8    Shell Features

This chapter functions as a reference source for C shell and Bourne, Korn, or POSIX shell features. Unlike other chapters of this guide that present conceptual or tutorial information, or both, the purpose of this chapter is to provide very brief reference information about each shell.

To get the most out this chapter, you already should be familiar with the introductory shell overview information in Chapter 7.

After completing this chapter, you should be able to:

• Understand the main differences between operating system shells

• Understand specific features of each operating system shell

• Understand the specifics of local login scripts for each shell

8.1    Comparison of C, Bourne, Korn, and POSIX Shell Features

Table 8-1 compares selected features of the C shell and the Bourne, Korn, and POSIX shells.

Table 8-1:  C, Bourne, Korn, and POSIX Shell Features

For detailed information on shell features, see the appropriate shell reference pages sh(1b), sh(1p), csh(1), or ksh(1).

8.2    C Shell Features

This section describes the following C shell features:

• Sample .cshrc and .login scripts

• Metacharacters

• Command history and aliases

• Built-in variables and commands

8.2.1    Sample .cshrc and .login Scripts

The .cshrc login script sets up your C shell environment by defining variables and operating parameters for the local shell process. The .login script defines variables and operating parameters that you want executed at the beginning of your session, and that you want to be valid for all shell processes during the current login session.

When you log in, the operating system executes the .cshrc file in your home directory first, and the .login file second. The .login script is executed only when you log in. However, the .cshrc file is executed each time you create a subshell.

In the following .cshrc script, shell variables, command aliases, and command history variables are set. Table 8-2 explains each part of the script.

# Set shell variables
set noclobber
set ignoreeof
set notify
 
# Set command aliases
alias h 'history \!* | more'
alias l 'ls -l'
alias c clear
 
# Set history variables
set history=40
set savehist=40
 
# Set prompt
set prompt = "\! % "

Table 8-2:  Example .cshrc Script

In the following .login script, the permissions for file creation are set, the PATH environment variable is set, and the editor and printer are specified. Table 8-3 explains each part of the script.

# Set file creation permissions
umask 027
 
# Set environment variables
set path=/usr/bin:/usr/local/bin:
set cdpath=.:..:$HOME
setenv EDITOR emacs
setenv MAILHOST boston
setenv PRINTER sales

Table 8-3:  Example .login Script

8.2.2    Metacharacters

Table 8-4 describes C shell metacharacters (characters that have special meaning to the shell). The meaning of these metacharacters are grouped by interpretation when they appear in a shell script, in a Filename specification, when used to quote other characters, in an Input/Output specification, or when used to indicate variable substitution.

Table 8-4:  C Shell Metacharacters

8.2.3    Command History

The command history buffer stores the commands you enter and lets you display them at any time. As a result, you can select a previous command, or parts of previous commands, and then reexecute them. This feature may save you time because it lets you reuse long commands instead of reentering them.

You may want to enter the following three commands in your .cshrc file:

• set history=n

  Creates a history buffer that stores the command lines you enter. The n entry specifies the number of command lines you want to store in the history buffer.

• set savehist=n

  Saves the command lines you entered during the current login session and makes them available for the next login session. The n entry specifies the number of command lines you want to store in the history buffer when you log out.

• set prompt=[\!] %

  Causes your C shell prompt to display the number of each command line.

To see the contents of the history buffer, use the history command. The displayed output will be similar to the following (your output will vary):

[18] % history
  3 set history=15
  4 pwd
  5 cd /usr/sales
  6 ls -l
  7 cp report report5
  8 mv /usr/accounts/new .
  9 cd /usr/accounts/new
  10 mkdir june
  11 cd june
  12 mv /usr/accounts/new/june .
  13 ls -l
  14 cd /usr/sales/Q1
  15 vi earnings
  16 cd /usr/chang
  17 vi status
  18 history
[19] % 

To reexecute any command in the command history buffer, use the commands listed in Table 8-5. Each command starts with an exclamation point (!), which tells the C shell that you are using commands in the history buffer.

