Unix Basics Part-VI

Features (csh)

Each shell has its own set of features. Those of the C Shell are discussed below.

Command history

The history mechanism maintains a list of recently used command lines, called events. Provides a shorthand for reexecuting previous commands. To use history:

1. Set the history variable:

set history = 100

2. Issue the history command and view the output:

 %history
 35  12:34   sort < unsorted > sorted.list
 36  12:34   cat sorted.list
 37  12:35   ls * > unsorted
 38  12:35   cat unsorted
 39  12:35   ls
 40  13:06   history
 41  13:08   alias
 42  13:11   ls
 43  13:11   vi .cshrc

3. To save history events across all login sessions, set the savehistory variable:

set savehistory = 50

Event reexecution

Allows you to specify a shorthand for reexecuting a previous event. Works with the history list.

!! - repeats last command

!number - repeats numbered command from history list

!string - repeats last command starting with string

Modifying previous events

Allows you to correct typos in previous command, or modify it to create a new command.

^old^new - changes the string "old" to the string "new" in the last command issued

!number:s/old/new/ - changes numbered command from history list; substitutes the string "old" with the string "new". Note that there is no space between number and :

Aliases

The alias command allows you to define new commands. Useful for creating shorthands for longer commands. The syntax is.

alias entered_command executed_command

Some examples:

alias m more

alias rm "rm -i"

alias h "history -r | more"

alias www "cd /home/deepak/asic-world"

To view all current aliases:

alias

To remove a previously defined alias:

unalias alias_name

Filename Generation

When you give the shell abbreviated filenames which contain special characters (metacharacters), it can generate filenames which match existing files. Some examples appear below:

Command	Description
ls *.txt	list files with .txt suffix
ls [abc]*	list files with names that start with a, b or c
lpr prog?.c	print files named prog?.c where ? is any character
cd ~jsmith	change to user jsmith's home directory

You can "turn off" filename generation by setting the noglob variable. This will permit special characters to be interpreted literally. For example:

set noglob

Filename Completion

The shell will complete a filename for you if you type in only enough characters to make the name unique. This feature is a great time saver and helps to prevent typos when trying to type long filenames.

To use filename completion, you need to set filec, either on the command line or in one of your initialization files.

set filec

Then, when specifying a filename, type the part which is unique and hit the escape key (C shell) or tab key (TC Shell). For example, if you had a directory with a long name, such as "Introduction.UNIX.Filesystems", you could cd to that directory by using the cd command with only a portion of the file's name, provided that the portion you specify is unique (no other files have similar names)

cd Intro<ESC>

Note: typing a portion of a filename and then hitting CTRL-D instead of ESCape or TAB will display a list of the filenames which match.

Variables (csh)

Each shell has its own set of variables and rules for using variables. Those associated with the C Shell are discussed below.

The shell has variables which are predefined as well as variables which you define (user defined).
Shell variables control many aspects of how your shell environment behaves. Modifying these variables (and creating new ones) allows you to customize your shell environment.
Shell variables are used extensively when creating shell scripts (covered later).

Variables can be

local - current shell only
global - current shell and child processes/shells
string - treated as character
numeric - treated as numbers
arrays - contain more than one value

Commands used to declare and manipulate shell variables:

Command	Description
set	assigns non-numeric string variables locally
unset	removes a previously "set" variable
set	shows all "set" variables
setenv	assigns non-numeric string variables globally
unsetenv	removes a previously setenv variable
setenv	shows all setenv variables
@	assigns numeric variables locally
echo $variable	displays value of variable

Examples

Command	Description
set name=Deepak	Sets variable name to value of Deepak
unset name	Unsets the variable name
set path=($path . ~/bin)	Adds to current setting of string array variable path
echo $HOME	Displays value of variable HOME
set colors=(red green blue)	Assigns three values to the string array variable colors
@ count = 1	Sets numeric variable count to one
@ count = ($count + 1)	Adds one to numeric variable count
set counts = (1 22 4 9)	Assigns 4 values to numeric array variable counts
@ counts[2] = 5	Assigns values to second element of numeric array variable counts
@ echo $counts[3]	Displays value of third element of numeric array variable counts

Predefined Shell Variables.

