Introduction to Unix Shell & Scripting with csh/tcsh

 Brief Unix History  Unix Shell & Flavor  CSH/TCSH Scripts

Unix Family Tree

Unix Architecture

Unix History and Motivation

   The first version of Unix came from AT&T in the early 1970s (Unix is old!).

Unix was developed by programmers and

for programmers

.

Unix is designed so that users can extend the functionality     To build new tools easily and efficiently To customize the shell and user interface. To string together a series of Unix commands to create new functionality.

To create custom commands that do exactly what we want.

What is Shell?

 Shell is  Command Interpreter that turns text that you type (at the command line) in to actions:  User Interface: take the command from user  Programming Shell can do  Customization of a Unix session  Scripting

Customization of a Session

 Each shell supports some customization.

   User prompt Where to find mail Shortcuts (alias)  The customization takes place in

startup

files   Startup files are read by the shell when it starts up The Startup files can differ for different shell

Types of Shells

 Interactive vs. Non-interactive; login or not  Interactive login shell started after login  Non-interactive shell   Present when shell script is running Just inherits parent’s environment  Interactive non-login shell started  Started from a command line  Copies parent environment then invokes ~/.bash_rc (or ~/.cshrc or ~/.tcshrc)

Popular Shells

   

sh ksh csh,tcsh bash

Bourne Shell Korn Shell C Shell (for this course) Bourne-Again Shell

Families of Shells

Flavors of Unix Shells

 Two main flavors of Unix Shells   Bourne (or Standard Shell): sh, ksh, bash, zsh • • Fast $ for command prompt C shell : csh, tcsh • • better for user customization and scripting %, > for command prompt  To check shell:  % echo $SHELL (shell is a pre-defined variable)  To switch shell:  % exec shellname (e.g., % exec bash)

  

Startup files

sh,ksh: /etc/profile /etc/profile.local (addtnl. local system settings) ~/.profile (out-of-the-box login shell settings) (addtnl. user customized settings) ~/.kcshrc

(non-login shell user customization) bash: /etc/profile (out-of-the-box login shell settings) /etc/bash.bashrc (out-of-box non-login settings) /etc/bash.bashrc.local (global non-login settings) ~/.bash_profile

(login shell user customization) ~/.bashrc

(non-login shell user customization) ~/.bash_logout (user exits from interactive login shell) csh/tcsh: /etc/login (out-of-the-box login shell settings) /etc/csh.login (non-login shell customizations) /etc/csh.login.local

(global non-login settings) ~/.login: (login shell user customizations) ~/.cshrc: (non-login shell user customizations) ~/.cshrc.logout: (non-login shells at logout) ~/.logout: (read by login shells at logout)

Some Special Keys Under tcsh

 Ctrl-U = Delete everything on the command line  Ctrl-A = Move cursor to the front  Ctrl-E = Move cursor to the end  Ctrl-P = Set the current command-line to the previous command  Ctrl-N = Set the current command-line to the next command  TAB = Filename completion

Don’t forget your Best Friend

 % man command (e.g., % man ls)  shows information about the command  usually space or Enter for more information  q to quit  % man man

Create a shell script

   Creating a simple shell script  A shell script is a file that contains commands that the shell can execute.

• Any commands you enter in response to a shell prompt.

•    A utility A compiled program Another shell script Control flow commands Run a shell script   Enter the script filename on the command line The shell interprets and execute the commands one after another Why shell script?

 Simply and quickly initiate a complex series of tasks or a repetitive procedure.

Shell programming

 Make the file executable  When you create a shell script using a editor, does it have execute permission typically? • Example: (Make sure you are using tcsh/csh script!...) willow> echo $SHELL /bin/tcsh willow> ./test ./test: Permission denied.

willow> ls -l test -rw-r--r-- 1 student ums 33 Sep 18 16:33 test willow> chmod +x test willow> ./test This is Test!

Invoking a Shell script

 Give the shell a command on the command line  The shell forks a process • Which creates a non-interactive duplicate of the shell process  The newly forked process attempts to exec the command • If the command is an executable program •  Exec succeeds  System overlays the newly created subshell with the executable program The command is a shell script    Exec failed The command is assumed to be a shell script The subshell runs the commands in the shell.

Invoking a Shell script

 The shell itself is program  It can be run as a command in a shell and also accepts arguments. Note: Let’s find your default shell executing “echo $SHELL” willow> echo $SHELL /bin/tcsh  To run a shell script  Which does not have executable permission Ex: willow>tcsh test  Run the script with different shell other than your interactive shell Ex: willow>sh test

Invoking a Shell script

  Put special characters on the first line of a shell script   To tell OS checks what kind of file it is before attempting to exec it To tell which utility to use (sh, csh, tcsh , …) Special sequence   The firsts two character of a script are #! Then followed by the absolute pathname of the program that should execute the script Ex:

willow> more test #!/bin/tcsh # This line will not run since it is commented out...

echo 'This is Test!‘

Make a comment #

 Comments make shell scripts easier to read and maintain  Pound sign (#) start a comment line until the end of that line as second line in previous example, except  #! In the first line.  Or inside quotes

Parameters and Variables

 A shell parameter is associated with a value that is accessible to the user.

