What does /dev/null mean in a shell script?

Posted by rishiag on Ask Ubuntu See other posts from Ask Ubuntu or by rishiag
Published on 2014-08-22T17:18:25Z Indexed on 2014/08/23 16:33 UTC
Read the original article Hit count: 207

Filed under:

command-line

|

bash

|

filesystem

|

shell-scripting

I've started learning bash scripting by using this guide: http://www.tldp.org/LDP/abs/abs-guide.pdf

However I got stuck at the first script:

cd /var/log
cat /dev/null > messages
cat /dev/null > wtmp
echo "Log files cleaned up."

What do lines 2 and 3 do in Ubuntu (I understand cat)? Is it only for other Linux distributions? After running this script as root, the output I get is Log files cleaned up. But /var/log still contains all the files.

© Ask Ubuntu or respective owner

Related posts about command-line

WMI permissions: Select CommandLine, ProcessId FROM Win32_Process returns no data for CommandLine

as seen on Server Fault - Search for 'Server Fault'
Hi all, I am gathering performance data via WMI and would like to avoid having to use an account in the Administrators group for this purpose. The target machine is running Windows Server 2003 with the latest SP/updates. I've done what I believe to be the appropriate configuration to allow our user… >>> More
WMI permissions: Select CommandLine, ProcessId FROM Win32_Process returns no data for CommandLine

as seen on Server Fault - Search for 'Server Fault'
I am gathering performance data via WMI and would like to avoid having to use an account in the Administrators group for this purpose. The target machine is running Windows Server 2003 with the latest SP/updates. I've done what I believe to be the appropriate configuration to allow our user access… >>> More
Java 1.5 Command Line Password Masking

as seen on Stack Overflow - Search for 'Stack Overflow'
All, Our server is running Java 1.5 and I am having difficulty trying to mask user input from the command line. I am executing a jar file (java -jar my.jar) and am working through command line prompts via printlns. I cannot use Java Console. Thanks >>> More
Modifying AD Schema permissions from the command line

as seen on Geeks with Blogs - Search for 'Geeks with Blogs'
Recently while making some changes for a client, I accidently dug myself into a pretty deep hole. I was trying to explicitly deny a certain user from reading a few group policies including the Default Domain Policy. When I went in to make the change I accidently denied Authenticated Users rather… >>> More
Ask the Readers: Do You Use the Command Line?

as seen on How to geek - Search for 'How to geek'
Most people have heard of it but not everyone is familiar or comfortable with how to use this bastion of geekdom. This week we would like to know if you use the command line or not. The command line…the bastion of ultimate geekery in many peoples’ eyes. You often hear people referring to doing… >>> More

Related posts about bash

launching a program from bash causes bash to go to new prompt

as seen on Ask Ubuntu - Search for 'Ask Ubuntu'
When I run a program from the console, e.g. me@box:~$ firefox I expect the console to log error messages (I think this is std out or std err?) and other items from the program, firefox in this case. But today I notice that bash just opens the program and goes to a new prompt, e.g. me@box:~$… >>> More
How to debug a .bash_profile

as seen on Ask Ubuntu - Search for 'Ask Ubuntu'
I was updating my .bash_profile, and unfortunetly I made a few updates and now I am getting: env: bash: No such file or directory env: bash: No such file or directory env: bash: No such file or directory env: bash: No such file or directory env: bash: No such file or directory -bash: tar: command… >>> More
Every command fails with "command not found" after changing .bash_profile?

as seen on Ask Ubuntu - Search for 'Ask Ubuntu'
I was updating my .bash_profile, and unfortunetly I made a few updates and now I am getting: env: bash: No such file or directory env: bash: No such file or directory env: bash: No such file or directory env: bash: No such file or directory env: bash: No such file or directory -bash: tar: command… >>> More
Is there any fundamental difference between piping in mac and linux?

as seen on Super User - Search for 'Super User'
ps -e | grep bash sample output from a linux machine: 1128 pts/14 00:00:00 bash 7491 pts/7 00:00:00 bash 12651 pts/14 00:00:00 bash 16145 pts/2 00:00:00 bash sample output from a mac machine: 58352 ttys000 0:00.09 login -pfl username /bin/bash -c exec -la bash /bin/bash 58353 ttys000… >>> More
why is $0 set to -bash?

as seen on Ask Ubuntu - Search for 'Ask Ubuntu'
First login process name seems to be set to "-bash", but if I subshell then it becomes "bash". for example: root@nowere:~# echo $0 -bash root@nowere:~# bash root@nowere:~# echo $0 bash -bash is causing some scripts to fail, such as . /usr/share/debconf/confmodule exec /usr/share/debconf/frontend… >>> More