Bash redirection explanation

Sang -

Intuitively, there are 3 concepts in file descriptor in bash: file descriptor number, file descriptor, and the file.

File descriptor numbers are 0, 1, 2, ..., which usually refers to a file descriptor or nothing.
A file descriptor is an object issued by operating system to access a file.
A file is a piece of data, can be a file on a hard disk, a TCP stream, a UDP stream, ...

File can be seen as a warehouse which contains goods ( data), A warehouse can be attached with many (or zero) gate ( file descriptor). The gate is issued (making, destroying) by the manager ( OS). Each gate has a key ( file descriptor number) in order to use/open the door to access good (data) from/to the warehouse. A key can be used to access at most one gate.

With this understanding, this command command 2>&1 should be interpreted as following. And you will recognize that there is nothing to do with redirection.

  • 2>&1 is a file descriptor duplication operation. Read more about all other operations here.
  • &1 refer the file descriptor denoted by file descriptor number 1.
  • Duplicate this file descriptor and assign it to the file descriptor number 2.

With the warehouse, gate, key metaphor: 2>&1 means

  • Find the key number 1
  • Find the gate associated with this key
  • Duplicate this gate. In other words, issues the manager to create another gate which can be used to access the the same warehouse as the gate in the previous step.
  • Assign number 2 to the key associated with the new gate

Another example

N<&M-: duplicate the file descriptor denoted by file descriptor number M, assign to file descriptor number N, then destroy the old descriptor.

Or this can be interpreted as move the file descriptor denoted by file descriptor number M to the file descriptor number N.