Shell scripts

1
Chapter 14

• Shell scripts
• Putting it all together

2
The Concept of Scripts

• A script is nothing more than an executable file
  of shell commands
• If you can do it on the command line, you can do
  it in a script.
• Best working model
• Work in the C shell as your interface
• Have your scripts run in the Bourne shell

3
Examination of Shells for Scripting

• You can write scripts in any shell
• The Bourne shell is best for scripting because
• It is the standard shell for system
  administration
• All flavors of Unix have a Bourne shell or the
  equivalent
• The Bourne shell was the first shell

4
Command line execution vs. execution in a script

• You dont need to write scripts.
• You can keep reinventing the wheel over and over
  and over on the command line.
• However, if you want to be efficient you will
  discover that if you have to do the same thing
  more than once, building a script to do it will
  save you lots of time.

5
Scripts are tools

• This is a critical concept.
• When you build a script, you craft a tool to
  perform a specific task
• Once you have the tool, you can use it to easily
  perform the same task over and over.

6
Forcing Execution in a Specific Shell

• The shebang character ! as the first character
  of a file, directs the shell to fork a process
  and run the command that follows the shebang
• The rest of the file is used as input to the
  program run from the first line.
• It is possible to use this construct to run other
  programs besides the shell.

7
Scripting standards

• You should always write your scripts to a
  standard.
• If your instructor has a standard use that
• If not, use the standard in the book

8
Shell variables as they apply to scripting

• The shell has a collection of variables available
  for your use
• You can also create your own variables
• If you want to use the contents of a shell
  variable, you must precede the variable name with
  a
• In the Bourne shell, you may not put white space
  around the assignment operator

9
Shell variables an example

• You simply assign a value to a variable with the
  sign
• your variables should be lower case
• unless you export them, shell variables are only
  known the the shell that created them

10
readonly

• You can set a shell variable to be readonly
• Once a variable has been marked as readonly, it
  cannot be changed
• There is no un-readonly command

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Useful shell variables

• count of command line arguments
• ? Return status of the most recently executed
  command
• The process ID
• ! Process ID of the last background process

• All command line args in one long string
• Each command line arg in its own string
• 0 leftmost part of the command line, the command
  or script
• n (1-9) the command line args. There are only 9
  addressable Bourne shell command line arguments

12
set

• You can use set to change the values of the
  command line arguments
• It changes , , _at_, as well as 1 -n
• It is not used often but it is useful for testing

13
env

• displays the environmental variables
• The env command can change the value of the
  Bourne shell environmental variables

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export

• The export command will make the values of user
  variables available to shells forked from the
  current shell
• It is the way you can pass variables from a
  parent process to a child process

15
Commands Used in Scripts

• The commands that follow are use to create the
  skeleton upon which other commands are hung
• You will use all of the commands you have learned
  in the text at one time or another in scripts.
• Shell scripting is considered by some to be the
  best possible programming environment

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echo

• Used as the standard output command
• Usually forces a newline at the end of the text
• The -n option suppresses newlines in csh
• The \c output specifier suppresses newlines in
  the Bourne shell
• The \n and \t and others are also available for
  formatting

17
echo an example
18
sh

• You can invoke the shell outside the script and
  use it as a debugging tool
• -v (verbose) shows all of the lines of the script
• -x (executed) prepends a to the front of every
  line that is executed
• You can also imbed those options in the shebang
  line

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sh an example
20
Simple if

• Allows conditional execution
• The required format is if test
  condition then command1
  command2 fi

21
Simple if example
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Simple if alternate form

• Allows conditional execution
• The required format is if condition
  then command1
  command2 fi
• This form is NOT preferred since it can be
  confusing. Please avoid this form

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if then else

• This form of the if will always take some action.
  
