Variables and User Input

1. VARIABLES AND USER
INPUT

2. VARIABLES
• A variable is a names storage location in memory that can hold values.
• Three types of variables used by the shell:
• Environmental – Also called special shell variables, hold information about the computer
system that the shell needs to operate correctly.
• Ex. PATH, HOME, etc.
• Positional Parameters – Used by the shell to store values of command line arguments.
• Ex. Set ‘date’ – stores each component in a different variable, Day/Month/Year
• User Defined – variables that the user declares and assigns.
• These are the variables that you worked with in CIS120.

3. ENVIRONMENTAL VARIABLES
• The environmental variables provide information to the shell about the way your
account is set up.
• Some of these variables are set automatically when you log in.
• Others you may have to set yourself, which is usually done in the initialization file.
• Whenever you run a shell script, it creates a new process called a subshell and your
script will get executed using that subshell.
• A Subshell can be used to do parallel processing.

4. POSITIONAL PARAMETERS
• Positional parameters or automatic variables, are variables the shell assigns for you
automatically and are read-only.
• They assign the values of the command line arguments that are being used for any program.
• There are 10 positional parameters numbered, 0 – 9
• The $ is a special character to the shell to indicate when to substitute the stored value inside a
variable.
• To display positional parameters make sure to use the echo command.
• Arguments passed to the script from the command line at runtime:
• $1 $2 $3 … $9
• The command invoked : $0
• All the positional parameters passed to the script: $*
• The number of positional parameters passed to the script: $#
• The return value of the last command executed: $?
• 0 represents success
• Non 0 represents an error or failure

5. EXAMPLE
#! /bin/bash
#script that illustrates PP with the date command
set `date`
echo “Time: $4 $5”
echo “Day: $1”
echo “Date: $3 $2 $6”
*Note

6. EXAMPLE
#! /bin/bash
#script that illustrates PP with the date
command
echo “My name is: $1”
echo “My favorite # is: $2”
• ./Test.sh Jason 32
*Note

7. USER DEFINED VARIABLES
• User defined variables are variables created by the user and then assigned a value.
• Question: What are some reasons to create your own variables? What would you store in
there?
• Shell variables begin with an alphabetic or _ and are followed by alphanumeric or _
characters
• variable = value
• Unlike most other programming languages, the shell has no concept whatsoever of
data types. Whenever you assign a value to a shell variable, no matter what it is, the
shell simply interprets that value as a string of characters.
*Note

8. EXAMPLE

9. OTHER TIPS
• If a command is stored in a variable to execute the command just reference the
variable with a $, do not include echo.
• $stuff
• To include a command with piping or options enclose it in ` `.
• command=`who |sort`
• The difference between:
• ‘ ‘ – string literal
• “ “ – interpets , $ and ` `
• ` ` - execute and insert, command substituion

10. GETTING INPUT FROM THE USER
• read -p
• echo
read