a command, if it is not a built-in command, the shell forks a new process in which to run the command. The kernel schedules the process and gives it a process ID (PID). The shell keeps track of the process and gives it a job number.

Some processes are run in the foreground: they might be interactive or take only a very short time to run. Other processes are better run in the background, especially commands that take a long time to run, such as a large sort. You can move a process to the foreground or the background and get a list of the current jobs. You can also temporarily suspend a process or terminate it.

Table 11.13 lists the job control commands supported by Services for UNIX.

Table 11.13 Job Control Commands

UNIX Utilities

The following UNIX utilities are available as part of Services for UNIX. For more information on these commands, see Services for UNIX Help.

Table 11.14 UNIX Utilities

Using vi

The vi editor is an interactive text editor for creating and editing ASCII files. The vi editor requires you to enter a command to perform an action, such as entering text, deleting text, or moving the cursor. You can be in one of two modes when using vi: command mode or input mode. In command mode, you can enter commands to perform such actions as deleting text or moving the cursor in the file. In input mode, you can enter and change text. You enter input mode by entering a specific vi command. You leave input mode by pressing ESC. This section provides some basic information to get you started using vi. Once you understand the basic mechanics of using vi, you can explore its functionality. (The mechanics are simple; the details can seem obscure at first.) For further details on the complete functionality of vi, consult any of the available print or online sources. In addition, consult the Services for UNIX online help for vi.

To edit a file using vi, at the system prompt type:

vi file-name

and press ENTER.

If the file already exists, it appears on the screen. If the file does not exist, vi creates it.

Note

You can take advantage of a file recovery feature that is provided with vi. If the system saves a copy of the last saved version of your file in a buffer, you can access that copy of the file, by typing vi -r file-name and pressing ENTER.

What you see on the screen is the text of the file (if it exists), a blinking cursor in the left-hand corner of the screen, a column of tildes along the left margin of the file representing blank lines (if there are any in view), and the name of the file in the last line of the screen. (The bottom of the screen is also used to display messages, to show commands you enter that begin with /, ?, !, and :, and the indication of input mode if showmode option is set.)

To begin entering text, press i (to insert text). You can then begin typing. The text you enter appears, beginning at the position of the cursor. When you are done entering text, press ESC.

To save the file and exit vi, type:

:wq

and press ENTER.

Use the colon to escape to the shell so that you can enter a command at the bottom of the screen. Press w to write the file to disk. Press q to quit the vi editor.

Table 11.15 Starting and Quitting vi

After you have created a file using vi, you can move throughout the file. As the size of the size of the file increases, the ability to move at will to any place in the file becomes increasingly useful using the following commands in command mode.

Table 11.16 Moving the Cursor in Command Mode

Many ways are provided for inserting and changing text that allow for detailed control.

Table 11.17 Input Mode

Table 11.18 Changing Text

Table 11.9 shows the possible ways to delete text in vi.

Table 11.19 Deleting Text

You can yank and put — that is, copy and paste — text within a file and between files. The yank commands copies specified text and places it in a buffer. The put commands copy the text from the buffer to a specified place in the file. Named buffers and numbered buffers are available but are beyond the scope of this discussion.

Table 11.20 Yank and Put Commands

You can search for a character string within the file. Remember that the search tools are case-sensitive. If the pattern is not found, vi displays a message at the bottom of the screen telling you that it is unable to find the pattern.

Table 11.21 Search Commands

Global pattern substitution, a powerful tool, is available from the command line.

The command takes the form:

:s/string/replacement/g

and press ENTER. In this command, string represents any regular expression that you want to search for, replacement represents the text that will replace string, and g specifies global replacement of all occurrences of string. If the trailing g is omitted, only the first occurrence of the string in each line is replaced. If you want to be prompted to confirm each substitution, type a c after the g in the command, as follows:

:s/string/replacement/gc

Here are a few of the many other tools available in vi.

Table 11.22 Other Useful Commands

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