CSCI 330THE UNIX SYSTEM

C Shell

C SHELL SPECIFICS

Startup and initialization Shell variables Prompt Alias Redirections and pipe

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INVOKING CSH

On the command line:% csh

% tcsh as login shell

specified in /etc/passwd with file as argument

file is csh-script% csh scriptfile

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CUSTOMIZING CSH

Initialization files ~/.cshrc ~/.tcshrc ~/.login /etc/csh.cshrc /etc/csh.login

options: -f don’t run initialization files -l run as login shell

also: ~/.logout /etc/csh.logout

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C SHELL VARIABLES

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Name Contentscwd The current working directory

history The size of the history list

ignoreeof Prevents the shell from terminating when pressing Control-D. Use the “logout” or “exit” command.

noclobber Prevents existing files from being overridden by output redirection (>), and non-existent files from being appended by append (>>)

prompt The shell primary prompt

savehist The number of commands to save in the history file “$HOME/.history”

status The exit code of the last command

SETTING SHELL VARIABLES

create shell variable and initialize it to “value”

varname: 1-20 characters long, lower case by convention Letters, digits, and underscore First character cannot be a digit Should not be the same as one of the pre-

defined variables

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Syntax: set varname = value

EXAMPLES: SETTING VARIABLES

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Command Result Comment

set x = 123 x contains “123” x contains a number, but it is stored as string of digits

set x = Hello x contains “Hello” Storing a character string

set name = Jane Black x contains “Jane” Only “Jane” is stored

set name = "Jane Black" x contains

“Jane Black”

When a string contains spaces it must be quoted

set x = "Go Huskies!" x contains

“Go Huskies!”

When a string contains spaces and special characters, it must be quoted

ACCESSING THE VALUES OF A VARIABLE

$ symbol precedes variable name

Examples:% set count = 5

% set number = $count

% echo count contains: $count

count contains: 5

% echo number contains: $number

number contains: 5

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SETTING ENVIRONMENT VARIABLES

create environment variable, initialize to “value”

If envname does not exist, it is created; otherwise, it is overwritten

Example:% setenv TERM vt100

% echo $TERM

vt100

% setenv EDITOR nano 9

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Syntax: setenv envname value

MULTI-VALUED SHELL VARIABLE

“list” shell variable allows multiple values

Syntax: set listvar = (val1 val2 val3 … valN)

variable can be used as whole single elements via index range of elements

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USING “LIST” VARIABLES

Examples:

% set ml=(mary ann bruce linda dara)

% echo $ml

mary ann bruce linda dara

% echo $ml[3]

bruce

% echo $ml[2-4]

ann bruce linda

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REMOVING A VARIABLE

use the “unset” command

Example:% set x = one

% echo "x contains:" $x

x contains: one

% unset x

% echo "x contains:" $x

x: Undefined variable

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SPECIAL BUILT-IN VARIABLES

Examples:% set days=(mon tue wed thr fri sat sun)% echo There are $#days days in a weekThere are 7 days in a week% echo $?days1 13

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Variable Name Contents/Meaning$#listvar Number of elements in list variable

$?var Determine if a variable has been declared

Return 1, if declared; 0, if not declared

ARITHMETIC VARIABLES

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Symbol Meaning Example

@ num = 10 echo $r

+ Addition @ r = $num + 4

- Subtraction @ r = $num - 5

* Multiplication @ r = $num * 10

/ Division @ r = $num / 4

% Modulo @ r = $num % 3

Syntax: @ varname = numberWhere number is an integer

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PATHNAME MODIFIERS

used on variables that contain pathnames four parts to a path:

head (h): everything from the beginning of the path up to the last slash in the path

tail (t): the last directory or file in the path root (r): the filename without an extension extension (e): the filename name extension

use a colon to separate the pathname modifier from the variable name

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EXAMPLE: PATHNAME MODIFIER

% set cf="/home/turing/ege/unix/assign1.txt"

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h

r

t

/home/turing/ege/unix/assign1.txt

e

% echo $cf:tassign1.txt

LISTING ALL VARIABLES

To list shell variables: set or @

To list environment variables: printenv

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C SHELL VARIABLES - SUMMARY

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To … Use the command:

Create an environment variable

setenv VARNAME value

Create a shell variable set varname=value

Create a numeric variable @ varname = number

Create a list variable set listname=(value1 … valueN)

List all variables set or @ or printenv

C SHELL PROMPT

default prompt: %

can be set via “prompt” shell variable special character “!” shows current event

number beware: use \! to avoid other meanings

Example: % set prompt = "$USER-\!: "

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C SHELL ALIASES

defines abbreviation for a command

Syntax: alias name ‘command-list’

use alias like any other command can rename existing commands can reference shell variables

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C SHELL ALIAS SPECIALTIES

alias can have arguments default: all arguments are appended use special character “!” to refer to arguments

beware: use \! to avoid other meanings

\!* all arguments\!^ first argument\!:n n-th argument

Examples:% alias ll "ls -al"% alias dir "ls -l"% alias count "ls -al \!* | wc -l" 21

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I/O REDIRECTION

Input Redirection (<)Syntax: command < file Command will read (take input) from file, instead

of from terminal Output Redirection (>)

Syntax: command > file Sends output of command to file, instead of to

terminal Appending Output (>>)

Syntax: command >> file Appends output of command to existing file 22

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“NOCLOBBER” VARIABLE

if set, then does not allow output redirection to existing file does not allow appending to non-existing file

Examples:% set noclobber

% who > current-users

current-users: File exists

% date >> usage-status

status: No such file or directory23

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OVERRIDE THE EXTRA PROTECTION

Add ! To the redirection! Means “do what I mean!”

Examples:% set noclobber

% who >! current-users Allows redirection even if ‘current-users’ exists

% date >>! usage-status Creates the file named “usage-status” even if it does

not already exist

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HANDLING STANDARD ERRORS

Sometimes a command has special output to inform of problems% gcc gets.c > compile.log

"gets.c":8: syntax error

Syntax: command >& file Redirect stdout and stderr to file

Syntax: command >>& file Append stdout and stderr to file

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EXAMPLES: REDIRECT/APPEND STDERR

% gcc gets.c >& compile.log

% cat compile.log

“gets.c”:8: syntax error

% gcc gets.c >>& compile.log

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PIPE: STDOUT AND STDERR

Syntax: command1 |& command2 Sends stdout and stderr of command1 to be stdin for command2

Example:% gcc gets.c |& more Sends stdout and stderr to ‘more’ for convenient

viewing% gcc gets.c |& lpr Produces a hardcopy of the messages

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SUMMARY: REDIRECTIONS AND PIPE

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Command Syntax Meaning

command < file Redirect input from file to command

command > file Redirect output from command to file

command >& file Redirect output and errors to file

command >> file Redirect output of command and appends it to file

command1 | command2 Take/pipe output of command1 as input to command2

command1 |& command2 Take/pipe output and errors of command1 as input to command2

SUMMARY: REDIRECTIONS AND PIPE

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Command Syntax Meaning

command >! file If the ‘noclobber’ variable is set, override its effects for this command and either open or overwrite file

command >>! file Override ‘noclobber’ variable; if file does not exist, it is created and output from command is appended to it

command >>&! file Override ‘noclobber’ variable; if file does not exist, it is created and both output and errors are appended to it