Assuming the commands succeeded,
/var/log/wtmp still exist but are now blank.
> is a redirection operator, implemented by the shell. Its syntax is:
command > file
command's standard output to
file may also be a device node.
file doesn't exist, it is created as a regular file.
file already exists as a regular file and is non-empty, it is overwritten. This is typically the case in the commands you ran, where you redirected the output of
cat /dev/null to
file already exists as a symbolic link, the link's target is used.
file already exists as a directory, you'll get an error like
bash: file: Is a directory.
(Of course, these operations may fail for other reasons, such as lack of permissions or a filesystem error.)
>> redirection operator is similar, but it appends to the end of non-empty regular files instead of overwriting their contents. (Another redirection operator is
command < file uses
command's standard input.)
/dev/null is a simple device (implemented in software and not corresponding to any hardware device on the system).
/dev/null looks empty when you read from it.
- Writing to
/dev/null does nothing: data written to this device simply "disappear."
Often a command's standard output is silenced by redirecting it to
/dev/null, and this is perhaps the
null device's commonest use in shell scripting:
command > /dev/null
cat /dev/null outputs the "contents" of
/dev/null, which is to say its output is blank.
> messages (or
> wtmp) causes this blank output to be redirected to the file on the right side of the
wtmp are regular files (rather than, for example, device nodes), they are turned into blank files (i.e., emptied).
You could use any command that does nothing and produces no output, to the left of
An alternative way to clear these files would be to run:
echo -n > messages
echo -n > wtmp
-n flag is required, or
echo writes a newline character.
(This always works in
bash. And I believe the default
sh in every GNU/Linux distribution and other Unix-like system popularly used today supports the
-n flag in its
echo builtin. But jlliagre is right that
echo -n should be avoided for a truly portable shell script, as it's not required to work. Maybe that's why the guide you're using teaches the
cat /dev/null way instead.)
echo -n way is equivalent in its effects but arguably is a better solution, in that it's simpler.
cat /dev/null > file opens three "files":
cat executable (usually
/bin/cat), a regular file.
echo -n > file opens only
echo is a shell builtin).
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.
Redirection and the trivial (blank/empty) command.
As jlliagre has pointed out (see also jlliagre's answer), this can be shortened further by simply omitting the command on the left of
> altogether. While you cannot omit the right side of a
>> expression, the blank command is valid (it's the command you're running when you just press Enter on an empty prompt), and in omitting the left side you're just redirecting the output of that command.
- Note that this output does not contain a newline. When you press Enter on a command prompt--whether or not you've typed anything--the shell (running interactively) prints a newline before running the command issued. This newline is not part of the command's output.
Redirecting from the blank command (instead of from
cat /dev/null or
echo -n) looks like: