Login or Register to Ask a Question and Join Our Community


Shell Programming and Scripting

Proper chaining and piping of commands

Login to Discuss or Reply to this Discussion in Our Community

 
Thread Tools Search this Thread
Top Forums Shell Programming and Scripting Proper chaining and piping of commands
# 1  
Old 01-07-2016
Proper chaining and piping of commands
can someone please help me fix the below one liner?

what i want to do is run a command, and if that command is successful, pipe the output of the command to another command. i'd prefer not to use any temp files for this.

Code:
who -blahblah ; if [ $? -ne 0 ] ; then echo exit; fi | egrep username

# 2  
Old 01-07-2016
I do not know if what you just posted is a "loose" interpretation of what you want but not real or you have actually tried to run that command.

You may do it as:
Code:
who | grep username

You can not check if the command is successful and then try to pipe it to another program. That's not how it works.
When you use a PIPE both programs run in parallel and the output of the first becomes the input of the second.
Last edited by Aia; 01-08-2016 at 12:41 AM..
# 3  
Old 01-08-2016
You haven't said what operating system or shell you're using.

If you're on a system using a recent version of bash, you might want to look at the bash manage for the definition of the PIPESTATUS array. With the commands:
Code:
who | grep name
echo ${PIPESTATUS[@]}

with a recent bash you would get the output from the grep followed by a line showing the exit value of both commands in the pipeline. For example:
Code:
bash-3.2$ who | grep dwc
dwc          console  Dec  8 21:19 
dwc          ttys000  Dec  8 21:22 
dwc          ttys001  Dec  8 21:22 
dwc          ttys002  Dec  8 21:22 
dwc          ttys003  Dec  8 21:22 
dwc          ttys004  Dec  8 21:22 
dwc          ttys005  Dec  8 21:22 
dwc          ttys006  Dec  8 21:22 
dwc          ttys007  Dec  8 21:22 
bash-3.2$ echo ${PIPESTATUS[@]}
0 0
bash-3.2$ who | grep unknown
bash-3.2$ echo ${PIPESTATUS[@]}
0 1
bash-3.2$ who -x | grep dwc
who: illegal option -- x
Usage: who [-abdHlmpqrsTtu] [file]
	who am i
bash-3.2$  echo ${PIPESTATUS[@]}
1 1
bash-3.2$

Login or Register to Ask a Question

Previous Thread | Next Thread

10 More Discussions You Might Find Interesting

1. UNIX for Beginners Questions & Answers

Piping commands using xargs

Need help in piping commands using xargs I have several .tar.gz files that I need to list the folder content in a subdirectory. For example, a.tar.gz b.tar.gz c.tar.gz The following command works great for each .tar.gz file but it's a pain to run the tar command for each file. tar -tf... (11 Replies)
Discussion started by: april
11 Replies

2. Shell Programming and Scripting

Chaining together exec within find

I need to do the following with a find command on my AIX box Find all files that are -type f Then do the following steps:- Take a listing of them, and write them to a log in /directory/backup/log Tar them up in /directory/backup/tar and remove the files. Here is what I have... (22 Replies)
Discussion started by: jeffs42885
22 Replies

3. UNIX for Dummies Questions & Answers

Piping commands

Hi I am tryin to undertand piping command1|command2 from what i learn output of cammand 2 is an intput for command 1 right? If so . What dose next sequence do cat f1 >> f2 | grep '^' I think it takes context of f1 and Concatenate's it to f2 and then looks for ....i don't know..... (7 Replies)
Discussion started by: iliya24
7 Replies

4. Shell Programming and Scripting

[Solved] BASH - chaining TEST and COMMAND with && and II

Can you explain what this line of script is doing. What I have understood is : -- variable C is the name of a software which is either not installed, so it must be installed or allready installed and then should be update if newer version found -- branch B="$B $C" is to install the software --... (4 Replies)
Discussion started by: jcdole
4 Replies

5. UNIX for Dummies Questions & Answers

Piping multiple commands

Using the below code I want to find all .sff files and extract them. This works but it seems very cheap. Is there a safer more efficient way to go about this? #!/bin/bash G1=(/home/dirone) find ${G1} -type f -name \*.sff | xargs python /usr/local/bin/sff_extract.py (3 Replies)
Discussion started by: jrymer
3 Replies

6. UNIX for Dummies Questions & Answers

Proper syntax

I'm new to Unix, and just had a quick question. I'm writing a bash script, and I was wondering what proper programming etiquette was for piping. How many pipes is too many pipes? OLDEST=$(find . -maxdepth 1 -type d -newermt 2012-07-01 ! -newermt 2012-07-30 | xargs ls -1td | tail -2) echo... (1 Reply)
Discussion started by: jrymer
1 Replies

7. Shell Programming and Scripting

bash aliases and command chaining with ; (semi-colon)

What am I doing wrong here? Or is this not possible? A bug? alias f='find . >found 2>/dev/null &' f ; sleep 20 ; ls -l -bash: syntax error near unexpected token `;' (2 Replies)
Discussion started by: star_man
2 Replies

8. Shell Programming and Scripting

Piping through commands read as variables from file

This is going to be part of a longer script with more features, but I have boiled it down to the one thing that is presently stumping me. The goal is a script which checks for updates to web pages that can be run as a cron job. The script reads (from a tab-delim file) a URL, an MD5 digest, and an... (1 Reply)
Discussion started by: fitzwilliam
1 Replies

9. UNIX for Dummies Questions & Answers

Proper use of prune...

My goal was to find any directories inside of any directory called "09_Client Original" not modified in the last 30 days. $ find /Volumes/Jobs_Volume/ -type d -name "09_Client Original" -exec find {} -mtime +30 -type d -maxdepth 1 \; The results of this find are passed along in a perl script... (1 Reply)
Discussion started by: guriboy
1 Replies

10. IP Networking

Proper routing

I have a series of new machines that are internet facing (have IP's that are accessible via the 'net) and it has internal facing interfaces. I need to be able to communicate back to the internal network to a specific server which processes monitoring and e-mail traffic. I've been told that I should... (3 Replies)
Discussion started by: BOFH
3 Replies
Login or Register to Ask a Question