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 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.

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 parent’s 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 • %, > 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. 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

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!...) 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!

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: 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

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: willow> 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 • 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 • Example:

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: 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

25. 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.

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 $@ • $* 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)

27. BASH SCRIPT WITH $*and $@ willow> more for_test.bash #!/bin/bash echo "using \$* " for arg in "$*" do echo "$arg" done echo "using \$@ " for arg in "$@" do echo "$arg" done 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.

28. Special Parameters • The number of arguments: $# • Return a decimal number • Use the test to perform logical test on this number willow> more num_args echo this script is called with $# arguments. willow> chmod +x num_args willow> ./num_args this script is called with 0 arguments. willow> ./num_args 1 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.

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. 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

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/substitution/pathname expansion • Quote ( ‘ ‘ “ “ \ ) • “ “ all but parameter, variable expansion and \ • ‘ ‘ suppress all types of expansion • \ escaping the following special character

32. 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.bashfor_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’

33. Killing BAD Processes • The “kill” command: kill [-<signal>] <pid> Send <signal> to process <pid> • The “killall” command: killall [-<signal>] <command> Send <signal> to all processes that start with <command> • 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 • $* • $@ • $# • $$ • $! • $?

35. Summary • Special Characters • NEWLINE • ; • () • & • | • > • >> • < • << • * • ? • \ • ‘ • “ • ` ` • [] • $ • . • # • && • || • !