The C Shell tutorialUniversity of Hawaii at Manoa
College of Engineering
What is a shell?A shell is a program which provides a user interface.
With a shell, users can type in commands and run programs on a Unix system.
Basically, the main function a shell performs is to read in from the terminal
what one types, run the commands, and show the output of the commands.
What's so good about C Shell?The C shell was written by Bill Joy at the
University of California at Berkeley. His main intent for writing the C shell
was to create a shell with C language-like syntax.
What can one do with C Shell?The main use of the C shell is as an
interactive shell, but one can write programs using the C shell. These programs
are called shell scripts.
Features of C ShellSome of the features of the C shell are listed here:
Features of the shell environmentThe C shell provides programming
features listed below:
The C
shell has three separate files which are used for customizing its environment.
These three files are .cshrc, .login, and .logout. Because
these files begin with a period (.) they do not usually appear when one types
the ls command. In order to see all files beginning with periods, the
-a option is used with the ls command.
The .cshrc file contains commands, variable definitions and aliases
used any time the C shell is run. When one logs in, the C shell starts by
reading the .cshrc file, and sets up any variables and aliases.
The C shell reads the .login file after it has read the .cshrc
file. This file is read once only for login shells. This file should be used to
set up terminal settings, for example, backspace, suspend, and interrupt
characters.
The .logout file contains commands that are run when the user logs out
of the system.
Sample .cshrc file #!/bin/csh
# Sample .cshrc file
setenv EXINIT 'set smd sw=4 wm=2'
set history=50
set savehist=50
set ignoreeof noclobber
if ($?prompt) then
set prompt='[\!]% '
alias f finger -R
alias lo logout
endif
Sample .login file #!/bin/csh
# Sample .login file
stty erase ^H intr ^C susp ^Z
echo "Welcome to Wiliki\!"
frm -s n
Sample .logout file #!/bin/csh
# Sample .logout file
echo -n "Logged out of Wiliki "
date
Special characters in C ShellSome characters are special to the shell,
and in order to enter them, one has to precede it with a backslash (\). Some are
listed here with their meaning to the shell.
- !
- History substitution.
- < >
- Output redirection.
- |
- Pipes.
- *
- Matches any string of zero or more characters.
- ?
- Matches any single character.
- [ ]
- Matches any set of characters contained in brackets.
- { }
- Matches any comma-separated list of words.
- ;
- Used to separate commands.
- &
- Also used to separate commands, but puts them in the background.
- \
- Quote the following character.
- $
- Obtains the value of the variable.
- '
- Take text enclosed within quotes literally.
- `
- Take text enclosed within quotes as a command, and replace with output.
- "
- Take text enclosed within quotes literally, after substituting any
variables.
Variables in C shell are defined using the
internal set command. C shell supports both regular and array variables.
Some examples are given below: set var1=a3 #sets var1's value to a3.
set var2=(a b c)
# sets the array variable var2 to a b, and c.
Using variablesVariables can be used in C shell by typing a dollar sign
($) before the variable name. If the variable is an array, the subscript can be
specified using brackets, and the number of elements can be obtained using the
form $#var2.
The existence of variables can be checked using the form $?variable.
If the variable exists, the expression evaluates to a one (true), otherwise, it
evaluates to a zero (false). Simple integer calculations can be performed by C
shell, using C language-type operators. To assign a calculated value, the
@ command is used as follows: @ var = $a + $x * $z
Certain variables control
the behavior of the C shell, and some of these don't require a value. (I.e., can
be set simply by using set command by itself without any value.) The
unset command can be used to unset any undesirable variables.
- argv
- Special variable used in shell scripts to hold the value of arguments.
- autologout
- Contains the number of minutes the shell can be idle before it automatically
logs out.
- history
- Sets how many lines of history (previous commands) to
remember.
- ignoreeof
- Prevents logging out with a control-D.
- noclobber
- Prevents overwriting of files when using redirection.
- path
- Contains a list of directories to be searched when running programs or shell
scripts.
- prompt
- Sets the prompt string.
- term
- Contains the current terminal type.
If the history variable is set to a
numerical value, that many commands typed previous would be remembered in a
history list. Commands from the history are numbered from the first command
being 1. To see the history, the history command is used.
Commands from the history can be recalled using the exclamation point. For
example, !! repeats the previous command, !25 re-types command
number 25 from the history, and !-2 re-types the second line previous
to the current line.
Individual words from these command lines can also be retrieved using this
history. For example, !25:$ returns the last argument (word) from
command 25, !!:* returns all the arguments (all words but the first
one) from the last command, and !-2:0 returns the command (the first
word) of the second line previous.
A shorthand can be assigned to a command or
sequence of commands which are frequently used. By assigning an alias with the
alias command, one can essentially create their own commands, and even
"overwrite" existing commands. For example: alias cc cc -Aa -D_HPUX_SOURCE
This alias definition will substitute the cc with the ANSI compiler
option on an HP System (such as Wiliki) whenever cc is typed. To
undefine an alias, the unalias command is used.
