Compare variable to array in Bash script

Jackoalltrades · 04-26-2017, 02:59 PM

I am relatively inexperienced writing bash scripts and I'm trying to develop a script to take user input to create AWS CloudWatch metrics. Part of this script requires checking the input to confirm it is expected input that the rest of the script can use. I'm comparing the input to strings and figuring creating an array with all of the comparison values would be fastest, but I am having a difficult time formulating the if loop to determine if the variable provided by the user matches an element in the array.

Code:

echo "Please select the unit of measurement: Percent, Gigabytes, Count, etc. Make sure first letter is capitalized."
read UNIT
unitarray=(Seconds Microseconds Milliseconds Bytes Kilobytes Megabytes Gigabytes Terabytes Bits Kilobits Megabits Gigabits Terabits Percent Count Bytes/Second Kilobytes/Second Megabytes/Second Gigabytes/Second Terabytes/Second Bits/Second Kilobits/Second Megabits/Second Gigabits/Second Terabits/Second Count/Second)
if [[ ! $UNIT =~ $unitarray ]]; then
        echo "Please select a unit of measurement from the following list:"
        printf '%s\n' "${unitarray[@]}"
        exit 1
fi

This is the current iteration of the if loop. It works if the UNIT variable is "Seconds" but if I input any other string (like Microseconds or Bytes or Farts) it spits out the full list, meaning it didn't find it in the list, even using strings that do exist.

I've read a few dozen forum posts and read any number of instructional pieces on working with arrays, but I can't quite get it to work.

Thanks in advance for any assistance.

michaelk · 04-26-2017, 04:17 PM

Can't see the forest for the trees... Look at the output of $unitarray vs ${unitarray[@]} i.e.

Code:

echo $unitarray

echo ${unitarray[@]}

Which is why your string match only works for Seconds. The other gotcha is that a match will be true for any occurrence unless you include the field separator i.e a space". Since your a beginner at script writing try this:

Code:

if [[ ! " ${unitarray[@]} " =~ " ${UNIT} " ]]; then

This is only one of many ways to accomplish a string comparison.

BW-userx · 04-26-2017, 05:23 PM

if you notice in @michaelk post the use of { } curly brackets. it is a got a be to show the contents of inside an array. to get the everything in the array{@] is used
array.length for conditional in a loop is better then a hard coded number because that gives the number you want to end with if going through the entire array elements. Because what you stated is in error, This is the current iteration of the if loop.

"if [conditions]" are not loops.

Code:

for (( i = 0 ; i < "${#array[@]}" ; i++ ))
{
      echo "${array[i]}"
}

point is the use of the # and @ in an array. Along with { curly brackets }

this too can be taken care of so you do not have to worry about Caps in first letter words.

Code:

echo "Please select the unit of measurement: Percent, Gigabytes, Count, etc. Make sure first letter is capitalized."

Code:

if [ "${UNIT,,}" =~ "${unitarray[@],,}" ]; then

this lower cases everything.
example

Code:

"Solid" Variable

userx%slackwhere ⚡ ~ ⚡> var="TheEW Thoe"
userx%slackwhere ⚡ ~ ⚡> var2=${var,,}
userx%slackwhere ⚡ ~ ⚡> echo $var2
theew thoe

an Array
userx%slackwhere ⚡ ~ ⚡> var3=(t t t t R ewe Trush)
userx%slackwhere ⚡ ~ ⚡> var4=${var3[@],,}
userx%slackwhere ⚡ ~ ⚡> echo $var4
t t t t r ewe trush
userx%slackwhere ⚡ ~ ⚡>

example 2

Code:

userx%slackwhere ⚡ ~ ⚡> var="HeLLo YoU"
userx%slackwhere ⚡ ~ ⚡> var1=(HELLO you)
userx%slackwhere ⚡ ~ ⚡> [[ "${var,,}" =~ "${var1[@],,}" ]] && echo "hey got a match" 
hey got a match

userx%slackwhere ⚡ ~ ⚡> [[ "${var1[@],,}" =~ "${var,,}" ]] && echo "hey got a match"
hey got a match

as you can see on either end it still matches.

