Pipes and Filters

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Notes:

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1. Pipes. A pipe takes the output from one command and uses it as input to the next command.  Example:
   • ls  -l  /etc  |   more
     display a long listing of the /etc directory and pipe the output through the more command so that it is displayed one screen at a time.
      
2. Remember:
   • to send the output of a command to a file, use redirection
   • to send the output of a command to another command, use piping
       
3. Commands that are used in pipelines to modify the output from other commands are being used as filters. All of these commands can be used on their own as well. Commands that are frequently used in pipelines include: grep, sort, wc
    
4. wc: Count bytes, words and lines
   • wc /etc/passwd
     Count the number of bytes, words and lines in /etc/passwd
   • wc -l /etc/passwd
     Count the number of lines in /etc/passwd
   • wc -w /etc/login.defs
     Count the number of words in /etc/passwd
      
5. sort: sort lines in a file
   • sort /etc/group
     Sort the contents of /etc/group
   • sort  -r  /etc/group
     Sort the contents of /etc/group in reverse order
   • ls -l | sort -k5n
     Sort the output of ls -l on the size of the files. k5 specifies sort starting on field 5 and n selects a sort in numerical order
      
6. More examples:
   • What command will count the number of files in the directory var/spool/mail?
      
     Answer:  ls /var/spool/mail | wc -w
      
   • What command will count the number of users whose login shell is /bin/bash? (Recall that the login shell is the last entry in the line for each user in the file /etc/passwd)
      
     Answer:  grep /bin/bash$ /etc/passwd | wc -l
      
   • What command will display a list of everyone who is logged on sorted by their usernames?
      
     Answer:  who | sort 

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Exercises:

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1. Copy the file /etc/services to your own home directory. This file contains a list of the port numbers used for Internet network services.
    
2. How many lines are in the file.
    
3. Display all of the lines for tcp services.
   Count the number of lines for tcp services.
    
4. Display all of the lines for udp services.
   Count the number of lines for udp services.
    
5. Use the "last" command to display the record of all login sessions.
   What command counts the number of login sessions for floopy?
    
6. Display a long directory listing for the /etc directory.
   Pipe the output of the long directory listing through a grep command that will only display the entries for directories in /etc.

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Questions & Answers:

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The file called "commands" contains:
cal     /usr/bin    9416
date    /bin        2860
who     /usr/bin    8692
cat     /bin        18056
lpd     /usr/sbin   42012

1. What is the output of the pipeline:
    sort -r  commands  |  head  -1
    
   Answer:  who     /usr/bin    8692
    
2. What is the output of the pipeline:
   sort  -bk3n  commands  |  head  -1
    
   Answer:  date    /bin        2860
    
3. What is the output of the pipeline:
   grep  ^c  commands  |  wc  -l
    
   Answer:   2
     
4. What is the output of the pipeline:
   grep  usr  commands  |  wc  -l
    
   Answer: 3
    
5. What is the output of the pipeline:
   grep  at  commands  |  sort  |  head  -1
    
   Answer: cat     /bin        18056

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