| (pipe operator)

command1 | command2 [| command3 ...]

The pipe operator is used in bash to chain multiple commands together. The two commands are run sequentially and the ouput of command1 becomes the input of command2.

$ ls | wc -l
3

Useful Options / Examples

ls | grep

$ ls | grep hello
hello.cpp

Break it down

• The pipe operator is often used to connect a command and grep to search for things in output. In the above example, ls would usually output “hello.cpp goodbye.cpp” - this output is passed to grep, which searches it for the string “hello”and outputs the matching word, “hello.cpp”.

cat | wc

$ cat * | wc -l
10390

Break it down

• The pipe will take the output from cat * (the text of every file in the current folder) and pass it to the wc command to count the number of lines in that output.

sort | uniq

$ sort breakfast.txt | uniq
bacon
cheese
eggs
sausage

• If we assume breakfast.txt contains the following:
  • eggs
  • bacon
  • cheese
  • sausage
  • eggs
• the pipe operator sorts the file alphabetically and passes the output to uniq, which requires sorted input. Uniq then filters out the repeated lines (in this case, “eggs”) and prints out the resulting sorted list.