A number of shell variables are predefined by the shell itself or inherited from the system environment. Some of the more common ones are described below.

Variable	Description
$	Contains the process id of the current shell
argv	Contains the command line arguments for an invoked command. argv is an array, with $argv[0] set to the name of the invoked command, $argv[1] set to the first argument, $argv[2] set to the second argument...and so on. $argv[*] can be used to specify all arguments. You may also use the shorthand $n where n is the number of the argument.
#argv	Set to the actual number of arguments in argv, excluding argv[0]
cdpath	Expands the search path for the cd command. By default, the cd command issued with a simple filename will search only the working directory. Use cdpath to increase the number of directories searched.
CWD or cwd	Holds that name of the working/current directory.
echo	Causes the shell to echo the command before executing it. Use set/unset to turn this on/off.
filec	Enables file completion. Use set/unset to turn this on/off.
history	Controls the size of the history list. 100 is recommended as safe size. If the number is too large, the shell may run out of memory.
HOME or home	The pathname of your home directory
ignoreeof	Prevents exiting the shell by typing CTRL-D, and thus, prevents accidentally logging off. Use set/unset to turn this on/off.
noclobber	Prevents you from accidentally overwriting a file when you redirect output. Use set/unset to turn this on/off.
noglob	Prevents the shell from generating/expanding ambiguous filenames. Use set/unset to turn this on/off.
notify	Tells the shell to notify you immediately when a background job completes. Ordinarily, notification will wait until the next shell prompt to prevent interruption of work. Use set/unset to turn this on/off.
PATH or path	Specifies the path that the shell searches when asked to execute a command. If an executable is not found in the path, you must specify its full pathname.
prompt	Allows you to customize the shell prompt. By default, the C shell prompt is simply a percent sign (%). For example, to display the machine name you are logged into as part of your prompt set prompt = "`hostname -s`% "
savehist	Specifies how many the number of command events to save as history after you logout. These events are saved in a file called .history in your home directory. The shell uses these as your initial history after you login again.
shell	Contains the pathname of the shell
status	Contains the exit status of the last executed command
USER or user	Contains you login userid
verbose	Causes the shell to display each command after a history substitution. Use set/unset to turn this on/off.

Initialization Files

System-wide shell initialization files are common on UNIX systems. These files can be "sourced" automatically by the shell and are typically setup by the System Administrator for the local environment.

Some examples of system-wide initialization files might be:

/etc/environment

/etc/profile

/etc/cshrc

/etc/login

Every shell provides a means for the user to customize certain aspects of the shell's behavior. These customizations usually permit you to augment and/or override the system-wide defaults. User customizations are specified in initialization files located in the top level of your home directory. Naming: Depending upon the shell, you must name your initialization file(s) accordingly.

Executed during interactive login

.login	csh, tcsh
.profile	sh, ksh, bash
.bash_profile	bash (alternative 1)
.bash_login	bash (alternative 2)

Executed for every new shell

.cshrc	csh, tcsh
.tcshrc	tcsh
.kshrc	ksh
.bashrc	bash

The C shell uses two files to set user preferences:

* .cshrc

runs at each login before the .login file
runs whenever another shell is created
runs when a new window is opened
runs when many utilities are invoked
typically sets alias information, path variable, filec, prompt, etc.

* .login

runs at invocation of login shell, after the .cshrc file
typically sets terminal characteristics and one time shell options and environment variables

After changing your .login or .cshrc files, you must "source" them for the changes to take effect:

source .login

source .cshrc

The system administrator may/may not provide you with default .cshrc and .login files when you first obtain your userid. If they are provided, be careful about modifying them - especially removing specifications which are required for your local system.

Example .cshrc and .login files are provided below:

* .cshrc file

* .login file

Logout Files

You are able to specify commands which the shell will execute upon logout. These commands are kept in a file located in the top level of your home directory.

.logout - csh, tcsh

.bash_logout - bash

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