Agenda

1. Agenda Tools for Shell Scripting – Part I • Quoting Special Characters • Backslash \ , single quotes ‘’, double quotes “” • Variables • (user-created variables, Keyword Shell variables, Read-only Shell variables (a.k.a. positional parameters)

2. Special Characters • A special character is used to give special meaning to a file or string when used with a UNIX command or when executing shell scripts. • For example: • redirection (>, >>, < , 2> , 2>> , <) • ambiguous file reference ( * , ? , [ ] ) • etc…

3. Special Characters • Sometimes, conflicts arise when you use a Unix command to perform something with a special character as an argument, and you get a different result that you anticipate. • For Example: • echo *(you might think that this will display an asterisk * but a listing of all files will be displayed – not what you might have expected!) • What happened? • The shell thinks “*” means to match all or nothing and the this command will display all files.

4. Quoting text • To make the shell “ignore” or “turn-off” the meaning of the special character when using you use quotes. • Types of Quotes: • Single Quotation marks: echo ‘*’ • Double Quotation marks: echo “*” • Backslash: echo \* • Note: A backslash must appear for every occurrence of a special symbol – Your instructor will provide examples in class the advantages and limitations associated with each of these quotation methods.

5. Variables • So far, we discussed that shell scripts are simply a file consisting of Unix OS commands to be run automatically. • Shell scripts can become more powerful if the user could input data (either in the form of an argument or input from the keyboard). • Several variables are available to provide Shell scripts this power.

6. User-Created Variables • A variable that the user creates within the shell script. • A user-created variable must begin with a letter. • You must not include white-space on either side of the equals sign • For example: • cat >test.file • name=Murray • <CTRL><D>

7. User-Created Variables • In order to access or display the value of the variable, a dollar sign $ must appear before the variable name - otherwise, it would be interpreted as text. • For Example: • cat > test2.file • name=Murray • echo $name • <CTRL><D>

8. User-Created Variables • You can use single or double quotes to assign an entire string of text • For Example: • name=‘Murray Saul’ • name=“Murray Saul” • Note 1: Single or double quotes need to “ignore” or “turn-off” shells interpretation that space or tab between words are separate arguments!! • Note 2: Single quotes (‘ ’) and backslash (\) will turn-off meaning of special characters including the variable name eg. echo ‘$name’ (Double quotes will allow variable to display value!!)

9. User-Created Variables • To remove a previously stored variable, assign a “null” value to the variable or use unset command • Example: • name= • unset name

10. User-Created Variables • The read command is used to pause and prompt the user to enter a line of text. • Example: • echo –n “Enter your full name” • read fullname • echo $fullname echo –n means to prompt input to read on same line as displayed text.(this works in Linux OS)

11. Shell Variables • There are two categories of shell variables: • Keyword Variables • Variables that have been assigned an easy to remember word that are commonly used • Examples include PATH, USER, PWD, HOME, etc… • Read-only Shell Variables(Positional Parameters) • Variables that are assigned by a command (set) as opposed to assigning by using the equal sign (=) • Examples include $1, $2, $3, … $9

12. Keyword Shell Variables • Keyword Shell variables are used by the shell and many of these variable have values already assigned to them. • Keyword Shell variables are usually identified as UPPER-CASE letters. • The user can see the values assigned to these Keyword Shell variables by issuing the set command without an argument. • Some of these Keyword Shell variables can be changed by the user, some are assigned by the system and cannot be changed.

13. Keyword Shell Variables • Keyword shell variables can be used in the shell script with Unix commands to “customize” the script for the particular user. • Examples: • echo “Hi there, $USER” Displays current user’s name. • echo $PWD Displays user’s current directory. • mkdir $HOME/dir1 Creates a directory called dir1 that “branches-off” the user’s home directory.

14. Read-Only Shell Variables(Positional Parameters) • Read-Only shell variables have the feature to a script’s arguments (when script is issued like a command) into variables to be used within the script when it is running! • You can use the set command to assign values to these read-only shell variables inside the script as well (added feature!). • You can also store standard output from commands issued while script is running to into variables (this is called “command substitution” which is covered later).

15. Read-Only Shell Variables • Read only variables range from $0 to $9 • $0 is script name (can also represent shell name if set command is issued as command from shell prompt (ie. not running a script!) • $1 to $9 are first nine positional parameters, or command line arguments • Read-Only Shell Variables need to be set • For Example (type from shell prompt) • set name1 name2 name3 echo $0 = setecho $1 = name1echo $2 = name2echo $3 = name3

16. Read-Only Shell Variables • Try to determine output from the following commands: • set one two three • echo $0 $1 $2 • set “2 1” • echo $1 $2 • set name1 name2 name3 • echo $1 $2

17. Read-Only Shell Variables • Read-Only Shell Variables are also referred to as “Positional Parameters”. • The term “Positional” in Positional Parameters means that the arguments are stored in a particular position(i.e. $1 - $9)

18. Positional Parameters • $0 is name of command used to call script $1 to $9 are first nine positional parameters • $* represents all positional parameters as one long string • "$*" will put single pair of double quotes around string • $@ same as $*, except "$@" will put double quotes around each parameter • $# contains number of parameters

19. Positional Parameters • can also access beyond $9 by using set braces, eg. ${10} will access 10th argument set, etc... • ${var} can be used instead of $var - useful if followed by alpha-numerics • $? is the exit status of the last command, 0 means successful script can be ended with exit n where n is the exit status or condition code of the script

20. Positional Parameters • shift shifts parameters left, so that the second one which was $2 becomes the first which is $1, etc., allowing parameters beyond the first nine to be accessed • Arguments "falling off" the left side are no longer available, so should be saved in other variables if they are required later in the script • shift nn, where nn is a number, has the same effect as that number of shiftcommands