leejunkim

Pipes

3 min read

What

  • A mechanism that redirects the standard output (stdout) of one command into the standard input (stdin) of another command
  • So you can chain multiple commands together and build more complex functionalities
  • General syntax: command | command | .... | command
  • Motivation
    • Without pipes, how do we count the number of files in a directory?
    • ls > output.txt, wc -l output.txt, rm output.txt
    • the output.txt should be in a different folder because ls will count it too
    • Use pipes, it makes your life easier by allowing you to combine multiple programs together
    • no need for temporary file
  • Pipe diagram

Examples

Example 1

  • ls | wc -l
  • ls prints the files to stdout, then the pipe redirects the stdout is redirected to the stdin to the 2nd program wc -l. Then the result will be printed
  • ls | cat

Example 2

leejun@leejun-VirtualBox:~/Desktop$ du -h text.txt not-exist.txt 2>&1 >/dev/null
du: cannot access 'not-exist.txt': No such file or directory
 
leejun@leejun-VirtualBox:~/Desktop$ du -h text.txt not-exist.txt 2>&1 > /dev/null | wc -l
1
  • 1st command - redirects error to current stdout (terminal) then redirects stdout to /dev/null
  • 2nd command - you can use this to count the number of errors
    • > /dev/null → Discards stdout, but stderr is still printed to the terminal (as it was redirected to previous current stdout (terminal)).
    • the pipe receives this stderr input for wc -l
    • You’re not piping from /dev/null—you redirected only stdout there. The remaining stderr output is still available for piping.

Common programs to use with |

tee

  • With | and the tee command, you can create a stdout and write it to a file at the SAME time!
  • Example
    • echo 'Hello Bash' | tee hello.txt
      • You want Hello Bash to be printed to stdout and also written into hello.txt
    • echo 'Hello Bash' | tee hello.txt | wc -c
      • multiple pipes
      • counting the text
  • Appending
    • echo 'Hello Bash' | tee -a hello.txt

ping

  • Allows us to send a ping packet to another server (ex. a remote server).
  • We can use to test if our connection is reachable or disrupted
  • Example
    • ping google.com
      • sends a ping packet to google.com
    • ping non_existing.com 2>&1 | tee ping.txt
      • Redirects stderr (2) to stdout (1), so both error messages and normal output are now part of stdout.
      • The tee command takes stdin and writes it to both terminal (stdout) and ping.txt

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