Table 8-5:  Reexecuting History Buffer Commands

The command history buffer also lets you reuse previous command arguments as well as modify previous command lines. For information on these features, see the csh(1) reference page.

8.2.4    File Name Completion

The C shell lets you enter a portion of a file name or pathname at the shell prompt, and the shell automatically will match and complete the name. This feature saves you time when you are trying to display long, unique file names.

For example, assume that you have the file meetings_sales_status in your current directory. To display a long listing of the file, enter the following command:

% ls -l meetings[Escape]

The system displays the following on the same command line:

% ls -l meetings_sales_status

You can now execute the command by pressing Return.

For more detailed information on file name completion, see the csh(1) reference page.

8.2.5    Aliases

The command aliases feature lets you abbreviate long command lines or rename commands. You do this by creating aliases for long command lines that you frequently use.

For example, assume that you often need to move to the directory /usr/chang/reports/status. You can create an alias status, which will move you to that directory whenever you enter it on the command line.

In addition, aliases let you make up more descriptive names for commands. For example, you could define an alias named rename for the mv command.

To create aliases, use the alias command. The format of the alias command is:

alias aliasname command

The aliasname entry specifies the name you want to use. The command entry specifies either the original command or a series of commands. If the command has more than one part (has spaces), enclose the whole expression in single quotes (' ').

For example, to create the alias status that moves you to the directory /usr/chang/reports/status, enter the following command:

% alias status 'cd /usr/chang/reports/status'

The usual way to define aliases is to make them a permanent part of your environment by including them in your .cshrc file. As a result, you can use the aliases whenever you log in or start a new shell. See Section 8.2.1 for an example.

To display all alias definitions, enter the following command:

% alias

To display the definition of a particular alias, enter the following command:

% alias aliasname

The aliasname entry specifies the particular alias for which you are requesting a definition.

To remove an alias for the current login session, use the unalias command. The general format of the unalias command is the following:

unalias aliasname

The aliasname entry specifies the alias you want to remove.

To remove an alias for the current and all future login sessions, do the following:

1. Enter the following command:

   
   % unalias aliasname
   

   The aliasname entry specifies the alias you want to remove.

2. Edit the .cshrc file and remove the alias definition. Then, save the file.

3. Enter the following command to reexecute the .cshrc file:

   % source .cshrc
   

For complete information on using aliases with the C shell, see the csh(1) reference page.

8.2.6    Built-In Variables

The C shell provides variables that can be assigned values. These variables can be very useful for storing values that can be used later in commands. In addition, you can affect directly shell behavior by setting those variables to which the shell itself refers.

Table 8-6 describes selected C shell built-in variables that are of the most interest to general users. For a complete list of C shell built-in variables, see the csh(1) reference page.

Table 8-6:  C Shell Built-In Variables

8.2.7    Built-In Commands

Table 8-7 describes selected C shell commands that are of the most interest to general users. For a complete list of C shell built-in commands, or for more information on the commands listed here, see the csh(1) reference page.

Table 8-7:  Built-In C Shell Commands

8.3    Bourne Shell Features

This section describes the following Bourne shell features:

• A sample .profile login script

• Metacharacters

• Built-in variables

• Built-in commands

8.3.1    Sample .profile Login Script

If your login shell is the Bourne shell, the operating system executes the .profile login script to set up your environment.

The .profile login script variables that are exported are passed to any subshells and subprocesses that are created. Variables that are not exported are used only by the login shell.

In the following .profile login script, shell variables are set and exported, a trap is set for the logout script, and the system is instructed to display information. Table 8-8 explains each part of the script.

# Set PATH
PATH=/usr/bin:/usr/local/bin:
# Export global variables
export PATH
# Set shell variables
PS1='$LOGNAME $ '
CDPATH=.:..:$HOME
# Set up for logout script
trap "echo logout; $HOME/.logout" 0
# Display status information
date
echo "Currently logged in users:" ; users

Table 8-8:  Example Bourne Shell .profile Script

8.3.2    Metacharacters

Table 8-9 describes Bourne shell metacharacters (characters that have special meaning to the shell). The meaning of these meta characters are grouped by interpretation when they appear in a shell script, in a Filename specification, when used to quote other characters, in an Input/Output specification, or when used to indicate variable substitution.

