Bash for loop syntax explanation

Question

I am working in Bash shell, Linux OS. I have the following for loop:

for ((i=0;i<${#listModels[@]};i++))
do
var=${listVersion[$i]}
if [ ${!var} ]
then
    export MY_LIBRARY_PATH=$MY_LIBRARY_PATH:$ROOT_PATH/${listModels[$i]}/${listModels[$i]}_${!var}
else
    echo ">>>> No ${listModels[$i]} version! <<<<"
fi
done

Before this, I have the following in the script:

listModels=(model1
model2
model3)

listVersion=(MODEL1
MODEL2
MODEL3)

The concept is MODEL1,MODEL2 and MODEL3 keep changing every now and then, for eg it becomes MODEL1.1,MODEL2.2,.. And I expect the script to modify my path everytime the listVersion values change. But could anybody explain how exactly the for loop functions (especially [@] and [$i] stuff). I am new to Bash and I know only the basic commands. Thanks in advance!

choroba · Accepted Answer · 2016-05-23 15:37:21Z

3

As documented in man bash, ${#array[@]} returns the number of elements in the array. Similarly, ${array[$i]} (better written as ${array[i]}) returns the $i-th element of the array.

#! /bin/bash
array=( a b c )
echo Size: ${#array[@]}
echo First: ${array[0]}
echo Second: ${array[1]}
echo Last: ${array[-1]}  # Negative index counts from the right!

answered May 23, 2016 at 15:37

choroba

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2 Comments

sjsam Over a year ago

IIRC negative indexing is allowed from bash 4.2

Müller Over a year ago

The expression within the if statement if [ ${!var} ] means i-th element does not exist?

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