This only works accidentally. If the output of ls is not a terminal, then it is single-filename-per-line (and sorted alphabetically): $ ls nomatchfile1 no

This only works accidentally. If the output of ls is not a terminal, then it is single-filename-per-line (and sorted alphabetically): $ ls nomatchfile1 nomatchfile2 testfile1 testfile10 testfile2 testfile3 testfile4 $ ls | cat nomatchfile1 nomatchfile2 testfile1 testfile10 testfile2 testfile3 testfile4 The first argument to grep is the pattern, the following arguments to grep are the files to search. If there are any files specified, stdin is not read/searched. $ grep testfile1 testfile2 testfile3 $ ls | grep testfile1 testfile2 testfile3 Quoting affects whether you pass one or multiple arguments to grep: $ echo ${data} testfile1 testfile2 $ echo "$data" testfile1 testfile2 Finally, newlines in the pattern cause grep to treat each line as a separate pattern: $ ls | grep "$data" testfile1 testfile10 testfile2 Backticks alone do not quote their contents, so the expanded backticks output is treated as multiple arguments. etc. etc. lots of ways to mix this all up :)