   Shell variables • Names consist of letters, digits and underscores • •  By convention, environment variables use UPPERCASE User created variables (create and assign value) Keyword shell variables  Have special meaning to the shell  Being created and initialized by the startup file Positional parameters • Allow you to access command line arguments Special parameters • Such as    The name of last command The status of most recently executed command The number of command-line arguments

Positional Parameters

 The command name and arguments are the positional parameters.  Because you can reference them by their position on the command line  $0 : Name of the calling program  $1 - $9 : Command-line Arguments • • • • The first argument is represented by $1 The second argument is represented by $2 And so on up to $9 The rest of arguments have to be shifted to be able to use $1- $9 parameters.

Positional Parameters

 Example: •Change directory to your assigned numbered subdirectory willow> cd 1 •List the directory contents, confirming

display_5args

willow> ls -l display_5args •Change mode of

display_5args

to executable willow> chmod +x display_5args •Execute the script willow> ./display_5args 1 2 3 4 5 you are running script ./display_5args with parameter 1 2 3 4 5

Positional Parameters

 $1-$9 allows you to access 10 arguments  How to access others?

 Promote command-line arguments: shift    Built-in command shift promotes each of the command-line arguments.

• The first argument ( which was $1) is discarded • • • The second argument ( which was $2) becomes $1 The third becomes the second And so on Makes additional arguments available Repeatedly using shift is a convenient way to loop over all the command-line arguments

Positional Parameters

 Example:

willow> more demo_shift #!/bin/tcsh echo $1 $2 $3 shift echo $1 $2 shift echo $1 willow> ./demo_shift 1 2 3 1 2 3 2 3 3

willow> more demo_shift #!/bin/tcsh echo $1 $2 $3 shift echo $1 $2 shift echo $1 shift echo $?

shift echo $?

shift echo $?

willow> ./demo_shift 1 2 3 4 1 2 3 2 3 3 0 0 shift: No more words.

Special Parameters

 Useful values  Command-line arguments   Execution of shell commands Can not change the value directly, like positional parameters  Value of Command-line arguments: $* and $@    $* and $@represent all the command_line arguments ( not just the first nine) “$*” : treats the entire list of arguments as a single argument “$@” : produce a list of separate arguments (Only bash/ksh/sh)

BASH SCRIPT WITH $*and $@

willow> more for_test.bash

#!/bin/bash echo "using \$* " for arg in "$*" do done echo "$arg" echo "using \$@ " for arg in "$@" do done echo "$arg" willow> ./for_test.bash 1 2 3 using $* 1 2 3 using $@ 1 2 3

TCSH SCRIPT WITH $*and $@

willow> more for_test #!/bin/tcsh echo 'using $*' foreach arg ($*) echo "$arg" end echo 'using $@' foreach arg ($@) echo "$arg" end willow> ./for_test 1 2 3 using $* 1 2 3 using $@ Illegal variable name .

Special Parameters

 The number of arguments: $#  Return a decimal number  Use the test to perform logical test on this number willow> more num_args willow> ./num_args 1 echo this script is called with $# arguments.

willow> chmod +x num_args willow> ./num_args this script is called with 0 arguments.

this script is called with 1 arguments.

willow> ./num_args 2 this script is called with 1 arguments.

willow> ./num_args 0 this script is called with 1 arguments.

Special Parameters

 Exit status: $?

   When a process stops executing for any reason, it returns an exit status to its parent process. By convention, • Nonzero represents a false value that the command failed.

• A zero value is true and means that the command was successful You can specify the exit status that a shell script returns by using the exit built-in followed by a number • Otherwise, the exit status of the script is the exit status of the last command the script ran.

willow> ls a a: No such file or directory willow> echo $?

2 willow> echo olemiss olemiss willow> echo $?

0 willow> more exit_status echo this program will have the exit code of 8.

exit 8 willow> ./exit_status this program will have the exit code of 8.

willow> echo $?

8 willow> echo $?

0

Summary

    A shell is both a command interpreter and a programming language.

Job control  Control-z/fg/bg/& Variables    Local and environment variables Declare and initialize a variable ( no type) Export unset Command line expansion   Parameter expansion/variable expansion/command/substitution/pathname expansion Quote ( ‘ ‘ “ “ \ ) • “ “ all but parameter, variable expansion and \ • • ‘ ‘ suppress all types of expansion \ escaping the following special character

Basic Script Example

willow> more basic_script #!/bin/tcsh echo 'Listing the files in long format appending due date/time' echo ls -lrtah echo echo 'Listing the files in long format appending due date/time' echo df -k # Using diff to find two files differences and writing them to another file diff -c for_test.bash for_test >> file_differences & echo echo 'sleeping mode for 4 seconds. Please wait!' echo sleep 4 echo echo 'GO REBELS' echo 'To find out the differences of files for_test and for_test.bash, ' echo 'Please open file_differences via using cat command as shown below:' echo 'cat file_differences ’

Killing

BAD

Processes

   The “kill” command:

kill [-]

Send

to process

The “killall” command:

killall [-]

Send

to all processes that start with

Useful signals (

kill –l

for the complete list): TERM nicely the default, “

term

inate”, kills things KILL HUP configurations STOP will kill anything, but not nicely “

h

ang

up

”, used to reload stops (suspends) a running process

 Shell parameters             HOME PATH PS1 SHELL $0 $n $* $@ $# $$ $!

$?

Summary

Summary

 Special Characters  NEWLINE  ;  ()  &  |  >  >>  <  <<              * ?

\ ‘ “ ` ` [] $ . # !

&& ||