Bash Index : G - The 'G' Bash commands : description, flags and examples

1. Each time it is invoked, getopts places the next option in the shell variable name, initializing name if it does not exist, and the index of the next argument to be processed into the variable OPTIND. OPTIND is initialized to 1 each time the shell or a shell script is invoked.
2. When an option requires an argument, getopts places that argument into the variable OPTARG. The shell does not reset OPTIND automatically; it must be manually reset between multiple calls to getopts within the same shell invocation if a new set of parameters is to be used.
3. When the end of options is encountered, getopts exits with a return value greater than zero.

• which options to expect :
  • fAx means that options -f, -A and -x are expected
• which options must have an argument, by appending them a trailing colon (:) :
  • fA:x means that -A has an argument, i.e. the command line will look like myScript.sh -A something
  • This value is mandatory and will cause an error message when missing, unless silenced (see next point).
• how to handle errors :

  	verbose mode	silent mode
  enabled with	enabled by default	a leading colon :fA:x
  $OPTARG on invalid option	unset, which means : you have no option to handle this yourself getopts does it for you by displaying an error message like : ./test.sh -z ./test.sh: illegal option -- z	the invalid option character

#!/usr/bin/env bash

while getopts ':a' opt; do
	case "$opt" in
		a)
			echo '-a was triggered!'
			;;
		\?)
			echo "Invalid option: -$OPTARG"
			;;
	esac
done

./test.sh -z

Invalid option: -z

./test.sh -a

-a was triggered!

#!/usr/bin/env bash

while getopts ':a:' opt; do
	case "$opt" in
		a)
			echo "Value of parameter '$opt' : '$OPTARG'"
			;;
		\?)
			echo "Invalid option: -$OPTARG"
			;;
	esac
done

./test.sh -z

Invalid option: -z

./test.sh -a

(nothing)

./test.sh -a 12

Value of parameter 'a' : '12'

./test.sh -a 12 34

Value of parameter 'a' : '12'

#!/usr/bin/env bash

options=':ab'

if (! getopts "$options" opt); then			Read more about this below
	echo 'no option given'
	exit 1
fi

while getopts "$options" opt; do
	case "$opt" in
		a)  echo "'-a' was triggered!" ;;
		b)  echo "'-b' was triggered!" ;;
		\?) echo "Invalid option: '-$OPTARG'"; exit 1 ;;
	esac
done

getopts "$options" opt || { echo 'no option given'; exit 1; }

./test.sh
no option given			as expected

./test.sh -ab
'-b' was triggered!		where's the a ?

./test.sh -aab
'-a' was triggered!		Houston, we've had a problem 
'-b' was triggered!

• count lines having an a
  echo -e "apple\nbanana\ncoconut" | grep -c a

  2

• same method to (try to) count o
  echo -e "apple\nbanana\ncoconut" | grep -c o

  1

  Because only 1 line has an o (albeit several occurrences within the line)
• solution
  echo -e "apple\nbanana\ncoconut" | grep -o o | wc -l

  2

• and it actually returns the total number of occurrences within the input
  echo -e "apple\nbanana\ncoconut" | grep -o a | wc -l

  4

echo -e 'a\nb\nc' | grep 'b'; echo $?

b
0

echo -e 'a\nb\nc' | grep -q 'b'; echo $?

0

echo -e 'a\nb\nc' | grep 'z'; echo $?

1

echo -e 'a\nb\nc' | grep -q 'z'; echo $?

1

echo -e 'a\nb\nc' | grep ; echo $?

Usage: grep [OPTION]... PATTERN [FILE]...
Try 'grep --help' for more information.
2

echo -e 'a\nb\nc' | grep -q; echo $?

Usage: grep [OPTION]... PATTERN [FILE]...
Try 'grep --help' for more information.
2

• result found + no error = 0
• no result found + no error = 1
• if error : 2

1. Backup : getfacl -R /path/to/dirContainingFilesWithPermissionsToBackup > /path/to/permissions.txt
2. Play with file permissions
3. Restore : setfacl --restore=/path/to/permissions.txt

local account :

domain account :

• grep ^myGroup: /etc/group
• getent group myGroup

myGroup:x:myGroupId:joe,jack,william,averell