You’ve already used the STDIO redirection operators: <, >, 2>, 2>&1, and so on. You may also have used the appending operators: >> and 2>>, and maybe the most modern ones: &> and &>>, to redirect both STDOUT and STDERR at the same time.

These operators cover most of what you need day-to-day. But have you ever wondered what they actually do — why > affects output, < affects input, and 2> affects errors? The answer is file descriptors, and understanding them turns a collection of punctuation into a coherent system.

Coming from almost any other programming language, you’d expect if to work on boolean expressions. You write something that evaluates to true or false, and the if decides which branch to take.

Bash works in a way that is almost, but not quite, entirely unlike that.

In Bash, if runs a command and checks its exit status. Zero means success (the “then” branch runs). Non-zero means failure (the “else” branch runs, if present).

Pop quiz: what characters can appear in a filename on Linux?

If you answered “letters, numbers, dots, dashes, underscores,” you’re thinking of sensible filenames. The actual answer is: every character except / (path separator) and NUL (the zero byte).

That includes spaces. Tabs. Newlines. Quote marks. Backslashes. Asterisks. Emoji. Control characters. Bell characters that make your terminal beep. All perfectly legal in filenames.

And that’s a problem, because most UNIX tools assume newlines separate records. When a filename contains a newline, tools get confused: