Generic Variables
Like any language, variables allow you to store data temporarily in memory and reference it later in your script.
Variable assignment
Assignment is as follows:
#!/bin/bash
x=5
Variable Substitution
The name of a variable is a placeholder for its value, the data it holds. Referencing (retrieving) its value is called variable substitution.
There are two ways to reference a bash variable:
1) Simplified notation:
$x
2) Standard notation:
${x}
If you try to output the variable x without using variable substitution, you’ll simply output the name of the variable.
Length of Variables
Bash has a simple method of allowing you to get the length of a variable. For example, it provides the number of characters in a string, or the number of indices in an array.
If x is the name of my variable, the syntax for length of a string is:
${#x}
If y is the name of an array variable, the syntax for array length is:
${#y[*]}
Constants
Constants are created by making a variable read-only. The readonly built-in keyword marks each specified variable as unchangeable.
The syntax is as follows:
readonly OPTION VARIABLE(s)
Example:
readonly HELLOWORLD='Hello, World!'
Again, once a readonly variable is assigned a value, it cannot be changed.
Using the declare built-in
The syntax for declare is the following:
declare OPTION(s) VARIABLE=value
| Option | Meaning |
|---|---|
| -a | declare an array type |
| -f | Use function names only |
| -i | Treat the variable as an integer; arithmetic evaluation is performed when the variable is assigned a value |
| -p | Display the attributes and values of each variable. When –p is used, additional options are ignored. |
| -r | Make this variable read-only. These variables cannot then be assigned values by subsequent assignment statements, and they cannot be unset. |
| -t | Gives each variable the trace attribute |
| -x | Mark each variable for export to subsequent commands via the environment. |
Using + instead of - turns off the attribute instead. When used in a function, declare creates local variables.
Examples
#!/bin/bash
# integers
declare -i i=5+5
# arrays
declare -a fruits=(apples oranges bananas)
# readonly (constant)
declare -r constant="taxes"
# Output the values
echo $i
echo ${fruits[@]}
echo $constant
Special Variables
Exit Status
The ? variable holds the exit status of the previously executed command (the most recently completed foreground process).
Any command or program or script that executed successfully without errors is supposed to return an exit code of 0. We say “supposed to” because of course the exit code isn’t enforced in all cases, but it would be non-standard and wrong to not do so.
Examples:
Testing exit status in an if statement
Here is an example written directly from the command-line.
[user@localhost]$ if [ $? -eq 0 ]; then echo 'Process exited normally.'; fi
Process exited normally.
[user@localahost]$
A Foreground process exiting successfully
[user@localhost]$ ls
Desktop Documents Downloads Music Pictures Public Templates Videos
[user@localhost]$ echo $?
0
[user@localhost]$
A Foreground process exiting unsuccessfully
[user@localhost]$ lv
bash: lv: command not found...
[user@localhost]$ echo $?
127
[user@localhost]$
Script Arguments
This will be covered in more detail in the input section, but suffice it to say that any variable whose name is a number will refer to an argument passed to the current script (or the script name itself).
Also, any variable whose name is @ or * will refer to an array containing all arguments passed to the script.