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Comment by pjot | Hacker News Reader

To illustrate why, consider a file name with spaces.

    filename="my file.txt"
    cat $filename   # Incorrect: Tries to 'cat' two files: 'my' and 'file.txt'

    cat "$filename" # Correct: Treats 'my file.txt' as one file

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Additionally,

    $ f=-x
    $ cat "$f"
    cat: invalid option -- 'x'

    $ cat -- "$f"
    cat: -x: No such file or directory
    # ^-- correct, but:

    $ f=-
    $ cat -- "$f"
    reading stdin...
    reading stdin...
    ^C

    $ f=./-
    $ cat -- "$f"
    cat: ./-: No such file or directory

...better to glob with ./* than *

Also for sanitizing user input.

  input=$1  # User input, potentially dangerous
  rm $input # Incorrect: Risky if input contains something like '*'

  rm "$input" # Correct: Safer, treats the user input as a single item

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