how to manipulate array in shell script

I want my script to define an empty array. array values should be added if predefined condition gets true. for this what i have done is

declare -a FILES
file_count=0
if [ "$file_ext" != "$SUPPORTED_FILE_TYPE" ] ; then
        echo "$file_ext is not supported for this task."
else
        $FILES[$file_count] = $filename
        file_count=$file_count+1
fi

when executing this script i am getting some error like this

linux-softwares/lau开发者_StackOverflow社区nchers/join_files.sh: 51: [0]: not found

When settings data in array does not recall with $:

declare -a FILES
file_count=0
if [ "$file_ext" != "$SUPPORTED_FILE_TYPE" ] ; then
        echo "$file_ext is not supported for this task."
else
        FILES[$file_count]=$filename
        file_count=$file_count+1
fi

FILES without $.

This works for me:

#!/bin/bash
declare -a FILES
file_count=0

file_ext='jpg'
SUPPORTED_FILE_TYPE='jpg'
filename='test.jpg'

if [ "$file_ext" != "$SUPPORTED_FILE_TYPE" ] ; then
        echo "$file_ext is not supported for this task."
else
        FILES[$file_count]=$filename
        file_count=$(($file_count+1))
fi

As you see, a little modification $(( )) for math operation, but the FILES assignements is the same...

As pointed out after lots of tests, Ubuntu default shell seems to be dash, which raised the error.

To add an element to the end of an array, use the += operator (since bash 3.1 in 2004):

files+=( "$file" )

you can write it this way as well

declare -a FILES
file_count=0
if [ "$file_ext" != "$SUPPORTED_FILE_TYPE" ] ; then
        echo "$file_ext is not supported for this task."
else
        FILES[((file_count++))]=$filename
fi

To: Vijay

tiny demonstration, list *.txt files in directory and put to array FILES

declare -a FILES
i=0
for file in *.txt
do
  FILES[((i++))]=$file 
done
# display the array
for((o=0;o<${#FILES};o++))
do
    echo ${FILES[$o]} $o
done

output

$ ./shell.sh
A.txt 0
B.txt 1
file1.txt 2
file2.txt 3
file3.txt 4