Option parsing with getopts in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of getopts in bash scripting?

getopts helps scripts read and handle command-line options (flags) easily and consistently.

It parses options like -a or -b value so your script can respond accordingly.

beginner

How do you specify that an option requires an argument in getopts?

In the options string passed to getopts, add a colon : right after the option letter.

For example, getopts "a:b" means -a needs an argument, -b does not.

intermediate

What variable does getopts use to store the current option letter?

getopts stores the current option letter in the shell variable given as its second argument (typically named opt).

OPTARG holds the argument value for options that require one, while OPTIND tracks the next index to process.

intermediate

What does the variable OPTIND represent in getopts usage?

OPTIND is the index of the next argument to be processed by getopts.

It starts at 1 and increments as options are parsed, helping your script know where options end and positional arguments begin.

beginner

Write a simple getopts loop to handle options -a (no argument) and -b (requires argument).

while getopts "ab:" opt; do
  case $opt in
    a) echo "Option a triggered" ;;
    b) echo "Option b with value: $OPTARG" ;;
    *) echo "Invalid option" ;;
  esac
done

In getopts, how do you indicate that an option requires an argument?

A Add a colon ':' after the option letter in the options string

B Add a semicolon ';' after the option letter

C Use uppercase letters for options with arguments

D Put the option letter inside square brackets

Adding a colon ':' after the option letter tells getopts that this option needs an argument.

Which variable holds the argument value for an option in getopts?

A OPTARG

B OPTIND

C OPTVAL

D OPTOPT

OPTARG stores the argument value for options that require one.

What does OPTIND represent in getopts?

A The total number of options parsed

B The current option letter

C The index of the next argument to process

D The exit status of the last command

OPTIND tracks the next argument index to be processed by getopts.

What happens if getopts encounters an unknown option?

A It throws a syntax error

B It stops the script immediately

C It ignores the option silently

D It sets the option variable to '?'

getopts sets the option variable to '?' when it finds an invalid option.

Which command correctly starts a getopts loop for options -x and -y with -y requiring an argument?

A while getopts "xy" opt; do ... done

B while getopts "xy:" opt; do ... done

C while getopts "x:y:" opt; do ... done

D while getopts "x:y" opt; do ... done

Option y requires an argument, so it is followed by a colon ':' in the options string.

Explain how to use getopts to parse command-line options in a bash script.

Think about how you tell the script which options need arguments and how you handle each option inside the loop.

Describe the roles of OPTARG and OPTIND when using getopts.

One stores values, the other tracks position.

Practice

1. What is the main purpose of the getopts command in bash scripting?

easy

A. To read input from a file

B. To parse command-line options and their arguments easily

C. To execute commands in parallel

D. To create new files automatically

Solution

1. Step 1: Understand the role of getopts

   getopts is a built-in bash command designed to handle command-line options and their arguments in scripts.

2. Step 2: Compare with other options

   Other options like executing commands or reading files are not related to option parsing, which is the specific task of getopts.

3. Final Answer:

   To parse command-line options and their arguments easily -> Option B

4. Quick Check:

   getopts = option parsing [OK]

Hint: Remember: getopts reads options, not files or commands [OK]

Common Mistakes:

• Confusing getopts with file reading commands
• Thinking getopts runs commands in parallel
• Assuming getopts creates files

2. Which of the following is the correct syntax to start parsing options with getopts in a bash script?

easy

A. while getopts "ab:" opt; do case $opt in a) echo "Option a" ;; b) echo "Option b with $OPTARG" ;; esac; done

B. for opt in getopts "ab:"; do echo $opt; done

C. if getopts "ab:" opt; then echo $opt; fi

D. getopts -a -b

Solution

1. Step 1: Identify correct getopts usage

   The correct way to use getopts is inside a while loop with a case statement to handle each option.

2. Step 2: Check each option

   The correct syntax uses while getopts "ab:" opt; do ... case $opt in a)... b)... esac; done. Other options use invalid control structures like for or if, or incorrect command syntax.