Table 8-9:  Bourne Shell Metacharacters

8.3.3    Built-In Variables

The Bourne shell provides variables that can be assigned values. The shell sets some of these variables, and you can set or reset all of them.

Table 8-10 describes selected Bourne shell built-in variables that are of most interest to general users. For complete information on all Bourne shell built-in variables, see the sh(1b) reference page.

Table 8-10:  Bourne Shell Built-In Variables

8.3.4    Built-In Commands

Table 8-11 describes selected Bourne shell commands that are of the most interest to general users. For a complete list of Bourne shell built-in commands, see the sh(1b) reference page.

Table 8-11:  Bourne Shell Built-In Commands

8.4    Korn or POSIX Shell Features

The POSIX shell is another designator for the Korn shell that signifies compliance with the IEEE POSIX.2 standard. This section describes the following Korn or POSIX shell features:

• Sample .profile and .kshrc login scripts

• Metacharacters

• Command history

• Editing command lines

• File name completion

• Aliases

• Built-in variables and commands

8.4.1    Sample .profile and .kshrc Login Scripts

If your login shell is the Korn or POSIX shell, the operating system processes the .profile login script in your home directory. The .profile login script defines environment variables. These variables are used by your login shell as well as any subshells and subprocess that are created. The .profile login script is executed only when you log in.

The .kshrc login script sets up your Korn or POSIX shell environment by defining variables and operating parameters for the local shell process. It is executed each time you create a subshell.

Note

Before creating a .kshrc file in your home directory, make sure that the ENV=$HOME/.kshrc environment variable is set and exported in your .profile. Once this is done, the .kshrc login script will execute each time you log in and each time you create a subshell.

In the following .profile login script, global environment variables are set and exported, and shell variables are set. Table 8-12 explains each part of the script.

# Set environment variables
PATH=/usr/bin:/usr/local/bin:
ENV=$HOME/.kshrc
EDITOR=vi
FCEDIT=vi
PS1="'hostname' [!] $ "
 
# Export global variables
export PATH ENV EDITOR FCEDIT PS1
 
# Set mail variables
MAIL=/usr/spool/mail/$LOGNAME
MAILCHECK=300

Table 8-12:  Example Korn or POSIX Shell .profile Script

In the following .kshrc login script, shell variables, command aliases, and command history variables are set, as well as the permissions for file creation. Table 8-13 explains each part of the script.

# Set shell variables
set -o monitor
set -o trackall
 
# Set command aliases
alias rm='rm -i '
alias rename='mv '
alias l 'ls -l'
alias c clear
 
# Set history variables
HISTSIZE=40
 
# Set file creation permissions
umask 027

Table 8-13:  Example .kshrc Script

8.4.2    Metacharacters

Table 8-14 describes Korn or POSIX shell metacharacters (characters that have special meaning to the shell). The meaning of these meta characters are grouped by interpretation when they appear in a shell script, in a Filename specification, when used to quote other characters, in an Input/Output specification, or when used to indicate variable substitution.

Table 8-14:  Korn or POSIX Shell Metacharacters

8.4.3    Command History

The command history buffer stores the commands you enter and lets you display them at any time. As a result, you can select a previous command, or parts of previous commands, and then reexecute them. This feature may save you time because it lets you reuse long commands instead of reentering them.

To see the contents of the history buffer, use the history command. The displayed output will be similar to Example 8-1 (your output will vary).

Example 8-1:  Sample ksh history Output

[18] $ history
  3 ls -l
  4 pwd
  5 cd /usr/sales
  6 ls -l
  7 cp report report5
  8 mv /usr/accounts/new .
  9 cd /usr/accounts/new
  10 mkdir june
  11 cd june
  12 mv /usr/accounts/new/june .
  13 ls -l
  14 cd /usr/sales/Q1
  15 vi earnings
  16 cd /usr/chang
  17 vi status
[19] $ 

To reexecute any command in the command history buffer, use the commands listed in Table 8-15. Each command starts with the letter r.

