Introduction to Unix Shell & Scripting with csh/tcsh

1
Introduction to Unix Shell Scripting with
csh/tcsh

• Brief Unix History
• Unix Shell Flavor
• CSH/TCSH Scripts

2
Unix Family Tree
3
Unix Architecture
4
Unix History and Motivation

• The first version of Unix came from ATT 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.

5
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

6
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

7
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 parents environment
• Interactive non-login shell started
• Started from a command line
• Copies parent environment then invokes
  /.bash_rc (or /.cshrc or /.tcshrc)

8
Popular Shells

• sh Bourne Shell
• ksh Korn Shell
• csh,tcsh C Shell (for this course)
• bash Bourne-Again Shell

9
Families of Shells
10
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
• , gt for command prompt
• To check shell
• echo SHELL (shell is a pre-defined variable)
• To switch shell
• exec shellname (e.g., exec bash)

11
Startup files

• sh,ksh
• /etc/profile (out-of-the-box login shell
  settings)/etc/profile.local (addtnl. local
  system settings)/.profile (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)

12
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

13
Dont 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

14
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.

15
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!...)
• willowgt echo SHELL
• /bin/tcsh
• willowgt ./test
• ./test Permission denied.
• willowgt ls -l test
• -rw-r--r-- 1 student ums 33 Sep 18
  1633 test
• willowgt chmod x test
• willowgt ./test
• This is Test!

16
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.

17
Invoking a Shell script

• The shell itself is program
• It can be run as a command in a shell and also
  accepts arguments. Note Lets find your default
  shell executing echo SHELL
• willowgt echo SHELL
• /bin/tcsh
• To run a shell script
• Which does not have executable permission
• Ex willowgttcsh test
• Run the script with different shell other than
  your interactive shell
• Ex willowgtsh test

18
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
• willowgt more test
• !/bin/tcsh
• This line will not run since it is commented
  out...
• echo 'This is Test!

19
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

20
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

21
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.

22
Positional Parameters

• Example

• Change directory to your assigned numbered
  subdirectory
• willowgt cd 1
• List the directory contents, confirming
  display_5args
• willowgt ls -l display_5args
• Change mode of display_5args to executable
• willowgt chmod x display_5args
• Execute the script
• willowgt ./display_5args 1 2 3 4 5
• you are running script ./display_5args with
  parameter 1 2 3 4 5

23
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

24
Positional Parameters

• Example
• willowgt more demo_shift
• !/bin/tcsh
• echo 1 2 3
• shift
• echo 1 2
• shift
• echo 1
• willowgt ./demo_shift 1 2 3
• 1 2 3
• 2 3
• 3

25

• willowgt more demo_shift
• !/bin/tcsh
• echo 1 2 3
• shift
• echo 1 2
• shift
• echo 1
• shift
• echo ?
• shift
• echo ?
• shift
• echo ?
• willowgt ./demo_shift 1 2 3 4
• 1 2 3
• 2 3
• 3
• 0
• 0

26
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 _at_
• and _at_represent all the command_line arguments
  ( not just the first nine)
• treats the entire list of arguments as a
  single argument
• _at_ produce a list of separate arguments (Only
  bash/ksh/sh)

27
TCSH SCRIPT WITH and _at_ willowgt more
for_test !/bin/tcsh echo 'using ' foreach arg
() echo "arg" end echo 'using
_at_' foreach arg (_at_) echo
"arg" end willowgt ./for_test 1 2 3 using
1 2 3 using _at_ Illegal variable name.
BASH SCRIPT WITH and _at_ willowgt more
for_test.bash !/bin/bash echo "using \ " for
arg in "" do echo "arg" done echo
"using \_at_ " for arg in "_at_" do echo
"arg" done willowgt ./for_test.bash 1 2 3 using
1 2 3 using _at_ 1 2 3
28
Special Parameters

• The number of arguments
• Return a decimal number
• Use the test to perform logical test on this
  number

willowgt more num_args echo this script is called
with arguments. willowgt chmod x
num_args willowgt ./num_args this script is
called with 0 arguments.
willowgt ./num_args 1 this script is called with 1
arguments. willowgt ./num_args 2 this script is
called with 1 arguments. willowgt ./num_args
0 this script is called with 1 arguments.
29
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.

30

• willowgt ls a
• a No such file or directory
• willowgt echo ?
• 2
• willowgt echo olemiss
• olemiss
• willowgt echo ?
• 0
• willowgt more exit_status
• echo this program will have the exit code of 8.
• exit 8
• willowgt ./exit_status
• this program will have the exit code of 8.
• willowgt echo ?
• 8
• willowgt echo ?
• 0

31
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/sub
  stitution/pathname expansion
• Quote ( \ )
• all but parameter, variable expansion and \
• suppress all types of expansion
• \ escaping the following special character

32
Basic Script Example

• willowgt 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 gtgt
  file_differences
• echo
• echo 'sleeping mode for 4 seconds. Please wait!'
• echo
• sleep 4

33
Killing BAD Processes

• The kill command
• kill -ltsignalgt ltpidgt
• Send ltsignalgt to process ltpidgt
• The killall command
• killall -ltsignalgt ltcommandgt
• Send ltsignalgt to all processes that start with
  ltcommandgt
• Useful signals (kill l for the complete list)
• TERM the default, terminate, kills things
  nicely
• KILL will kill anything, but not nicely
• HUP hangup, used to reload configurations
• STOP stops (suspends) a running process

34
Summary

• Shell parameters
• HOME
• PATH
• PS1
• SHELL
• 0
• n
• _at_
• !
• ?

35
Summary

• Special Characters
• NEWLINE
• ()
• gt
• gtgt
• lt
• ltlt

• ?
• \
• .
• !