1. Shell Scripting
March 1st, 2004
Class Meeting 7

2. Shell Script (Program)
What is a shell script?
A series of shell commands placed in an ASCII text file
Commands include . . .
anything you can type on the command line
shell variables
control statements (if, while, for, )

3. Script Execution
Provide script as an argument to the shell program (i.e. bash my_script)
Or specify which shell to use within the script
First line of the script: #!/bin/bash
Make sure that the script is executable
Run directly from the command line
No compilation is necessary

4. Simple Script #!/bin/bash echo “Hello, World!” cd ~ pwd Output:
/home/grads/callgood

5. Shell Variables Numeric Strings Arrays
var refers to the name of the variable, $var refers to the value
var=100 # sets the value to 100
(( var2=1+$var ))
Variable names begin with an alphabetic character and can include alpha, numeric, and the underscore

6. Numeric Variables
Integer variables are the only pure numeric variables that can be used in bash
Declaration and setting value
declare –i var=100
Numeric expressions are enclosed in double parentheses
(( var+=1 ))
Operators are the same as in C/C++
+, -, *, /, %, &, |, <, >, <=, >=, ==, !=, &&, ||

7. String Variables
Unless variables are explicitly declared as another type, they are considered to be strings
var=100 makes the var the string 100
However, placing the variable within double parentheses will treat it as an integer
(( var2=1+$var ))

8. String Variables (cont)
Using substrings
${string:n}
${string:5} # first five chars
${string:-2} # last two chars
${string:n:m}
${string:0:4} # first to fifth
${string:1:3} # second to fourth
${#string} # length of string
Concatenating strings
string_var1=“$string_var1 $string_var2”

9. Array Variables
Array is a list of values – do not have to declare size
Reference a value by ${a[index]}
${a[3]} # value in fourth position
$a # same as ${a[0]}
Use the declare –a command to declare an array
declare –a sports
sports=(basketball football soccer)
sports[3]=hockey

10. Array Variables (cont)
Array initialization
sports=(football basketball)
moresports=($sports tennis)
or ${array[*]} refers to the entire contents of the array
echo ${moresports[*]}
Output: football basketball tennis

11. Command Line Arguments
If arguments are passed to a script, they can be referenced by $1, $2, $3, . . .
$0 refers to the name of the script
refers to all of the arguments – not an array – space separated group of strings
$# refers to the number of arguments

12. Output and Quoting
echo command is used to output information to standard out
echo –n does not print a newline
Shell interprets $ and `` within double quotes
$ - variable substitution
` - command substitution
echo “`date +%D`” # 03/01/04
Shell does not interpret special charactes within single quotes
echo ‘`date +%D`” # `date +%D`

13. Redirecting Output to File
Redirecting output to a file is the same as on the command line
Examples
echo “$var” > $OUTFILE # overwrite
echo “$var” >> $OUTFILE # append
String expansion
echo $’\n\n\n’
echo $’\t’

14. Conditions If using integers: (( condition ))
If using strings: [[ condition ]]
Examples
(( a == 10 ))
(( b >= 3 ))
[[ $1 = -n ]]
[[ ($v != fun) && ($v != games) ]]
Special conditions for file existence, file permissions, ownership, file type, . . .

15. Conditions (cont) [[ -e $file ]] – File exists?
[[ -f $file ]] – Regular file?
[[ -d $file ]] – Directory?
[[ -L $file ]] – Symbolic link?
[[ -r $file ]] – Read permission?
[[ -w $file ]] – Write permission?
[[ -x $file ]] – Execute permission?

16. If Statement
Syntax
if condition
then
statements
elif
else fi
optional

17. If Statement (cont) Example if [[ -r $file ]] then
echo “$file is readable”
elif [[ (-w $file) && (-x $file) ]]
echo “$file is writeable and exectuable” fi

18. For Loops Syntax If list is omitted, $@ is assumed
for var [in list] do
statements
done
If list is omitted, is assumed
Otherwise ${list[*]}, where list is an array variable

19. For Loops (cont) Example colors=(yellow red blue green orange)
for color in ${colors[*]} do
echo $color
done

20. While Loops
Syntax
while condition do
statements
done
The keywords break, continue, and return have the same meaning as in C/C++

21. Case Statements Syntax case expression in pattern1) statements ;;
... *)
esac

22. Case Statements (cont)
Example
case $1 in
-a)
# statements related to option a ;;
-b)
# statements related to option b *)
# all other options
esac

23. Command Substitution
Use the command of a command in a variable, condition, . . .
`command`
$(command)
Example
files=(`ls`)
for file in ${files[*]} do
echo $file
done

24. Functions Syntax function function_name { statements . . . }
Functions can take arguments, in the same way that the script takes arguments (i.e. $1 represents first argument, $2 the second, )

25. Functions (cont) Example function add_two{ (( $sum=$1+$2 ))
return $sum }
. . .
# calling a function
add_two 1 3
echo $?
$? represents the return value of the last function call or command
Function definition needs to occur before the function is called in the script

26. Assignment Suggestions
One approach to the script:
Handle arguments, setting flags for each
i.e., if recursive option is given, set a variable that indicates that it was set
Write part of script that generates the makefile in a function; call this function recursively, if necessary (i.e. if recursive option was set)

27. Assignment Suggestions (cont)
Within function, store the output of ls command in an array; loop through this array and test filename of each file in the current directory for valid filenames and/or extensions
files=(`ls`)
for file in ${files[*]} do
# no need to check directories
if [[ -f $file ]]
then
# test for valid filename and/or extensions fi
done

28. Assignment Suggestions (cont)
Remember that you need to know all of the valid filenames before you can begin writing any of the makefile; you may need to store parts of the output in temporary files, and then concatenate these files into your makefile; be sure that you remove and temporary files that you create