Parameters

Arguments to a script are called parameters. You can access the parameters using the special variables $0, $1, $2, and so forth. $0 contains the name of the script, $1 contains the first parameter, $2 contains the second parameter, and so forth.

The special variable $# contains the total number of parameters.

Here is a simple script which shows you what parameters have been received:

#!/usr/bin/bash​
echo "Number of parameters:  $#"​
echo "Parameter 0:           $0"​
echo "Parameter 1:           $1"​
echo "Parameter 2:           $2"​
echo "Parameter 3:           $3"​
echo "Parameter 4:           $4"

When you run this script with three parameters (red, green, and blue), you get this output:

$ ./params red green blue​
Number of parameters:  3​
Parameter 0:           ./params
Parameter 1:           red​
Parameter 2:           green​
Parameter 3:           blue​
Parameter 4:

The shift command discards parameter $1 and moves each of the remaining parameters to the previous position (so the value in parameter $2 is moved to $1, and $3 is moved to $2). We can modify the previous script to demonstrate this:

$ cat params2​
#!/usr/bin/bash​
echo "Number of parameters:  $#"​

echo "Parameter 0:           $0"​
echo "Parameter 1:           $1"​
echo "Parameter 2:           $2"​
echo "Parameter 3:           $3"​
echo "Parameter 4:           $4"

echo "---- Performing shift ----"
shift

echo "Parameter 0:           $0"​
echo "Parameter 1:           $1"​
echo "Parameter 2:           $2"​
echo "Parameter 3:           $3"​
echo "Parameter 4:           $4"

$ ./params2 red green blue
Number of parameters:  3
Parameter 0:           ./params2
Parameter 1:           red
Parameter 2:           green
Parameter 3:           blue
Parameter 4:           
---- Performing shift ----
Parameter 0:           ./params2
Parameter 1:           green
Parameter 2:           blue
Parameter 3:           
Parameter 4:  

The shift command is useful for looping through parameters, and for accessing parameters higher than number 9.