CSc 352 Shell Scripts

1. CSc 352Shell Scripts Saumya Debray Dept. of Computer Science The University of Arizona, Tucson debray@cs.arizona.edu

2. What is a shell script? • A shell script is a list of commands to be run by a shell • basically a program, but using shell commands instead of C or Java statements • Why? • automate repetitious tasks • e.g.: testing a program on a large set of test inputs • package up commonly executed command sequences • create our own commands

3. Creating and executing a shell script • Create a file, say “foo”, containing the commands to be executed by the script • Execute the script: • invoke the appropriate shell with foo as argument, e.g.: • bash foo • csh foo • chmod a+x foo ./foo

4. A simple example • a shell script to print “hello world!” the “echo” command, as executed from the command line contents of the shell script file “hello_world.script” this script behaves the same for a variety of different shells with appropriate permissions, the script can be executed directly

5. Syntax issues across shells Different shells have slightly different syntax rules • the same script may not work on all shells: script contents different behaviors

6. Which shell runs my script? my current shell is csh the script executes as expected when csh is invoked explicitly if not invoked explicitly, the default shell /bin/shis used

7. Solution: specify shell in the script #!pathToShellon first line specifies which shell to use executes as expected when invoked in csh executes as expected when invoked in bash

8. Specifying the shell interpreter • ‘#!’ at the beginning of a file indicates that it is a script • The rest of the first line specifies (absolute) path to the interpreter • What if the interpreter is at a different location on a different system? • use /usr/bin/env to improve portability: #!/bin/bash #!/usr/bin/envbash

9. Variables • Variables different than in C or Java: • don’t have to be declared in advance • they are untyped: the same variable can hold an integer value or a string • Syntax for using variables (bash): • Defining the value of a variable X: X=value • Using the variable X: $X or ${X}

10. Example variable definitions variable uses undefined variable evaluates to empty string

11. Guarding against uninitialized variables -u : “exit if using an unitialized variable” • -u : • works if we specify /bin/bash • but not with /usr/bin/env

12. Guarding against uninitialized variables

13. Bash scripts: whitespace without whitespace  the command works as expected • with whitespace  • weird error message • variable value not set as expected

14. Different kinds of quotes back quotes: evaluates to the output of the enclosed commands single quotes: variables not expanded double quotes: variables inside the quotes are expanded

15. A simple scripting problem • Problem: • Iterate over the files in a given directory: • for each file, express its size in human-readable form (i.e., using KB and MB where appropriate): • give 2 decimal places for MB sizes; 1 decimal place for KB sizes • functionality similar to using “ls –lh” • Requires: • list files in a directory [valid directory?  if-then-else] • iterate over these files • obtain the size of each file • perform arithmetic [KB/MB decision  if-then-else]

16. The test command • Used to guide control flow in conditionals and loops • Syntax: testoperand1 operator operand2 testoperand • often abbreviated to: [ operand1 operator operand2] [ operand ] • see “man test” for a list of operators • Returns a nonzero status if the test is true; zero o/w

17. Example: test

18. Example: if-then-else if-then-else command

19. Example: if-then-else need a ‘;’ between the test expression and ‘then’

20. Example: if-then-else

21. Alternative formulation of test

22. For Loops • Syntax 1: for X inword ; do cmds done Convenient for iterating over lists • Syntax 2: for (( e1; e2; e3 )) ; do cmds done Convenient for iteration involving arithmetic expressions

23. For Loops: Example 1

24. For Loops: Example 2

25. Iterating over lists: pitfalls ! print out the first couple of lines of a file listing

26. Iterating over lists use quotes to prevent splitting at whitespace Note: the loop iterates only once – not once per file!

27. Arithmetic arithmetic evaluation doesn’t happen automatically; must be specified explicitly get file size (see “man stat” for other file properties)

28. Arithmetic integer arithmetic only! arithmetic evaluation: $(( expr ))

29. Arithmetic get bc to do the floating point math for us

30. Command-line arguments • Arguments at known positions, e.g., 1st, 2nd, ..., can be accessed using $1, $2, … • $0 = the name of the script file • Total no. of arguments: $# • However, we can’t get to the ith argument this way • $i  value of variable i

31. Example

32. Iterating over the argument list

33. Getting at the entire list of arguments

34. Iterating over the argument list Placement of quotes matters: all of the arguments are seen as a single word

35. Iterating over the argument list Placement of quotes matters:

36. Arguments with whitespace • $@ is similar to $*, but each argument is a quoted string • no interpretation or expansion • allows us to properly access arguments that may contain whitespace • needs to be quoted “…”

37. Arguments with whitespace

38. Debugging

39. Debugging: -v prints shell input lines as they are read

40. Debugging: -v these are the lines from the script that are being executed Note that variable values etc. are not being shown

41. Debugging: -x prints out command traces before they are executed

42. Debugging: -x variable values, command arguments are printed out

43. Debugging: summary • Debugging can be done by observing what commends get executed • “set –v” • prints out shell commands as they are read in • does not print out runtime variable values • “set –x” • prints out a trace of the command before it is actually executed • prints out variable and argument values • Debugging can be turned off using “set +v”, “set +x” • useful for localizing which parts of the script are observed

44. String manipulation • shell scripts often involve string manipulation • e.g., to pattern match on file names, create new files based on existing files, etc. • bash gives extensive facilities for string matching and manipulation • see “man expr”

45. A sample problem Write a bash script to convert Unix make files to “mymake” make files unix make mymake a : b.oc.od.o gcc *.o –o a b.o : b.cb.h gcc –Wall –c b.c @target a : b.oc.od.o @cmdgcc *.o –o a @target b.o : b.cb.h @cmdgcc –Wall –c b.c 

46. Attempt 1 • Ignore definitions • assume make file does not have stuff like “CC = gcc” • Assume • user has added whitespace where necessary • e.g.: on either side of ‘ : ’ in rules • all dependencies have been made explicit • Issues: • need to process a line at a time • lines may contain whitespace • need to check whether a line starts with a ‘\t’

47. Attempt 1

48. Attempt 1 IFS = “Internal Field Separator” used by bash to split the input into words the new definition of IFS means that the input is now split at newlines identify lines that begin with a tab

49. Attempt 1

50. Attempt 2 • relax assumption about whitespace around : • want: • input: “a.o: a.c b.h c.h” • output: “@target a.o : a.c b.h c.h” • Issue: • replace ‘:’ by ‘ : ’ • bash offers two replacement operators: • ${str/substr/replacement} • replaces the first occurrence of substr in str with replacement • ${str//substr/replacement} • replaces all occurrences of substr in str with replacement