Table 8-15:  Reexecuting History Buffer Commands

For more information on reexecuting history buffer commands, see the ksh(1) reference page.

If you want to increase or decrease the number of commands stored in your history buffer, set the HISTSIZE variable in your .profile file. This variable has the following format:

HISTSIZE= n

The n entry specifies the number of command lines you want to store in the history buffer.

For example, to store 15 commands in the history buffer, use the following command:

HISTSIZE=15

The Korn or POSIX shell also lets you edit current command lines as well as reuse those already entered in the command history buffer. To use this feature, you must know how to use a text editor such as vi or emacs. For information on these features, see the following section.

8.4.4    Command Line Editing Using the fc Command

The Korn or POSIX shell lets you list or edit or both the command lines in your command history buffer. As a result, you may modify any element of a previous command line and then reexecute the command line.

The command line editing functions for the Korn or POSIX shell are extensive. This section covers only the most basic functions. For more detailed information, see the ksh(1) or sh(1p) reference pages.

To display the command history buffer or to edit its contents, or both, use the built-in command fc (fix command). The fc command has the following two formats:

fc [ -e editor ] [ -nlr ] [ first ] [ last ]

This command format lets you display and edit any number of command lines in your buffer.

• The -e editor entry specifies the editor (usually vi or emacs) you want to use in editing the command line. If you do not specify -e, the fc command displays the lines, but does not let you edit them.

• The -n flag specifies that you want to list the command lines in the buffer without numbers. The -l flag specifies that you want to list the command lines in the buffer with numbers. If you do not specify a line number or a range of line numbers, the last 16 lines you entered will be listed.

• The -r flag specifies that you want to list the command in the buffer in reverse order.

• The first and last entries specify a range of command lines in the buffer. You may specify them either with numbers or with strings.

If you want to specify a default editor for the -e flag, define the FCEDIT variable in your .profile script. For example, if you want to make emacs your default editor, enter the following variable definition:

FCEDIT=emacs

fc -e - [old=new] [string]

This command lets you immediately replace an old string with a new string within any previous command line.

• The -e - entry specifies that you want to make a replacement.

• The old=new entry specifies that you want to replace the old string with the new string.

• The string entry specifies that the Korn or POSIX shell should make the edit to the most recent command line in the buffer containing the string.

The following section contains some examples of fc use.

The Korn or POSIX shell also lets you edit individual command lines at the shell prompt by using a command set similar to the vi or the emacs editors. For more information on this feature, see the ksh(1) or sh(1p) reference pages.

8.4.4.1    Examples of Command Line Editing

To display command lines 15 to 18, enter the following command:

$ fc -l 15 18
15 ls -la
16 pwd
17 cd /u/ben/reports
18 more sales
$

You also may list the same command lines by specifying command strings instead of line numbers, as in the following example:

$ fc -l ls more
15 ls -la
16 pwd
17 cd /u/ben/reports
18 more sales
$

To display and edit command lines 15 to 18 with the vi editor, enter the following command:

$ fc -e vi 15 18
ls -la
pwd
cd /u/ben/reports
more sales
~
~
~
~

After making your edits, write and exit the file with the :wq! command. The command lines in the file are then reexecuted.

Assume that you have just entered the echo hello command, and now want to replace hello with goodbye. To do the replacement and reexecute the command line, enter the following command:

$ echo hello
hello
$ fc -e - hello=goodbye echo
echo goodbye
goodbye

For more detailed information on the fc command and command line editing, see the ksh(1) reference page.

8.4.5    File Name Completion

The Korn or POSIX shell lets you enter a portion of a file name or pathname at the shell prompt and the shell automatically will match and complete the name. If there is more than one file name or pathname that matches the criterion, the shell will provide you with a list of possible matches.