3. Final Answer:

   while getopts "ab:" opt; do case $opt in a) echo "Option a" ;; b) echo "Option b with $OPTARG" ;; esac; done -> Option A

4. Quick Check:

   while + getopts + case = correct syntax [OK]

Hint: Use while loop with getopts and case for options [OK]

Common Mistakes:

• Using for loop instead of while
• Missing case statement for options
• Incorrect getopts command syntax

3. What will be the output of the following script if run as ./script.sh -a -b value?

while getopts "ab:" opt; do
  case $opt in
    a) echo "Option a set" ;;
    b) echo "Option b set with $OPTARG" ;;
  esac
done

medium

A. Option a set
Option b set with value

B. Option a set
Option b set with -b

C. Option b set with value

D. No output

Solution

1. Step 1: Analyze the command line arguments

   The script is called with -a and -b value. Option a has no argument, b requires an argument.

2. Step 2: Trace getopts parsing

   getopts processes the arguments sequentially. It first encounters -a, sets $opt to a, and echoes Option a set Option a set Option b set with -b set with value set. Next, -b value sets $opt to b and $OPTARG to value, echoing Option a set Option b set with -b set with value.

3. Final Answer:

   Option a set Option b set with value -> Option A

4. Quick Check:

   Options a and b with argument = output A [OK]

Hint: Remember colon means option needs argument [OK]

Common Mistakes:

• Confusing option argument with option name
• Ignoring option arguments
• Assuming options print nothing

4. Given this snippet:

while getopts "ab:" opt; do
  case $opt in
    a) echo "Option a" ;;
    b) echo "Option b with $OPTARG" ;;
    ?) echo "Invalid option" ;;
  esac
done

What is the problem if the script is run with -b but no argument?

medium

A. getopts will crash with syntax error

B. getopts will ignore -b and continue silently

C. getopts will treat -b as option without argument and print "Option b with"

D. getopts will set opt to '?' and print "Invalid option"

Solution

1. Step 1: Understand getopts behavior for missing argument

   If an option that requires an argument (like b:) is given without one, getopts sets opt to '?' to indicate an error.

2. Step 2: Check case handling

   The case statement has a '?' option that echoes "Invalid option". So the script will print that message.

3. Final Answer:

   getopts will set opt to '?' and print "Invalid option" -> Option D

4. Quick Check:

   Missing argument triggers '?' and error message [OK]

Hint: Missing argument triggers '?' option in getopts [OK]

Common Mistakes:

• Assuming getopts crashes on missing argument
• Thinking option is accepted without argument
• Ignoring '?' case in script

5. You want to write a script that accepts options -f (file name) and -v (verbose flag). Which getopts string and case structure correctly handle these options, where -f requires an argument and -v does not?

hard

A. getopts string: "fv"; case: v) verbose=1 ;; f) file=$OPTARG ;;

B. getopts string: "fv:"; case: f) file=$OPTARG ;; v) verbose=1 ;;

C. getopts string: "vf:"; case: v) verbose=1 ;; f) file=$OPTARG ;;

D. getopts string: "vf"; case: f) file=$OPTARG ;; v) verbose=1 ;;

Solution

1. Step 1: Determine correct optstring

   Option -f requires an argument, so it must be followed by a colon: f:. Option -v is a flag, no colon needed. The correct string has ':' immediately after 'f', such as "vf:". Strings like "fv:" place ':' after 'v', making -v require an argument, which is wrong; missing ':' means no arguments expected.

2. Step 2: Match case statement to options

   Case must handle v by setting verbose flag and f by assigning $OPTARG to file variable. getopts string: "vf:"; case: v) verbose=1 ;; f) file=$OPTARG ;; matches this correctly.

3. Final Answer:

   getopts string: "vf:"; case: v) verbose=1 ;; f) file=$OPTARG ;; -> Option C

4. Quick Check:

   Option with colon needs argument, flag no colon [OK]

Hint: Put colon after options needing arguments only [OK]

Common Mistakes:

• Placing colon after flag option
• Swapping option letters and arguments
• Not assigning $OPTARG for options needing arguments