topic Re: command difference in Operating System - HP-UX

command difference

kholikt — Tue, 20 Mar 2001 13:37:22 GMT

Hi,

Just wondering what is the different to reset a file to zero with the following commands.

1. cat /dev/null > file
2. > file
3. touch file

All of the above seems to achieve the same purpose however for some log it is recommended to use > file instead of cat/dev/null what is the reason for this?

Re: command difference

John Palmer — Tue, 20 Mar 2001 13:49:10 GMT

> file causes the shell to truncate file

The cat /dev/null does precisely nothing and so can be omittred.

touch file doesn't truncate the file or change the file contents at all. It just (by default) changes the time it was last modified. However it will create an empty file if it didn't already exist.

Regards,
John

Re: command difference

Joseph C. Denman — Tue, 20 Mar 2001 14:17:02 GMT

The above answer is correct. When I want to set a file to zero. I copy /dev/null to the file.

cp /dev/null file1

Re: command difference

James A. Donovan — Tue, 20 Mar 2001 14:59:51 GMT

errr..if 'cat /dev/null' isn't supposed to work, then why does it work on mine?

# echo "Testing">test
# ll test
-rw-r--r-- 1 root sys 8 Mar 20 06:57 test
# cat /dev/null >test
# ll test
-rw-r--r-- 1 root sys 0 Mar 20 06:57 test

Re: command difference

Tracey — Tue, 20 Mar 2001 15:13:50 GMT

To truncate/archive a file I use:

mv file file.old
touch file

Re: command difference

John Palmer — Tue, 20 Mar 2001 15:14:52 GMT

Jim,

It isn't the 'cat /dev/null' that truncates the file. It's the '> file' which is actioned by the shell that does the truncating before the cat process is forked.

Regards,
John

Re: command difference

Patrick Wallek — Tue, 20 Mar 2001 15:25:17 GMT

The '> file' portion of the command line is basic output redirection. If you recall the > tells the shell to always create a new file before writing any output to it.

If you did something like 'cat /dev/null >> file' then you wouldn't see any change in the file because >> says to append. So John is correct when he says the /dev/null doesn't do anything. If you want, just think of it as a place holder in the command so that you always use the command syntax you are used to (ie. 'cat filename > another_file') which is probably what most of us are used to.

Re: command difference

James A. Donovan — Tue, 20 Mar 2001 15:49:44 GMT

John, Patrick,

thanks for clearing that up for me, i never really gave much thought to it before...to think, i've been typing all those extra characters for years now...my poor carpal-tunneled hands. :-)