Understanding default parameters

Many times, we may pass certain parameters from the command line, but, sometimes, we may not pass any parameters at all. We may need to initialize certain default values to certain variables.

We will review this concept through the following script.

Create script default_argument_1.sh, as follows:

#!/bin/bash 
MY_PARAM=${1:-default} 
echo $MY_PARAM 

Execute the script and check the output:

$ chmod +x default_argument_1.sh One$ ./default_argument_1.sh OneOne$ ./default_argument_1.shdefault

Create another default_argument_2.sh script:

#!/bin/bash 
variable1=$1 
variable2=${2:-$variable1} 
echo $variable1 
echo $variable2 

The output is as follows:

We executed the script two times:

1. When we passed two arguments, then variable1 was $1 and variable2 was $2.
2. In the second case, when we passed only one argument, then $1 was taken as the default argument for $2. Therefore, variable1 was used as the default for variable2. If we do not give a second parameter, then the first parameter is taken as the default for the second parameter.