Why doesnt sudo /dev/null > /var/log/syslog and sudo > /var/log/syslog work, while sudo rm /var/log/syslog works?!
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5 Answers
truncate -s 0 /var/log/syslog
working for me on 18.04. Got it from here:
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There are two main problems.
One problem is that /dev/null isn't a command, so running sudo /dev/null can't succeed. You need sudo [a command]. In this case, you probably want sudo cat /dev/null.
The other problem is that > separates things into a full command on the left and a file on the right, so the full command on the left is sudo cat /dev/null, and sudo's job is now done once it runs cat /dev/null.
That means that the > is running as your user, not under sudo. Your user doesn't have permission to write to /var/log/syslog, so this will fail.
You need some way to run the entire line cat /dev/null > /var/log/syslog under sudo. Well, > isn't a command or anything. It's something the shell handles, so you need to have a shell handle that redirection symbol properly. You can do that with sh's -c option: sh -c 'cat /dev/null > /var/log/syslog'.
Now that you have everything together as one command, you can have sudo run the entire thing:
sudo sh -c 'cat /dev/null > /var/log/syslog'
answered Mar 18, 2014 at 20:11
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No command is needed, the
>is enough. The OP just got an error but did truncate the target file, see my answer.– terdonMar 18, 2014 at 20:25 -
Agreed. Except that "sudo allows a permitted user to execute a command as the superuser or another user, as specified by the security policy." Mar 18, 2014 at 20:28
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True, but irrelevant. The OP would get the same error with or without
sudo, it comes frombash, not fromsudo.– terdonMar 18, 2014 at 20:32 -
The command you are thinking of is probably
> /var/log/syslog
Nothing else is needed. In bash and other shells, the > will immediately truncate the file, emptying it. However, when you run this:
sudo /dev/null > /var/log/syslog
The system is attempting to run /dev/null as a command and you will get this error:
sudo: /dev/null: command not found
Note, however, that despite this, /var/log/syslog has actually been emptied because, as I said above, the > is enough, no command is necessary.
truncate: The truncation process basically removes all the contents of the file. It does not remove the file itself, but it leaves it on the disk as a zero byte file.
Clear ALL Content of Syslog with:
sudo truncate -s 0 /var/log/syslog
Another way to do this is
sudo tee </dev/null /var/log/syslog
Or if you prefer a useless use of cat:
cat /dev/null | sudo tee /var/log/syslog
