Capture output of certain Unix commands
Today's hint will probably only appeal to those of you learning to use the Unix side of OS X. A while back, I was trying to capture the output of the Unix command httpd -t (which runs a syntax check on the Apache web server's configuration files) to the clipboard. Typically, you do this in OS X by sending the command's output through the pbcopy (pasteboard copy) command:
httpd -t | pbcopy
That should, in theory, put the output of the command on the clipboard. But in this case, it wasn't working. After much investigation, what I learned is that some Unix commands, httpd -t included, don't send their output to the Terminal in the usual way. The usual way in Unix is to send output to something called the "standard output," which for all intents in this discussion, means the Terminal window.
Instead, certain commands send their output to standard error (STDERROR, in Unix parlance). Although this text is also displayed in Terminal, you can't do anything with the output as you can if it were sent to standard output. (Unix wizards, please feel free to correct any of my mistakes in the above; it's meant to be a summary view of the situation.)
The solution is to reroute what gets sent to standard error to standard output, using this syntax:
httpd -t 2>&1
The 2 represents standard error, and the 1 represents standard output. The bit in the middle, >& is the usual Unix redirect command (the greater than sign), followed by the ampersand, which changes the redirect's output from a file (which is typical) to a file descriptor (in this case, the standard output).
I know that's a mouthful, and I'll admit that I don't fully understand it--but I know it works, at least in the bash shell that's been the default for a while now. Once the output is in standard output, it can just be fed through pbcopy as usual:
httpd -t 2>&1 | pbcopy
The end result, assuming there's nothing wrong with your Apache configuration files, will be the text Syntax OK on your clipboard. I know that ssh -v (verbose mode ssh) also sends its output to standard error, as does time, and I'm sure there are many more.
If you're a Unix expert and can clarify or expand on this in the comments, please feel free!
See more like this: unix-linux, Terminal, Mac OS
» posted by ITworld staff
Macworld.com
Sign up for ITworld's Daily newsletter
Follow ITworld on Twitter @IT_world
On Twitter now
mac os
Powered by Twitter
jfruh
Apple syncing patent can't come soon enough
pasmith
New Twitter features borrow from 3rd party clients
Esther Schindler
Open Source Changes the Software Acquisition Process
mikelgan
How to set up continuous podcast play on the new iTunes
David Strom
Five important Windows 7 mobility features
sjvn
Guard your Wi-Fi for your own sake
Sandra Henry-Stocker
Grepping on Whole Words
Sidekick: The Good News & the Bad News
Either way you look at it Microsoft Data Center management did not follow standards or best practices in this failure. In which case it makes me wonder more about the outsourcing of corporate data much less personal data.
- mburton325
Join the conversation here
Quick, practical advice for IT pros. Made fresh daily.
Want to cash in on your IT savvy? Send your tip to tips@itworld.com. If we post it, we'll send you a $25 Amazon e-gift card.
Clarification
Actually, the '&' goes with the '1' to specify that the target is standard output (&1). You could just a easily redirect the standard error to a different file with'2 > /somepath/somefile'.
This does not change your solution however, because the pipeline '|' will only feed the standard output to another program.