Advanced redirects and file descriptors

In the lesson on basic file-descriptors (04-pipes) you already saw >, >>, and |. Here are five idioms that show up in any serious bash script and somehow rarely get explained together:

• 2> and 2>&1: working with stderr separately
• tee: write to a file and keep the pipeline going
• <(cmd): process substitution, a command as a file
• here-doc and here-string: data right in the script
• exec N>file: custom FDs for long-running logs

All of these operators work by manipulating the process's integer file descriptors. There are three by default: 0 (stdin), 1 (stdout), 2 (stderr). The redirect 2>file means "open file and make it fd=2 for the command".