Nested if statements (that is, if statements within if statements) are cool and all, and will usually do the job nicely, but if your list of ifs for all possible outcomes gets bigger and bigger, then the code become unreadable quite quickly... case statements to the rescue!
case is an incredibly useful tool in your scripting arsenal, but so many sysadmins and bash developers don't use it to its full potential. Lets break it down here, and walk through a few of its uses to clear up some confusion,
case EXPRESSION in\n CASE1)\n COMMAND1\n COMMAND2\n ;;\n CASE2)\n COMMAND3\n ;;\n CASE3)\n COMMAND4\n COMMAND5\n COMMAND6\n ;;\n CASE4|CASE5)\n exit\n ;;\n *)\n OTHER COMMANDS HERE\n ;;\nesacLets break it down... (and dont worry if you dont get it at first look, all will become clearer further down)
\ncase EXPRESSION in
case is the command we are using ( the one we are learning about right now )EXPRESSION is the thing we want to test, it could be a string, a number, an input (eg: $1)in is a keyword for the case command to check if EXPRESSION is \"in\" the below listCASE1)
EXPRESSION matches CASE1 it will execute COMMAND1 and COMMAND2 below;; this symbol will tell case that it has finished executing the correct COMMANDS for this CASE and will now exit the case statement altogether. It's a very common mistake to forget to put ;; at the end of each CASE and can case some seriously unwanted code to execute so remember to put it there!
EXPRESSION didn't match CASE1, what now?well case will continue down the list of cases... so, onto the next CASE!
CASE2)
CASE1 then case will not execute any of the commands inside the CASE1 block, and will continue down the list to CASE2 looking for another match...CASE2 then it will execute COMMAND3 inside that block...case will continue looking further and further down the list until it finds a matchGreat question! thats where this line does its job...
\n*)\n OTHER COMMANDS HERE\n ;;
* symbol here is considered as a \"catch-all\", that is to say if no CASE matched our EXPRESSION then * will catch it, and any commands within the * block will execute. So we know every possible input will be caught somehow. * can be used to catch invalid input and return an error message to the user to wise-up ... (see below)and ....
\nesac ends the case statement, (its case backwards, geddit?), again its a common gotcha to forget to end with esac and it will make you crazy trying to figure out whats going wrong, so don't forget it!No problem my furry little friend.. a picture is worth a 1000 words... so lets look at 3000 words to see some examples of what we learned here...
\n\n
Above we can see if the user enters for user_input the value of \"4\" then the following code will execute
echo "you chose an even number, NICE!"user_input is equal to 2. Obviously 4 is not equal to 2, so this will not be true, and the if statement ends, as does the case statement, and we carry on with our lives...However if the user does enter \"2\" as user_input then this is the output of our script....\n\n
beautiful...
\n\n\nđź‘Ť Try to go ahead now and think how you can use
\ncasestatements insidecasestatements, to really fine-tune yourEXPRESSIONmatching
Have a look at this and think how it could help you automate some of your daily work as a sysadmin,\nall I will explain is that \"$1\" is a bash standard shortcut for the \"first value given after the script name...\" so in this example..
\nmy_terminal_prompt:> ./my_awesome_script.sh sean will help youand in this example
\nsystemctl status ntpso, keeping that in mind, have a look below...
\n\n
Now go and play my (script) kiddies! Have a great day!
\nIf Sean Helped You today, feel free to share this post or connect with us soon, available via gmail, slack or github.\nThanks for reading!
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