Create the constants variable

• You can create the constants variables using the readonly command or declare command.
• The readonly buitin syntax is as follows:

readonly var
readonly varName=value

• The declare builtin syntax is as follows:

declare -r var
declare -r varName=value

Example

• Create a constant variable called DATA and make its value always the same throughout the shell script i.e. it can't be changed:

readonly DATA=/home/sales/data/feb09.dat
echo $DATA
/home/sales/data/feb09.dat
DATA=/tmp/foo
# Error ... readonly variable

• You cannnot unset (delete) the readonly variable:

unset DATA

Sample outputs:

bash: unset: DATA: cannot unset: readonly variable