13.2.4 Shell redirection

Basic redirection to remember (here the [n] is an optional number to specify the file descriptor):

     [n]> file      Redirect stdout (or n) to file.
     [n]>> file     Append stdout (or n) to file.
     [n]< file      Redirect stdin (or n) from file.
     [n1]>&n2       Redirect stdout (or n1) to n2.
     2> file  >&2   Redirect stdout and stderr to file.
      > file 2>&1   Redirect stdout and stderr to file.
     | command      Pipe stdout to command.
     2>&1 | command Pipe stderr and stdout to command.

Here,

• stdin: standard input (file descriptor = 0)

• stdout: standard output (file descriptor = 1)

• stderr: standard error (file descriptor = 2)

The shell allows you to open files using the exec built-in with an arbitrary file descriptor.

     $ echo Hello >foo
     $ exec 3<foo 4>bar  # open files
     $ cat <&3 >&4       # redirect stdin to 3, stdout to 4
     $ exec 3<&- 4>&-    # close files
     $ cat bar
     Hello

Here n<&- and n>&- mean to close the file descriptor n.