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리눅스 : Lecture 6 Shell Programming

리눅스 : Lecture 6 Shell Programming. Extension of Functionality. 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.

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리눅스 : Lecture 6 Shell Programming

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  1. 리눅스: Lecture 6Shell Programming

  2. Extension of Functionality • 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.

  3. Shell • Command Interpreter that turns text that you type (at the command line) in to actions • User Interface: take the command from user • Shell Programming • We often want to do a number of commands together • And bundle them up into one new command. • Just like a batch file in MS-DOS

  4. Shell scripts Any collection of shell commands can be stored in a file called a shell script. Scripts have variables and flow control statements like other programming languages.

  5. Popular Shells • sh Bourne Shell • ksh Korn Shell • bash Bourne-Again Shell • csh,tcsh C Shell (for this course) • Shell scripts among those shells are slightly different

  6. 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.

  7. Shell script example #!/bin/csh echo “Current Time - `date`” echo I am `whoami` C Shell

  8. Invoking scripts There are two ways to launch scripts: 1) Direct interpretation csh scriptfile [args …] 2) Indirect interpretation The first line of the file must be #!/bin/csh and the file must be executable (permission). C Shell

  9. Shell Variables • Environment Variables • Used to provide information to programs • (Global) environment variable • New programs and shells inherit environment variables from their parent shell • (Local) shell variable • Used only by that shell • Not passed to other processes

  10. Environment Variables • “env” or “printenv” command • Display current environment variables • DISPLAY The graphical display to use, e.g. nyssa:0.0 • EDITOR The path to your default editor, e.g. /usr/bin/vi • GROUP Your login group, e.g. staff • HOME Path to your home directory, e.g. /home/frank • HOST The hostname of your system, e.g. nyssa • IFS Internal field separators, usually any white space (defaults to tab, space and <newline>) • LOGNAME The name you login with, e.g. frank • PATH Paths to be searched for commands, e.g. /usr/bin:/usr/ucb:/usr/local/bin • PS1 The primary prompt string, Bourne shell only (defaults to $) • PS2 The secondary prompt string, Bourne shell only (defaults to >) • SHELL The login shell you're using, e.g. /usr/bin/csh • TERM Your terminal type, e.g. xterm • USER Your username, e.g. frank

  11. Set Shell Variables • Mostly set automatically when log in • setenv • $ setenv NAME value # in C Shell • set • $ set name = value # in C Shell

  12. Variables To set variables: set X [= value] # processed as a string To unset variables : unset X Variable contents are accessed using ‘$’: echo $PATH C Shell

  13. Array To create lists: set Y = (abc 1 123) To set a list element: set Y[2] = 3 To view a list element: echo $Y[2] To count the number of variable elements: echo $#Y set fname = prog1 rm ${fname}.c C Shell

  14. Built-in Variables $user -- who am I? $path -- my execution path (list of directories to be searched for executables) $term -- what kind of terminal I am using $status -- a numeric variable, usually used to retun error codes $prompt -- what I am currently using for a prompt $shell -- which shell am I using (usu. either /bin/csh or /bin/sh) % set Will display the variable lists.

  15. Arithmetic (@) command • C shell provides arithmetic operaters • must be used with the arithmetic (@) command • Arithmetic command works only with integers. set count = 5 @ count += 2 echo $count 90

  16. Shell Arithmetic • expr op1 math-operator op2 • Example % expr 1 + 3 % expr 10 \* 3 % set A = `expr 3 + $B`

  17. Command arguments A shell script to swap files: #! /bin/csh –f cp $argv[1] tempfile cp $argv[2] $argv[1] cp tempfile $argv[2] rm –f tempfile Arguments : $argv The number of arguments to a script: $#argv -f option says we want fast startup (no read .cshrc) . C Shell

  18. if-then-else if ( expr ) simple-command if ( expr ) then commandlist-1 [else commandlist-2] endif C Shell

  19. if-then-else cont’d An example: if ($#argv != 2) then echo “we need two parameters!“ else set name1 = $argv[1] set name2 = $argv[2] endif C Shell

  20. Loops while ( expr ) commandlist end foreach var ( worddlist ) commandlist end C Shell

  21. switch switch ( str ) case string1: commandlist1 breaksw case string2: commandlist2 breaksw default commandlist endsw C Shell

  22. goto (Considered harmful!) To jump unconditionally: goto label A label is a line such as: label: The classic paper on why not to use goto: Go To Statement Considered HarmfulEdsger W. Dijkstra, CACM, March 1968 C Shell

  23. shift command • Moves the values in the parameters toward the beginning of the parameter list #!/bin/csh –f echo “There are” $#argv “parameters\n” while ($#argv > 0) echo –n “$argv[1] “ shift end echo “\n” echo “There are now” $#argv “parameters” echo “end of script” C Shell

  24. Input • Reading Line by Line % set x = $< This is a line. % echo $x This is a line.

  25. File Operators -e file: True if file exists -r file : True if file is readable -l file : True if file exists and is a symbolic link -w file : True if file exists and is writable -x file : True if file exists and is executable -o file : True if the user owns it -f file : True if the file exists and is a regular file -d file : True if the file exists and is a directory -s file : True if file exists and has a size greater than zero -z file : True if file length is zero (empty)

  26. Logical operator • ! : NEGATE • && : logical AND • || : logical OR • Ex) if (! -e somefile) then # does not exist

  27. Debugging %csh –vx somescript args -v : vervose -x : echoes the commands after all substitutions are made -n : syntax check. No execution

  28. example #!/bin/csh if (-e $argv[1]) then echo $argv[1] exists else echo $argv[1] does not exist and cannot be opened endif # rest of script here C Shell

  29. example #!/bin/csh set sum = 0 echo –n “Enter a number: ” set num = $< while ($num != “”) @ sum += $num echo –n “Enter the next number: ” set num = $< end echo “\nThe sum of the number is : $sum” C Shell

  30. Guidelines • Shell script is better than C program if the problem can be solved by using UNIX commands • Why script? • Easier to create and modify • Easy to debug • Good thing to do • Use redirection and pipe • Do validity check (argument number , type) • Check existence of files and directories • Display error messages

  31. example #!/bin/csh set j = (1 2 3 4 5) foreach i ($j) echo $i Hello end C Shell

  32. example #!/bin/csh set ary = `cat ary.dat` echo “The whole array : $ary” echo “The number of elements : $#ary” echo “The first element: $ary[1]” echo “The last element: $ary[$#ary]” C Shell

  33. Numeric validation example #!/bin/csh echo $argv[2] > temp grep ‘^[0-9]*$’ temp > /dev/null if ($status != 0) then echo “Month argument is not numeric” exit 1 endif if ($argv[2] < 1 || $argv[2] > 12) then echo “Month argument must be <1…12>” exit 2 endif echo “Validation is OK. We continue.” C Shell

  34. example #! /bin/csh -f foreach name ($argv) if ( -f $name ) then echo -n "delete the file '${name}' (y/n/q)?" else echo -n "delete the entire dir '${name}' (y/n/q)? " endif set ans = $< # $< means “read a line” switch ($ans) case n: continue case q: exit case y: rm -rf $name continue endsw end: C Shell

  35. Exercise 1 • Write a shell script that displays the number of files and directories in a given directory • format % ./fd_count.csh directory_name

  36. Exercise 2 • Write a shell script that removes duplicate words from an input text file. • Format • % remove_dup.csh in.txt out.txt Four Two One One Four Two Two Three Four Two One Three out.txt in.txt

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