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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.

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1  
I see 1 problem with this: the 2nd and 3rd line ONLY work with running as root. Otherwise you need sudo in front of it. –  Rinzwind Aug 22 at 17:50

4 Answers 4

up vote 16 down vote accepted

cat will list the contents of a file comming after cat to standard output and the > sends it to the file messages and wtmp where > means to first remove all contents of the file and >> would mean to ADD to the current file. In this case you are using > so the file will end up being empty.

Now for the kicker: /dev/null is a device that sends 'nothing' to the 2 files behind the >.

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.

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6  
cat "echo" > messages will give you a blank file, but also the message cat: echo: No such file or directory. cat will relay anything on STDIN, but it doesn't print parameters like echo does. I would personally recommend echo "" > file as the intent is clear and unambiguous. –  Mr. Llama Aug 22 at 20:56
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echo "" > wtmp will create an unusable wtmp file (as non empty) and then prevent programs that use it to work. –  jlliagre Aug 23 at 9:13
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@Mr.Llama I would recommend that truncate for this - why use an echo and a pipe when there is a command explicitly designed for this task. –  Boris the Spider Aug 23 at 10:38
    
@BoristheSpider Why use a explicitly designed command when no command at all do the same job ? ;-) –  jlliagre Aug 23 at 19:16
    
@jlliagre Ha! Although I would hate to see your scripts... –  Boris the Spider Aug 23 at 19:20

Assuming the commands succeeded, /var/log/messages and /var/log/wtmp still exist but are now blank.

Shell Redirection

> is a redirection operator, implemented by the shell. Its syntax is:

command > file

This redirects command's standard output to file.

  • file may also be a device node.
  • If file doesn't exist, it is created as a regular file.
  • If 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 messages and wtmp.
  • If file already exists as a symbolic link, the link's target is used.
  • If 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.)

The >> 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 file as command's standard input.)

The null Device

/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

You're using /dev/null differently. 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 > operator.

Since messages and 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

The -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.)

The 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":

  • The cat executable (usually /bin/cat), a regular file.
  • The /dev/null device.
  • file

In contrast, echo -n > file opens only file (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 > or >> 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:

> messages
> wtmp
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Wow! Thanks for extensive explanation. –  rishiag Aug 22 at 18:09
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You can go further and replace the useless echo -n by : or just nothing, given the fact the command does nothing anyway. –  jlliagre Aug 23 at 9:18
    
Rather than echo -n I would recommend you use truncate - using a command designed for the purpose is, in my option, a little clearer to future readers of the script. –  Boris the Spider Aug 23 at 10:42
    
@BoristheSpider I get your point about readability but the shell can do it a simpler way with an empty redirection. –  jlliagre Aug 23 at 13:22
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@jlliagre You're right: while I thought to suggest echo -n > messages as it's self-documenting, it's arguably even better and more instructive to use > messages, which is equivalent. Thank you for pointing this out--I've expanded my answer to cover this option. (: > messages is fine too, though personally I don't particularly care for it--it has the same meaning as true > messages, true also being a shell builtin. If I'm going to to redirect from a named command, echo -n is self-documenting as to its purpose lying in its output.) –  Eliah Kagan Aug 23 at 15:33

As already answered, these two lines are clearing the content of the /var/log/messages and /var/log/wtmp files, or are creating them in the unlikely event they do not already exist.

However, they are based on a well established urban legend that gives /dev/null "paranormal" powers.

It actually has none so cat /dev/null is a waste of keystrokes, time and CPU cycles as it outputs absolutely nothing. Eliah Kagan's reply suggests the better approach which is using echo -n instead. This is better but not portable to some shells/OSes where it might put the '-n' string into these files.

You can go further and replace these commands by the portable and simpler:

cd /var/log
: > messages
: > wtmp
echo "Log files cleaned up."

With most shells (but not the csh based ones), you can go even further and remove the no-op command : :

cd /var/log
> messages
> wtmp
echo "Log files cleaned up."
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I see the benefit of redirecting from : (rather than from nothing) in that it will work with non-Bourne-style shells, such as csh/tcsh. Are there also Bourne-style shells that won't redirect stdout from an empty command? –  Eliah Kagan Aug 23 at 16:16
    
@Rinzwind Those changes don't work, and severely break the script. sudo su starts a new interactive shell. Only after that shell has exited will the next command in the script be run. If this script is run by a non-root user, the commands after sudo su that need root privileges will still fail. Then exit will exit the original script, and echo "Log files cleaned up." will not run. (By the way, the context provided in the guide the OP's using is that the script will be run as root.) –  Eliah Kagan Aug 23 at 16:39
    
ah ok @EliahKagan I was doing this command line :D –  Rinzwind Aug 23 at 16:41
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@EliahKagan No Bourne style shell I'm aware of won't redirect stdout from an empty command. This was already the case with the Unix version 7 (1979) original Bourne shell. –  jlliagre Aug 23 at 19:30

> is output redirecting operator. It will redirect the output of command to file mentioned after it instead of standard output device, truncating or overwriting file's contents.

for example ls -l > demo.txt. After executing this command, "demo.txt" will contain th output ls -l command.

Now next thing is what is this /dev/null./dev/null is the null file Is is a special file which contains nothing.

So when you execute commands

cat /dev/null > messages
cat /dev/null > wtmp

it clears the contents of "messages" and "wtmp" file by overwriting with EOF(End Of File) character.

So here you are not clearing your /var/log directory, but you are clearing the contents of these two files.

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