If the filenames used behind an alias must come before text being
substituted, history substitution can be used, as follows: alias manl 'man \!* | less -p'
This form of the command places the arguments placed after the
manl alias between the man command and the | (pipe).
The input and output of
commands can be sent to or gotten from files using redirection. Some examples
are shown below: date > datefile
The output of the date command is saved into the contents of the
file, datefile. a.out < inputfile
The program, a.out receives its input from the input file,
inputfile. sort gradefile >> datafile
The sort command returns its output and appends it to the file,
datafile.
A special form of redirection is used in shell
scripts. calculate << END_OF_FILE
...
...
END_OF_FILE
In this form, the input is taken from the current file (usually the shell
script file) until the string following the "<<" is found.
If the special variable, noclobber is set,
if any redirection operation will overwrite an existing file, an error message
is given and the redirection will fail. In order to force an overwrite of an
existing file using redirection, append an exclamation point (!) after the
redirection command. For example for the command: date >! datefile
The file datefile will be overwritten regardless of its existence.
Adding an ampersand (&) to the end of an output redirection command will
combine both the standard error and the standard output and place the output
into the specified file.
The output of one command can be sent to the
input of another command. This is called piping. The commands which are to be
piped together are separated by the pipe character. For example: ls -l | sort -k 5n
This command takes the output of the ls -l command and puts the
output of it into the sort command.
By appending an ampersand (&) after the pipe character, one can combine
the standard error and standard output and send it to the standard input of the
program receiving the piped output.
The C shell handles job control, which
allows the user to run certain programs in the background, and recall them to
the foreground when necessary. In order to place a running process into the
background, the suspend character must be set by the stty command shown earlier.
Processes may be started in the background by following the command with an
ampersand (&).
When a job is placed in the background, information for the job is shown
similar to the example given below: [1] 15934
This specifies that the process has been placed in the background, and is
job 1. In order to recall jobs placed in the background, the fg command
is used, while the bg command places a recently stopped process into the
background. The jobs command gives a list of all processes under control
of the current shell. Also, typing a percent sign (%) with the job number brings
that particular job to the foreground.
The C shell has control
structures similar to the C programming language. These are foreach,
if, switch and while. These are usually used in shell scripts.
There are two forms of the if statement. The first one has a simple
command after the expression. This simple command cannot be an alias, nor can it
use statements that use the backquote (`). The second form
of the if command must have the word, then following the
expression. Several if statements can be chained together, through the
use of the else statement. This statement must have a corresponding
endif statement. if (expression) simple command
if (expression) then
...
else
...
endif
The switch statement can replace several if ... then
statements. For the string given in the switch statement's argument,
commands following the case statement with the matching pattern are
executed until the endsw statement. These patterns may contain ?
and * to match groups of characters or specific characters. switch (string)
case pattern1:
commands...
breaksw
case pattern2:
commands...
breaksw
default:
commands...
breaksw
endsw
The while statement will enter the loop only if the expression
evaluates to true (or non-zero). Once within the loop, the commands
within it will continue to execute until the expression evaluates to
false (zero). while (expression)
commands...
end
The foreach statement takes an array variable and places the contents
of each array element into the loop variable for each iteration. foreach variable (array variable or list)
...
end
The break statement breaks out of the current loop. break
The continue command returns to the top of the current loop after
testing the condition for the loop. continue
The shift command without arguments will shift the variable, argv down by one element. In other words, argv[2]
becomes argv[1] and so forth, with argv[1] being discarded.
With an array variable argument, the shift command performs the same
operation on the variable specified. shift
shift variable
The expressions used in the
while and if commands are similar to C language expressions, with
these exceptions:
- =~
- If the right hand side matches a pattern, (i.e., similar to filename
matching, with asterisks and question marks.) the condition is true.
- !~
- If the right hand side doesn't match a pattern, the condition is true.
- -d $var
- True if the file is a directory.
- -e $var
- True if the file exists.
- -f $var
- True if the file is a file. (I.e., not a directory)
- -o $var
- True if the file is owned by the user.
- -r $var
- True if the user has read access.
- -w $var
- True if the user has write access.
- -x $var
- True if the user has execute access.
- -z $var
- True if the file is zero-length.
Here are a few keys which may be
pressed to perform certain functions.
- <escape>
- The escape key preceded by a partial command or filename will attempt to
complete the filename. If there are more than one filename matching, the common
letters are completed, and the C shell beeps.
- Control-D
- When typed after a partial filename, C shell gives a list of all matching
filenames or commands.
- Control-W
- Erases over the previous word.
Shell scriptingShell scripts are programs written in C shell. They
are plain text files which can be edited and created by any text editor. There
are a few guidelines to follow, however.
- Create a file using any text editor. The first line must begin with
the string #!/bin/csh.
- Give yourself execute permission with the chmod u+x filename
command.
- You can run the shell script by simply typing filename as if it
were a regular command.
The shell script file can contain any commands
which can be typed in, as well as the control structures described above.
When you write a shell script, a
special array variable argv is created with the arguments entered. For
example, if the shell script tester is created, and is run by typing it
with the arguments as shown, tester one two jump, the array variable
argv will contain "one", "two", and "jump" in its three elements.
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