Variables

• Prefixed with "$" (when used as r-values)

Special variables

• $# : The number of command-line arguments
• $@ : The list of command-line arguments.
• $1..$9 : The first through ninth positional parameters (arguments) (can access additional arguments by enclosing the number in curly braces)
  
• $? : The return value of the most recently executed command.

Command-line arguments

shift

shift [n]

• Shifts the positional parameters by n (the default value of n is 1).

getopts

while getopts [letter[:]]+ <varname>; do
    commands;
done

• Iterates through a loop once for each command-line option that matches one of the letters in the specified set of letters.
• Stores the matching letter in the specified variable.
• If the letter is specified with a colon following it, then the command-line option is expected to have an argument, separated from it by a blank space, which will be stored in the special variable OPTARG.

• Each time through the loop, the variable OPTIND is used to store the index of the next command-line argument to be processed.
• OPTIND is not automatically reset by the getopts command.  Before using getopts again in the same shell invocation, the variable OPTIND must be manually reset.

# <script> -a -b apple -c
while getopts ab:c option; do
    echo "Option: $option $OPTARG"
done

OPTIND="1"    # Manually reset the $OPTIND variable
while getopts ab:c option; do
    echo "Option: $option $OPTARG"
done

Script variables

<varname>=<value>

• By default, variables defined in a shell script are only available within that script.

greeting="hello"
name="Buford"
echo "$greeting $name"

Local variables

local <varname>[=<value>]

• Creates local variables for a function.

local dest="/usr/bin"

Commands

declare

declare [-xi] varname[=value]

• Declares a variable
• -x : Exports the variable for use by subshells (child scripts)
• -i : Treats the variable as an integer

export

export [-n] <varname>[=<value>]

• Allows child scripts to access a variable.
• A copy of the variable will be created for each new subshell generated (for each child script).

export source="/usr/local/bin"
export -n dest    # Make the variable no longer be exported

readonly

readonly [varname]*

• Marks the variables as read-only

eval

eval [arguments]+

• Concatenates the arguments and runs them as a single command
• Arguments with blank spaces remain intact (they're not broken up into multiple arguments)