1 © 2001 John Urrutia. All rights reserved. Chapter 10 using the Bourne Again Shell

2 © 2001 John Urrutia. All rights reserved. Topics Control Structures The Here Document Expanding NULL or unset variables The Builtins Functions

3 © 2001 John Urrutia. All rights reserved. Control Structures Selections  if … then … fi  One-Way Selection  if … then … else … fi  Two-Way Selection  if … then … elif … fi  Multi -Way Selection The above builtins must be on separate lines or separated by ;

4 © 2001 John Urrutia. All rights reserved. Control Structures Syntax: if test expression or [ expression ] then command(s) fi … Must Evaluate True or False Boolean Expression Executes if expression is true test expression or [ expression ] if then Executes if expression is false fi

5 © 2001 John Urrutia. All rights reserved. Boolean Expressions Algebra created by George Boole Always evaluates to a binary state  Generally:  1 is TRUE  0 is FALSE

6 © 2001 John Urrutia. All rights reserved. Boolean or Logical Operators And  -a Or  -o Not  !

7 © 2001 John Urrutia. All rights reserved. and Boolean Truth Tables Expr 1Expr 2 -a True FalseTrue False TrueFalse

8 © 2001 John Urrutia. All rights reserved. or Boolean Truth Tables True FalseTrue FalseTrue False Expr 1Expr 2 -o

9 © 2001 John Urrutia. All rights reserved. not Boolean Truth Tables Expr ! FalseTrue FalseTrue

10 © 2001 John Urrutia. All rights reserved. test ing, test ing, test ing test expression or [ expression ]  Evaluates the expression and returns a Boolean true or false.  expression can be simple or compound  Criteria:  String  Integer  Filename  File descriptor number

11 © 2001 John Urrutia. All rights reserved. Test Criteria String expressions – applies to string variables or literals  Is null or Length is zero (-z) or Length is > 0 (-n)  string1 = or != string2  If you are comparing literals quote them

12 © 2001 John Urrutia. All rights reserved. Testing Strings …]$ if test "not" != "equal";then echo Not Equal;fi Not Equal …]$ if [ "not" = "equal“ ];then echo Equal;fi …]$

13 © 2001 John Urrutia. All rights reserved. Test Criteria (cont.) Integer relationship  -gt greater than  -ge greater than or equal to  -eq equal to  -ne not equal to  -le less than or equal to  -lt less than

14 © 2001 John Urrutia. All rights reserved. Testing Integers …]$ if test 123 -ne 234;then echo Not Equal;fi Not Equal …]$ if [ 123 -eq 234 ];then echo Equal;fi …]$

15 © 2001 John Urrutia. All rights reserved. Test Criteria (cont.) Filename expression – file exists and has specific attributes  drewsx  -d irectory  -r eadable  -e xists  -w ritable  -s ize – file has data (length >0)  -x executable

16 © 2001 John Urrutia. All rights reserved. Testing Files …]$ if [ ! -e “TestFile” ];then echo Not found;fi Not found …]$ …]$ if test -e “TestFile”;then echo Found it;fi …]$

17 © 2001 John Urrutia. All rights reserved. Bourne - if, then if  Establishes a control structure  Followed by a test command then  Commands executed if test is true

18 © 2001 John Urrutia. All rights reserved. Bourne – else, fi else  Commands executed if test is false fi  Terminates this if statement

19 © 2001 John Urrutia. All rights reserved. if …then … else … fi Start Test 1 False True

20 © 2001 John Urrutia. All rights reserved. Bourne - elif elif  “else if” structure  Linear in nature  Similar to the case or switch in selection but completely different in execution.

21 © 2001 John Urrutia. All rights reserved. if …then … elif … else … fi Start Test 2 False True Test 1 True if then elifelse

22 © 2001 John Urrutia. All rights reserved. if … then … [ else or elif ] … fi 2 structures  if […] then cmds if […] then cmds else cmds else cmds fi  if […] then cmds elif […] then cmds else cmds fi

23 © 2001 John Urrutia. All rights reserved. if [ $a –gt 2 ] then if [ $a –gt 3 ] then echo value is 4 else echo value is 3 fi else if [ $a –gt 1 ] then echo value is 2 else echo value is 1 fi if [ $a –gt 3 ] then echo value is 4 elif [ $a –gt 2 ] then echo value is 3 elif [ $a –gt 1 ] then echo value is 2 else echo value is 1 fi

24 © 2001 John Urrutia. All rights reserved. Control Structures Iterations  for … in  for

25 © 2001 John Urrutia. All rights reserved. for… and for …in Assign element Do commands More elements Done

26 © 2001 John Urrutia. All rights reserved. Here we go loop-de-loop for x in a b c  do cmd cmd cmd done Substitutes the x variable with each in variable and executes the commands between do and done

27 © 2001 John Urrutia. All rights reserved. California the land of … echo “California the land of – “ for info in fruits nuts cheese wine do echo –n “ $info” done echo –e “\n And of course Hollywood”

28 © 2001 John Urrutia. All rights reserved. Here we go loop-de-loop for x do cmd cmd cmd done Substitutes the x with command line arguments and executes the commands between do and done

