[bash] Using find command in bash script

Welcome to bash. It's an old, dark and mysterious thing, capable of great magic. :-)

The option you're asking about is for the find command though, not for bash. From your command line, you can man find to see the options.

The one you're looking for is -o for "or":

  list="$(find /home/user/Desktop -name '*.bmp' -o -name '*.txt')"

That said ... Don't do this. Storage like this may work for simple filenames, but as soon as you have to deal with special characters, like spaces and newlines, all bets are off. See ParsingLs for details.

$ touch 'one.txt' 'two three.txt' 'foo.bmp'
$ list="$(find . -name \*.txt -o -name \*.bmp -type f)"
$ for file in $list; do if [ ! -f "$file" ]; then echo "MISSING: $file"; fi; done
MISSING: ./two
MISSING: three.txt

Pathname expansion (globbing) provides a much better/safer way to keep track of files. Then you can also use bash arrays:

$ a=( *.txt *.bmp )
$ declare -p a
declare -a a=([0]="one.txt" [1]="two three.txt" [2]="foo.bmp")
$ for file in "${a[@]}"; do ls -l "$file"; done
-rw-r--r--  1 ghoti  staff  0 24 May 16:27 one.txt
-rw-r--r--  1 ghoti  staff  0 24 May 16:27 two three.txt
-rw-r--r--  1 ghoti  staff  0 24 May 16:27 foo.bmp

The Bash FAQ has lots of other excellent tips about programming in bash.