Bash – a simple explanation for how pipes work in Bash


I often use pipes in Bash, e.g.:

dmesg | less

Although I know what this outputs, it takes dmesg and lets me scroll through it with less, I do not understand what the | is doing. Is it simply the opposite of >?

  • Is there a simple, or metaphorical explanation for what | does?
  • What goes on when several pipes are used in a single line?
  • Is the behavior of pipes consistent everywhere it appears in a Bash script?

Best Solution

A Unix pipe connects the STDOUT (standard output) file descriptor of the first process to the STDIN (standard input) of the second. What happens then is that when the first process writes to its STDOUT, that output can be immediately read (from STDIN) by the second process.

Using multiple pipes is no different than using a single pipe. Each pipe is independent, and simply links the STDOUT and STDIN of the adjacent processes.

Your third question is a little bit ambiguous. Yes, pipes, as such, are consistent everywhere in a bash script. However, the pipe character | can represent different things. Double pipe (||), represents the "or" operator, for example.