29 © 2001 John Urrutia. All rights reserved. California the land of … echo “California the land of – “ for arg in fruits nuts cheese wine do echo –n “ $arg” done echo “ And of course Hollywood” This modification causes the script to display all command line arguments again

30 © 2001 John Urrutia. All rights reserved. Control Structures Iterations  while  until

31 © 2001 John Urrutia. All rights reserved. While I’m here – Until it’s gone while […] (initial state true - go) until […] (initial state true – stop)  do cmd cmd cmd done

32 © 2001 John Urrutia. All rights reserved. While you were out… cold While Test do commands Done Pre-Test Loop

33 © 2001 John Urrutia. All rights reserved. Until … The cows come home until Test do commands Done Pre-Test Loop

34 © 2001 John Urrutia. All rights reserved. Continue to Break continue and break  Used in for while and until structures  break – terminates the structure  continue – terminates this iteration

35 © 2001 John Urrutia. All rights reserved. continue for idx in do if [ $idx –le 3 ] ; then echo “continue” continue fi echo $idx done Continues three times Echoes 4 – 10

36 © 2001 John Urrutia. All rights reserved. break for idx in do if [ $idx –gt 6 ] ; then echo “break” break fi echo $idx done Stops for loop after sixth iteration Echoes 1 – 6

37 © 2001 John Urrutia. All rights reserved. Bourne – case The case command tests for multiple values in a variable Allows the use of “wild cards” First match wins

38 © 2001 John Urrutia. All rights reserved. Get on this case case $var in test-var1 ) cmd1;cmd2 cmd3 ;; test-var2 ) cmds ;;  ) cmds ;; esac

39 © 2001 John Urrutia. All rights reserved. case “$variable” in A|a ) echo Entered A or a ;; [0-9] ) echo Entered number ;; ?z* ) echo Entered z in 2 nd position ;; esac

40 © 2001 John Urrutia. All rights reserved. Topics Control Structures The Here Document Expanding NULL or unset variables The Builtins Functions

41 © 2001 John Urrutia. All rights reserved. Here boy << The Here Document  Allows in-stream data to feed a script.  Must start with << and a data delimiter character  Data delimiter character on line by itself - terminates

42 © 2001 John Urrutia. All rights reserved. Here in the script #!/bin/bash grep –i “$1” <<+ AlexJune 22 BabsFebruary 3 LeroyJanuary 20 + echo “All Done now”

43 © 2001 John Urrutia. All rights reserved. Bundle script #!/bin/bash echo “# To unbundle sh this file for i do echo “echo $i 1>&2” echo “cat >$i <<‘End of $i’” cat $i echo “End of $i” done

44 © 2001 John Urrutia. All rights reserved. Bundle script -- output # To unbundle sh this file echo 1 st filename 1>&2 cat > 1 st filename <<‘End of 1 st filename ’ Contents of 1 st filename.. End of 1 st filename

45 © 2001 John Urrutia. All rights reserved. Topics Control Structures The Here Document Expanding NULL or unset variables The Builtins Functions

46 © 2001 John Urrutia. All rights reserved. Expanding Null or Unset Vars. ${ name } – expands to the value of the variable. If variable is null or not set the expansion produces a null string (\0) ${ name :- default } – expands to the value of the variable or the default value if null.

47 © 2001 John Urrutia. All rights reserved. Expanding Null or Unset Vars. ${ name := default } – expands to the value of the variable or the default value if null and sets the variable to the default value. : ${ name := default } – if null sets the variable to the default value. ${ name :? message } – if null or unset sends message to stdout.

48 © 2001 John Urrutia. All rights reserved. Topics Control Structures The Here Document Expanding NULL or unset variables The Builtins Functions

49 © 2001 John Urrutia. All rights reserved. The execute command The exec command  Executes scripts or programs  Runs under the same PID  Provides access to the original environment variables  Terminates current process.

50 © 2001 John Urrutia. All rights reserved. The execute command The exec command  Can be used to redirect stdin, stdout and stderr from inside a script. exec < infile exec > outfile 2> errfile

51 © 2001 John Urrutia. All rights reserved. I feel like trap – let’s kill a PID trap ’ action ’ [ signal number ]  64 signals available 0 – 63  1 – Phone Hangup  2 – Pressing an interrupt key ( ^c)  3 – Pressing the quit key (Ctrl+Shft+| or \)  9 – Kill command (without mercy ^d)  15 – Terminate process (kill command)  20 – Job control STOP key (^z)

52 © 2001 John Urrutia. All rights reserved. I feel like trap – let’s kill a PID kill - [ signal number ] [ pid ]  Sends a signal to one or more PIDs  By default sends signal 15 (SIGTERM)  Using signal 9 is un-trappable Get-out … NOW!  kill –9 0

53 © 2001 John Urrutia. All rights reserved. Topics Control Structures The Here Document Expanding NULL or unset variables The Builtins Functions

54 © 2001 John Urrutia. All rights reserved. Play that Function music Functions are like pre-processed scripts. Can be stored in.profile or in script files. Generally used by more than 1 script.

55 © 2001 John Urrutia. All rights reserved. Function declaration Function-name () { commands } Functions can be called with arguments and process them the same way a command line would.