Now, we need to know about the file descriptor. In the UNIX ecosystem, these are integer values assigned to a file. Using the file descriptor 1 and 2 in this situation , we can redirect the stdout and stderr to other files.
For starter, the following example will redirect the stdout of the echo command to a text file. If not specified, bash will use stdout by default.
Now, using the redirection, we can get a clearer output. The output looks much better, right? How do we do that? In certain situations, the output may not be useful at all.
Using redirection, we can dump all the output into the void. If you tend to forget the file descriptor of stdout and stderr , the following command will do just fine. You could display a files contents by doing: cat filename by that doesn't use up the file. It's still there. To delete a file, do rm filename. Join Date: Jul Maybe a couple more examples will make it clear.
I think that you got that much. The third one is explicitly doing for the shell did automatically. Instead of being stored in a file on disk, a special little program called a driver is going to get that "hello" string.
The tty driver will figure out how to make the characters appear on your screen. And then it will tell the OS, ok The fourth one is also going to a special file and again a driver will get the string "hello". But this driver is just going to tell the OS, ok But there is no device and the null driver did not try to do anything. It just tosses the characters away.
And the driver will wait for you to type something and read those characters and send them on. Hi Experts, I Have a query. Its just a normal file. Shell Programming and Scripting.
I apologize if this question has been answered else where or is too elementary. How are these two different? They both prevent output and error from being displayed. Please help urgently. Is it only for other Linux distributions? After running this script as root, the output I get is Log files cleaned up.
This redirects command 's standard output to file. Of course, these operations may fail for other reasons, such as lack of permissions or a filesystem error. Since messages and wtmp are regular files rather than, for example, device nodes , they are turned into blank files i.
The -n flag is required, or echo writes a newline character. This always works in bash. But jlliagre is right that echo -n should be avoided for a truly portable shell script, as it's not required to work.
The echo -n way is equivalent in its effects but arguably is a better solution, in that it's simpler. Although this should be expected to improve performance, that's not the benefit--not when just running a couple of these commands by hand, anyway.
Instead, the benefit is that it's easier to understand what's going on. There is a reason to do it like this: the file is NOT removed from the system. If you would rm it and then do a touch messages the permissions might be wrong and if just after the rm something would want to write to the file it would be gone and error out. Depending on how the software is created it could crash.
Eliah Kagan's reply suggests the better approach which is using echo -n instead. With most shells but not the csh based ones , you can go even further and remove the no-op command ' : ' :. It will redirect the output of command to file mentioned after it instead of standard output device, truncating or overwriting file's contents.
After executing this command, "demo. Ubuntu Community Ask! Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group.
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