Session 5

1. Session 5 The Bourne Shell

2. Overview • Shell fundamentals • Streams revisited • Processes and Subshells • Shell script basics • Examples

3. Shell fundamentals • Command separation • Command grouping • Job control (limited in Bourne)

4. Command Separation • Newline (nl) X’0D0A’ • ends command and initiates execution • Semicolon (;) • just separates commands • Backslash (\) X’5C0D0A’ • at end of line and before you type return Allows command to be continued

5. Command Separation (cont.) • Ampersand (&) • execute task in the background • Pipe ( | ) • pipe

6. Command Grouping • Parenthesis used to group commands • causes Shell to create a subshell • additional processes are created as required when the subshell runs the commands within the parenthesis • (ls ; date; w) ; more • (ls ; date; w) | more

7. Job Control • Ampersand & • tells the Operating system to run the job in the background • User will still be able to interact with the shell • Pure Bourne shell has limited ability. Can not deal with a specific job it has put into background after initial creation. C shell much better.

8. Job Control (continued) • First two jobs in background, c in foreground • a & b & c • Entire sequence put into background • a | b | c & • All three jobs executed in background • a & b & c &

9. Streams Revisited • Three streams • standard in < or 0< • standard out > or 1> • standard error 2>

10. Streams standard I/O • cat x y • if x exists and y does not, contents of x and error message due to y are sent to terminal • both standard out and standard error default to the terminal

11. Streams Continued • cat x y 2>error.log • standard error is sent to a file to separate it from the expected results of the command • cat x y 2>>newfile 1>>newfile • standard out is redirected to newfile

12. Processes and Subshells • A process is the execution of a command • login to UNIX • execution of a UNIX utility • execution of a shell script creates a new process • script commands each start a new process • Process structure is hierarchical

13. Process Flow • User logs in: shell process is created • User issues command, enters return • Shell creates a subshell • child process is forked or spawned • unless the command is built into the bourne shell process

14. Process flow (cont.) • Subshell is a clone of the parent shell • Subshell tries to exec the command • If program, the program runs • If shell script, exec fails and subshell interprets commands.

15. Process Flow • Parent Shell sleeps until child shell finishes • (unless job was executed in background) • Variables that are used in a parent can be sent to a child, but the reverse is not true.

16. Process Flow • Shell Scripts need to have execute permission if you just type the file name as you would a command • Alternative (new subshell): sh file • Alternative (current shell): • file

17. Shell Script Basics • Creating a Shell Script • Keyword Shell Variables • User Created Variables • Readonly Shell Variables

18. Creating a Shell Script • Use a text editor like vi • First line should start with #! Followed by the absolute pathname of the shell that is to interpret the script. (default is C shell) • #!/bin/sh • Lines which start with a # are comments • (except the special line mentioned above)

19. Keyword Shell Variables • HOME (contains login directory) • PATH (Used by shell to locate commands you type in) • /usr/bin:/usr/sbin:/class/n01/bin: • MAIL (contains name of central post office file for your mail) • PS1, PS2 (primary and secondary prompts)

20. Keyword Variables (continued) • CDPATH • like PATH, except used by cd command • TZ • timezone • IFS • Internal field separator. Blanks and tabs are default

21. User Created Variables • Create a variable by giving a name of your choice and an optional value • name=charlie • NO blanks around the equal sign!! • Remove variable • unset name • Keep variable but remove value • name=

22. Readonly Shell Variables • Two types: user created variable that has been declared to be readonly • readonly name • keeps later statements from changing the value • Special Shell variables • Positional Variables • Miscellanous variables

23. Positional Variables • $1 through $9 • Keep the first nine arguments entered after the name of the shell script • myscript aardvark dog cat • $1 will contain the word aardvark • $2 will contain the word dog • $3 will contain the word cat

24. Miscellaneous Variables • $* contains all arguments (not just the first one) • $@ similar to $*, except that it internally quotes each argument. • $# total number of arguments • $$ process id number (pid) of current process

25. Shift command • Promotes values of each positional variable to the left. • Contents of $1 go to ‘bit bucket’ • Contents of $2 go to $1 • Contents of $3 go to $2 • etc (etcetera, not etci)

26. Boolean Expressions • Algebra created by George Boole • Always evaluates to a binary state • Generally: • 1 is TRUE • 0 is FALSE

27. Boolean Operators • And • -a • Or • -o • Not • !

28. and Boolean Truth Tables -a True True True False True False False True False False False False

29. or Boolean Truth Tables -o True True True True True False False True True False False False

30. not Boolean Truth Tables ! True False False True

31. test ing,test ing,testing • testexpression or [ … ] • Evaluates the expression and returns a Boolean true or false. • expression can be simple or compound

32. Test Criteria • String expresions • Is null orLength is zero (-z) orLength is > 0 (-n) • string1 = or != string2

33. Test Criteria (cont.) • Filename expression • File exists and has specific attributes • -block-character-directory • -size – file has data (length >0)

34. Test Criteria (cont.) • Integer relationship • -gtgreater than • -ge greater than or equal to • -eqequal to • -ne not equal to • -le less than or equal to • -lt less than

35. Bourne - if, then • if • Establishes a control structure • Followed by a test command • then • Commands executed if test is true

36. Bourne – else, fi • else • Commands executed if test is false • fi • Terminates this if statement

37. Bourne - elif • elif • “else if” structure • Linear in nature

38. 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 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

39. Bourne – case • The case command tests for multiple values in a variable • Allows the use of “wild cards” • First match wins

40. case “$variable” in A|a ) echo Entered A or a ;; [0-9] ) echo Entered number ;; ?z* ) echo Entered z in 2nd position ;; esac

41. Examples: Myscript #!/bin/sh #-----------------------------------------------------------------# # Script Name: myscript # Written by: Charlie Verboom # Date: 9/19/97 # Purpose: demonstrate the use of # variables and displays upon terminal # Arguments: 3 arguments used to set # variables to be displayed #-----------------------------------------------------------------#

42. Myscript (continued) echo the first argument is $1 echo the second argument is $2 echo the third argument is $3 echo $# arguments were typed in echo the current process number is $$ echo script executed successfully> log.$$

43. Additions to myscript echo Please type in your name read name echo You typed in $name echo Please type in your age age=`head -1` echo Your age is $age