getopts Command in Linux with Examples
getopts is a very convenient bash script utility, which helps you to conveniently and gracefully handle the passing of flags(short version), and arguments in a clean, standardized manner. Instead of having the developer manually handle the flags passed to a script, this provides a handy way of handling this when iterating through a while loop.
Prerequisites: It is recommended to have a basic idea of the following topics:
- Basic Understanding of bash script.
- Flags in bash.
- Arguments in bash.
Syntax:
getopts optstring name [arg]
To get the syntax through terminal:
getopts
Working with getopts command
1. Printing the help section: Enter the following command to print the help section of the getopts command.
getopts --help
The above command will print the details of the command along with some parameters and options that could be used to run the command.
2. Usage in a script: getopts could be used in a script in Linux as well. The following script depicts the same.
#!/bin/bash
while getopts ":a:bc:" flag;do
echo "flag -$flag, Argument $OPTARG";
done
This script runs a while loop, which iterates through the arguments that match with given optstring, in this case, “a:bc:”, and stores the value of the flag in the variable flag. If the flag had any associated argument, it is stored in a variable OPTARG.
The optstring works in the following way:
- For every option letter, getopts stores the option in the variable flag(declared just after the optstring), and iterates the loop.
- Every option letter followed by a colon expects an argument, which is stored in the variable OPTARG.
- If getopts was expecting an argument, but could not parse one, it prints an error. If it was not expecting one, OPTARG will be initialized to “”(an empty string).
- If the very first character of the optstring was “:”(a colon), the error message is not printed
3. Running the script by passing the arguments as expected.
./getoptsDemo.sh -a argA -b
Note: OPTARG for -b is blank, and not its previous value since it is initialized for every iteration. Also, -c is not passed, which is not an issue, as each flag in optstring is optional.
4. Running the script by skipping the parameter for a flag where it is required: This prints an error message. The flag is initialized to “?”, and OPTARG is “.
./getoptsDemo.sh -a
5. Running the script by adding a flag not present in the optstring: This prints an error message. Again, the flag is initialized to “?”, and OPTARG is “”.
./getoptsDemo.sh -d
If the first character was “:”, getopts goes into silent mode, the error messages are not printed, and the behavior changes in some cases. Consider the optstring “:a” for the following example-
6. Skipping the parameter for a flag where it is required: The flag is initialized to “:”, and OPTARG is set to the option character.
./getoptsDemo.sh -a
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