Various Bash guides recommend putting quotes around just about everything. I had a script that contained this line:
sudo apt-get install --yes $(cat "$BUILD/install_on_aws_ubuntu.txt")
While refactoring, I put another set of quotes around the $(cat ...) out of an abundance of caution:
sudo apt-get install --yes "$(cat "$BUILD/install_on_aws_ubuntu.txt")"
Several other changes later, I couldn’t figure out why my script had stopped working.
Here’s what happened:
$ cat $HOME/tmp/demo.txt foo bar quux $ echo $(cat "$HOME/tmp/demo.txt") foo bar quux $ echo "$(cat $HOME/tmp/demo.txt)" foo bar quux
The new outer quotes retained the newlines; the original replaced newlines with spaces.
The Bash FAQ has more: specifically Command Substition and Word Splitting.