Pipes

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
• Basically after the | there should be a command, and NOT A FILE
• Pipe diagram

Examples

• Bash commands
• Bash commands - Commands used with pipe

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
  • wc -l and cat can also read from stdin
    • related: What about stdin
• ls -l | tail -n3
  • show the last 3 lines of ls -l
• ls -1 | tail -n8 > test/ls.txt
  • get the last 8 lines from ls -1 and put that on test/ls.txt

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.