To activate the file name completion mechanism, define the EDITOR variable in your .profile file. For example, if you want to use the vi editor, enter the following variable definition in your .profile file:

EDITOR=vi

To demonstrate how file name completion works, assume that your editor is vi and that you have the salesreport1, salesreport2, and salesreport3 files in your current directory. To display a long listing and to activate file name completion, enter the following command:

$ ls -l salesreport [Escape] =
1) salesreportfeb
2) salesreportjan
3) salesreportmar
$ ls -l salesreport

The system redisplays your command, and the cursor is now at the end of salesreport. If you want to choose salesreportjan, type a (the vi append command) followed by jan, then press Return. The listing for salesreportjan will be displayed.

For more detailed information on file name completion, see the ksh(1) and sh(1p) reference page.

8.4.6    Aliases

The command aliases feature lets you abbreviate command lines or rename commands. You do this by creating aliases for command lines that you frequently use.

For example, assume that you often need to move to the directory /usr/chang/reports/status. You can create an alias status, which will move you to that directory whenever you enter it on the command line.

In addition, aliases let you make up more descriptive names for commands. For example, you could define an alias named rename for the mv command.

To create aliases, use the alias command. The general format of the alias command is the following:

alias aliasname=command

The aliasname entry specifies the name you want to use. The command entry specifies either the original command or a series of commands. If the command has more than one part (has spaces), enclose the whole expression in single quotes.

For example, to create the alias status that moves you to the directory /usr/chang/reports/status, enter the following command:

alias status='cd /usr/chang/reports/status'

The usual way to define aliases is to place them in your .kshrc file so that you can use them whenever you log in or start a new shell. See Section 8.4.1 for an example.

To display all alias definitions, enter the following command:

$ alias

To display the definition of a particular alias, enter the following command:

$ alias aliasname

The aliasname entry specifies the particular alias for which you are requesting a definition.

The Korn or POSIX shell lets you export the aliases you create. Aliases that are exported are passed to any subshells that are created so that when you execute a shell procedure or new shell, the alias remains defined. (Aliases that are not exported are used only by the login shell.)

To export an alias, use the following form of the alias command:

alias -x aliasname=command

The -x flag specifies that you want to export the alias. The aliasname entry specifies the name you want to use. The command entry specifies either the original command or a series of commands. If the command has more than one part, enclose the whole expression in single quotes.

For example, to export an alias definition for the rm command, enter the following:

alias -x rm='rm -i '

You can enter the preceding command in one of two ways:

• Edit the .kshrc or .profile file if you want an alias exported whenever you log in

• Export an alias on the command line if you want the alias exported only for the current login session

To remove an alias for the current login session, use the unalias command. The general format of the unalias command is the following:

unalias aliasname

The aliasname entry specifies the alias you want to remove.

To remove an alias for the current and all future login sessions, do the following:

1. Enter the following command:

   
   $ unalias aliasname
   

   The aliasname entry specifies the alias you want to remove.

2. Edit the .kshrc file (or the file on your system that contains alias definitions) and remove the alias definition. Then, save the file.

3. Enter the following command to reexecute the .kshrc file:

   $ . ./.kshrc
   

The Korn or POSIX shell provides additional aliasing features. For complete information on using aliases with the Korn or POSIX shell, see the ksh(1) or sh(1p) reference pages.

8.4.7    Built-In Variables

The Korn and POSIX shells provide variables that can be assigned values. The shell sets some of these variables, and you can set or reset all of them.

Table 8-16 describes selected Korn or POSIX shell built-in variables that are of the most interest to general users. For complete information on all Korn or POSIX shell built-in variables, see the ksh(1) or sh(1p) reference pages.

Table 8-16:  Built-In Korn or POSIX Shell Variables

8.4.8    Built-In Commands

Table 8-17 describes selected Korn or POSIX shell commands that are of the most interest to general users. For a complete list of shell built-in commands, or for more information on the commands listed, see the ksh(1) or sh(1p) reference pages. Most of these commands also have a reference page that you can access as described in Section 1.6.1.

Table 8-17:  Korn or POSIX Shell Built-In Commands