Solved

Pipe operator confusion!

Posted on 2014-03-19
8
266 Views
Last Modified: 2014-03-19
Ah hello.

I am reading a Linux book which gives the following example command

ls | diff /dev/fd/0 oldfilelist

It apparently will compare the output of ls to that stored in oldfilelist.  I understand that /dev/fd/0 is stdout, but I don't quite understand how it ties in with the use of the pipe.  

1) Can someone please clarify how this is working?

2) When I type

ls |

I get a single > prompt.  I have tried to find out what this means/what is happening but have failed.  Can someone also clarify this please?

TIA
0
Comment
Question by:mrwad99
8 Comments
 
LVL 19

Assisted Solution

by:Ken Butters
Ken Butters earned 200 total points
ID: 39940102
The pipe redirects the output from the command prior to the pipe, as input to the command following the pipe.

In your case, the output of "ls" is redirected as input to the next command which is the "diff"

if you type "ls |"  you are saying to redirect the output of "ls" command, but not saying where to redirect the output to... so it is lost.
0
 
LVL 19

Author Comment

by:mrwad99
ID: 39940119
Correction: /dev/fd/0 is stdin not, stdout.
0
 
LVL 19

Author Comment

by:mrwad99
ID: 39940120
Thanks Ken, and sorry because my question was unclear.  I understand that the pipe is for redirection, but what I don't understand is how the output from 'ls' suddenly gets treated as the first argument to diff, which is in fact standard input!
0
Master Your Team's Linux and Cloud Stack

Come see why top tech companies like Mailchimp and Media Temple use Linux Academy to build their employee training programs.

 
LVL 34

Assisted Solution

by:Dan Craciun
Dan Craciun earned 50 total points
ID: 39940148
Think in terms of inputs and outputs: the pipe takes the output of the command on the left and serves it as input to the command on the right of the pipe.

HTH,
Dan
0
 
LVL 19

Accepted Solution

by:
Ken Butters earned 200 total points
ID: 39940161
If I understand what you are asking... that is the whole purpose of Pipe...to redirect the stdout to the stdin.

"ls" creates a list of files which are directed to stdout
the pipe command redirects all of the stdout of the "ls" to stdin of the next command
The diff command is using "/dev/fd/0" to tell where the stdin comes into play
0
 
LVL 19

Author Comment

by:mrwad99
ID: 39940222
Argh I typed a huge comment then but accidentally navigated away and lost it!  Oh well..

In a nutshell, my confusion came from the fact that I have never had to explicitly refer to the input stream before, hence I was not seeing that /dev/fd/0 as an argument to a command after a | is in fact the output of the command prior to |.

Obvious now I know :)

I have figured out my second question: it seems that the prompt is waiting for me to enter whatever I want on the right hand side of the pipe: if I type ls -l, then hit enter, then type "more", it is as if I typed "ls -l | more" outright.

Thanks both for providing input :)
0
 

Expert Comment

by:TerryZumwalt
ID: 39940225
I do not believe that "ls |" is doing anything extra for you.   You can get the same results with just "diff item1 item2" as you will with " ls | diff item1 item2. " 

When you use the pipe operator the system expects to see a command or string after the pipe.  If you do not enter any information after the pipe and press enter, then you are prompt to enter it which is why you get the > .

1. For example, if you enter "ls |" and press enter your will get the >.
2. Then enter diff and your files to compare:  > "diff item1 item2"  then press enter.
3. Your second set of input completed the initial pipe requirements and you will see a list if your info was entered correctly or an error if not.
0
 
LVL 19

Author Comment

by:mrwad99
ID: 39940253
Thanks Terry, and welcome to EE.

Shame, if I was a bit slower you would have gotten your first points for that explanation...

The purpose of the "ls |" is to compare the current contents of the working directory with those stored in oldfilelist, as a mechanism for seeing what has changed in the directory without having to output the current directory contents first.
0

Featured Post

Active Directory Webinar

We all know we need to protect and secure our privileges, but where to start? Join Experts Exchange and ManageEngine on Tuesday, April 11, 2017 10:00 AM PDT to learn how to track and secure privileged users in Active Directory.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

It’s 2016. Password authentication should be dead — or at least close to dying. But, unfortunately, it has not traversed Quagga stage yet. Using password authentication is like laundering hotel guest linens with a washboard — it’s Passé.
Join Greg Farro and Ethan Banks from Packet Pushers (http://packetpushers.net/podcast/podcasts/pq-show-93-smart-network-monitoring-paessler-sponsored/) and Greg Ross from Paessler (https://www.paessler.com/prtg) for a discussion about smart network …
Connecting to an Amazon Linux EC2 Instance from Windows Using PuTTY.
This demo shows you how to set up the containerized NetScaler CPX with NetScaler Management and Analytics System in a non-routable Mesos/Marathon environment for use with Micro-Services applications.

821 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question