Now to nit pic your code. So you hopefully to pick up on a few things.
Hope you don't mind.
your code

Code:

echo "Please select the unit of measurement: Percent, Gigabytes, Count, etc. Make sure first letter is capitalized."
read UNIT
unitarray=(Seconds Microseconds Milliseconds Bytes Kilobytes Megabytes Gigabytes Terabytes Bits Kilobits Megabits Gigabits Terabits Percent Count Bytes/Second Kilobytes/Second Megabytes/Second Gigabytes/Second Terabytes/Second Bits/Second Kilobits/Second Megabits/Second Gigabits/Second Terabits/Second Count/Second)
if [[ ! $UNIT =~ $unitarray ]]; then
        echo "Please select a unit of measurement from the following list:"
        printf '%s\n' "${unitarray[@]}"
        exit 1
fi

can you justify the need for the use of an array when you can just make that a hard coded string?

because of this read UNIT that is the real variable yes? Whereas what you are using as an array is hard coded and will not change during run time, YES?

Code:

unitarray=(Seconds Microseconds Milliseconds Bytes Kilobytes Megabytes Gigabytes Terabytes Bits Kilobits Megabits Gigabits Terabits Percent Count Bytes/Second Kilobytes/Second Megabytes/Second Gigabytes/Second Terabytes/Second Bits/Second Kilobits/Second Megabits/Second Gigabits/Second Terabits/Second Count/Second)

which can be written just using " " quotes and the same conditional comparison used without an array.

Code:

unitarray="Seconds Microseconds Milliseconds Bytes Kilobytes Megabytes Gigabytes Terabytes Bits Kilobits Megabits Gigabits Terabits Percent Count Bytes/Second Kilobytes/Second Megabytes/Second Gigabytes/Second Terabytes/Second Bits/Second Kilobits/Second Megabits/Second Gigabits/Second Terabits/Second Count/Second"

too

Code:

read -p "Please select the unit of measurement: Percent, Gigabytes, Count, etc." UNIT

puts it on one line. less for the interpreter to evaluate. Faster run times. But you may not even need that whole string anymore. So I Shortened It In This Example.

one more thing, the use of string and substring when comparing strings. where to put which one?

Ok if you want to know this then pls read it.
test script

Code:

#!/bin/bash

substring="Terabits"

unitarray="Seconds Microseconds Milliseconds Bytes Kilobytes Megabytes Gigabytes \
Terabytes Bits Kilobits Megabits Gigabits Terabits Percent Count Bytes/Second Kilobytes \
/Second Megabytes/Second Gigabytes/Second Terabytes/Second Bits/Second Kilobits/Second Megabits/Second \
 Gigabits/Second Terabits/Second Count/Second"

echo "$unitarray"

if [[ ${unitarray,,} =~ ${substring,,}  ]] ; then
	echo "got a match"
else
	echo "nO match"
fi

notice the use of the \ to split the line so you can move it down onto another line and still have it read as one complete line. Along with no use of an array.

results

Code:

userx%slackwhere ⚡ scripts ⚡> ./LongString
Seconds Microseconds Milliseconds Bytes Kilobytes Megabytes Gigabytes Terabytes Bits Kilobits Megabits Gigabits Terabits Percent Count Bytes/Second Kilobytes /Second Megabytes/Second Gigabytes/Second Terabytes/Second Bits/Second Kilobits/Second Megabits/Second  Gigabits/Second Terabits/Second Count/Second
got a match

Jackoalltrades · 05-08-2017, 02:50 PM

Both of these responses were very helpful in understanding some of the differences between using strings and arrays.

Thank you both very much!

chrism01 · 05-10-2017, 08:55 PM

If you're going to mess with arrays, I highly recommend this http://wiki.bash-hackers.org/syntax/arrays