Overview

stdin, stdout, And The Pipe

More information

• I/O Redirection And Pipes

• Good Ol’ Unix philosopy

  A program does just one thing, and it does that well!

• Combining programs via the pipe

  • Connect one program’s standard output with another program’s standard input

  • ⟶ multiple tools connected toether in a pipeline

• Most text/line based

  • Configuration files

  • Logfile

  • Programm source

  • …

stdin, stdout: Examples

• Counting lines from stdin

  $ wc -l
  line 1
  line 2
  line 3
  ^D
  3
  

  ^D: end of file when stdin is terminal/console

• Counting lines from /etc/passwd

  $ wc -l < /etc/passwd
  47
  

  wc reads /etc/passwd from stdin, arranged by the shell

• Finally: redirecting stdout

  $ wc -l > num-lines
  line 1
  line 2
  line 3
  ^D
  $ cat num-lines
  3
  

Pipe: Examples

• How many groups am I member of?

  $ cat /etc/group | grep jfasch | wc -l
  2
  

  Equivalent (though more efficient):

  $ grep jfasch < /etc/group | wc -l
  2
  

  Equivalent:

  $ grep jfasch /etc/group | wc -l
  2
  

• Number of #include directives in Linux kernel source

  $ find ~/work/linux -name '*.[hc]' -exec cat {} \; | grep '^#include' | wc -l
  325324
  

  Equivalent (though more efficient):

  $ find ~/work/linux -name '*.[hc]' | xargs cat | grep '^#include' | wc -l
  325324
  

Basics Text Tools

Command	Description
cat	Write file content to stdout
head	Write first couple of line to stdout
tail	Write last couple of line to stdout
cut	Cut out fields from lines (according to configurable field separator)
less	Page file content to stdout
sort	Sort lines and write to stdout
uniq	Eliminate consecutive equal lines (ideally, in a pipe after sort)
grep	Filter lines by regular expression