• If the condition is true the code after the then
  is executed, (called the if or then block)
• If the condition is false the code after the else
  is executed, (called the else block)

24
if then else syntax

• The syntax of the if/then/else is if test
  condition then commands if condition
  is true else commands if condition is
  false fi

25
if then else example
26
Nested if

• Adding one more verb allows us to create very
  complex and confusing constructs called nested
  ifs
• Only one of the instances can be true
• Be very careful when constructing nested ifs,
  consider a case structure instead

27
Nested if syntax

• The syntax of the nested if if test
  condition then done when condition
  1 is true elif test condition 2
  then done when condition 2 is true
  else done if neither condition true fi

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Nested if example
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test

• The test command can evaluate 3 types of
  conditions
• file conditions
• string comparisons and conditions
• numeric comparisons
• The test command is worth focused study, the more
  you know about test the better your scripting
  will be

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testing file conditions

• The format is if test -cond file
• Some examples of -cond
• -d (the file is a directory)
• -f (the file is a regular file)
• -r (the file is readable)
• -s (the file has a size greater than 0)
• -w (the file may be written to)
• -x (the file is executable)

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testing string comparisons

• The following are the string comparisons used
  with the test comand
• -n string (true if string has contents)
• -z string (true if the string has no contents)
• s1 s2 (true if string 1 equals string2)
• s1 ! s2 (true if string 1 doesnt equal string
  2)
• string (true if string is not empty)

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Testing numeric relationships

• The numeric comparisons are what you would
  expect
• n1 -eq n2 (the two numbers are equal)
• n1 -le n2 (first number is less than or equal
  second)
• n1 -lt n2 (first number is less than the second)
• n1 -ge n2 (first number greater or equal the
  second)
• n1 -gt n2 (first number is greater than the
  second)
• n1 -ne n2 (the two numbers are not equal)

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A common logic error

• Many logic errors are caused when scriptors
  forget that there are three possible
  relationships between any two numbers
• the first is greater than the second
• the first is less than the second
• the two numbers are equal
• It is this last situation that many coders forget.

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An example of a common error
35
case

• The case structure adds structure to Bourne shell
  scripting
• Use the case anytime you have more than 2 nested
  ifs
• Do not use case to replace a series of ifs, only
  to replace nested ifs
• Only one of the patterns in a case will match

36
Nested if vs. case
37
Complex case condition example
38
read

• Besides taking input from the command line via
  the command line arguments, you can prompt the
  user and read data from the keyboard
• The shell parses the input stream on the normal
  field separator
• Each variable is given one string, with the last
  variable taking everything that is left

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read example
40
exit

• As you already know, the exit command will exit
  from the current shell
• You can use the exit command to set a return code
  or return status
• Good structure requires only one exit from any
  script

41
A sample exit script
42
A sleepy script
43
command substitution

• Command substitution allows you to store the
  output of a command or pipe in a variable.
• It is very useful for many applications
• You can create huge values if you are not careful
  about what you store
• Use the grave marks not apostrophes around
  the commands

44
command substitution example
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expr

• In the Bourne shell, all math is integer math
• The tool you use to do math is expr
• You must put whitespace between each part of the
  command
• If you use variables with expr, those variables
  must have a value

46
expr examples
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Other uses for expr

• Expr can also perform
• relational operations
• , !, gt, lt, gt, and lt
• logical operations
• whether one of the arguments is empty
• whether the arguments contain the same data
• if the second argument contains the first
  argument
• While expr can perform these tests, this type of
  usage is very uncommon

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for

• The for loop is a counted loop with the following
  format for var in list do
  code you want iterated done
• The variable var will take on each of the values
  in the list

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A simple sample for
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A common use for for
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A special form of for
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A special form of for

• Using the for variable form of for,
  you can access more than 9 command line
  arguments!
• This is one of two tricks to get around the 9
  addressable arguments situation.

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continue

• The continue command stops processing the current
  value of the loop variable, and restarts the loop
  with the next value.
• This is a way to skip processing a particular
  record

54
continue example
55
break

• The break command exits from the loop, passing
  control to the command following the end of the
  loop
• If there is no command following the loop, break
  causes the script to exit
• If you want to exit from the script, use the exit
  command

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break example
57
while

• The while loop is the conditional loop of choice
  since it evaluates the condition before executing
• The while loop has the following syntax while
  condition do what to iterate
  done

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while example
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More interesting while example
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shift

• The preceding while example showed you the second
  way to access by name more than 9 command line
  arguments
• The shift command performs a left shift logical
  on the command line arguments, moving the value
  of 1 out, the value of 2 into 1, the value of
  3 into 2, and so on.
• Notice that the previous script always addresses
  1, and that value changes

61
An elegant way to pass data

• If you need to pass the value of a variable to a
  script, you can use it on the command line, or
  you can send the data down a pipe with the echo
  command.
• Often the echo method is more efficient

62
A data passing example
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Congratulations!

• You have survived the entire course.
• Go out and script
• and most of all
• Have Fun