CSCI 330 The UNIX System

1. CSCI 330The UNIX System Unit VIII: Introduction to Shell Programming

2. Introduction to Shell Programming • shell programming is one of the most powerful features on any UNIX system • large portion on UNIX administration and house keeping is done via shell scripts • if you cannot find an existing utility to accomplish a task, you can build one using a shell script CSCI 330 - The Unix System

3. Shell Programs • A shell program contains high-level programming language features: • Variables for storing data • Decision-making control (e.g. if and case statements) • Looping abilities (e.g. for and while loops) • Function calls for modularity • A shell program can also contain: • any UNIX command • file manipulation: cp, mv, ls, cd, … • utilities: grep, sed, awk, … CSCI 330 - The Unix System

4. Shell Programs: tips • Naming of shell programs and their output • Give a meaningful name • example: findfile • Do not use: • script1, script2 • UNIX command names • Repository for shell programs • If you develop numerous shell programs, place them in a directory (e.g. ~/bin or ~/shellprogs) • Update your path to include the directory name where your shell programs are located CSCI 330 - The Unix System

5. Shell Programs: tips • Comments • comment lines start with a pound sign (#) • Include comments to describe sections of your program: about one comment for every 5 lines • Help you understand your program when you look at it later • Formatting of shell programs • Indent areas (3 or 4 spaces) of programs to indicate that commands are part of a group • To break up long lines, place a \ at the end of one line and continue the command on the next line CSCI 330 - The Unix System

6. Shell Programs: minimum required • create script that contains shell commands • data structure: variables • control structure: sequence, decision, loop • 1. line (shebang line) for bash shell script: #! /bin/bash #! /bin/sh • to run: • make executable: % chmod +x script • invoke via: % ./script CSCI 330 - The Unix System

7. bash Shell Programming Features • Variables • Input/output • command line parameters • prompting user • Decision • if-then-else • case • Repetition • do-while, repeat-until • for, select • Functions • Traps CSCI 330 - The Unix System

8. User-defined shell variables Syntax: varname=value Example: rate=moderate echo “Rate today is: $rate” • use double quotes if the value of a variable contains white spaces Example: name=“Thomas William Flowers” Note: no spaces CSCI 330 - The Unix System

9. Output via echo command • Simplest form of writing to standard output Syntax: echo [-ne] argument[s] -n suppresses trailing newline -e enables escape sequences: \t horizontal tab \b backspace \a alert \n newline CSCI 330 - The Unix System

10. Examples: shell scripts with output #! /bin/bash echo "You are running these processes:" ps #! /bin/bash echo -ne "Dear $USER:\nWhat's up this month:" cal CSCI 330 - The Unix System

11. Command line arguments • Use arguments to modify script behavior • command line arguments become positional parameters to shell script • positional parameters are numbered variables: $1, $2, $3 … CSCI 330 - The Unix System

12. Command line arguments Meaning $1 first parameter $2 second parameter ${10} 10th parameter { } prevents “$1” misunderstanding $0 name of the script $* all positional parameters $# the number of arguments CSCI 330 - The Unix System

13. Example: Command Line Arguments #! /bin/bash # Usage: greetings name1 name2 echo $0 to you $1 $2 echo Today is `date` echo Good Bye $1 CSCI 330 - The Unix System

14. Example: Command Line Arguments % chmod +x greetings % ./greetings Mark Flowers ./greetings to you Mark Flowers Today is Wed Oct 5 14:18:03 CST 2011 Good Bye Mark $0 => ./greetings $1 => Mark $2 => Flowers CSCI 330 - The Unix System

15. Script example • Use command line argument as input for command #! /bin/bash # counts characters in command argument echo -n "$1" | wc -c CSCI 330 - The Unix System

16. Numeric variables Syntax: let varname=value declare –ivarname=value • can be used for simple arithmetic: let count=1 let count=$count+20 let count+=1 • alternative syntax: ((count++)) CSCI 330 - The Unix System

17. Array variables Syntax: varname=(list of words) • accessed via index: ${varname[index]} ${varname[0]} first word in array ${varname[*]} all words in array CSCI 330 - The Unix System

18. Using array variables Examples: % ml=(maryannbrucelindadara) % echo $ml mary % echo ${ml[*]} maryannbrucelindadara % echo ${ml[2]} bruce % ml[2]=john % echo ${ml[*]} maryann john lindadara CSCI 330 - The Unix System

19. Exporting Variables • Environment variable is created by exporting shell variable Syntax: export varname declare –x varname CSCI 330 - The Unix System

20. Variables commands • To delete both local and environment variables unset varname • To prohibit change readonlyvarname • list all shell variables (including exported) set CSCI 330 - The Unix System

21. variable manipulation • use portion of a variable’s value via: ${name:offset:length} • name – the name of the variable • offset – beginning position of the value • length – the number of positions of the value Example: % SSN="123456789" % password=${SSN:5:4} % echo $password % 6789 CSCI 330 - The Unix System

22. Special variable uses • ${#variable} number of characters in variable’s value • ${variable:-value} if variable is undefined use “value” instead • ${variable:=value} if variable is undefined use “value” instead, and set variable’s value • ${varname:?message} if variable is undefined display error “message” CSCI 330 - The Unix System

23. Summary • Shell scripts can do what can be done on command line • Shell scripts simplify recurring tasks • Next: control structures CSCI 330 - The Unix System