[SOLVED] ">" vs "cat /dev/null >"

andrew.comly · 05-14-2017, 12:47 AM

BACKGROUND
I am reading Advanced Bash Scripting by Mendell Cooper. In this eBook there is a Chapter 16 External Filters, Programs and Commands. Within this chapter there is "Example 16-15. Using tail to monitor the system log".

INTRO
To create the file if it does not already exist, truncate it to zero length (if it exists) and to paste last ten lines of the file /var/log/messages to the aforesaid log file, the author chooses the below method:

TEXTBOOK

Code:

	#NAME LOG FILE
		filename=sys.log

	#TRUNCATES LOG TO ZERO LENGTH
		cat /dev/null > $filename; echo "Creating / cleaning out file."

	#PASTE LAST TEN MESSAGES INTO LOG
		tail /var/log/messages > $filename

But I use the following code line serving the same purpose:

ALTERNATE METHOD

Code:

	#NAME LOG FILE
		filename=sys.log

	#PASTE LAST TEN MESSAGES INTO LOG (Using the ">" to write over the old input with new input).
		tail /var/log/messages > $filename

ANALYSIS
Originally I thought: Isn't truncating the log to zero length by the command: '

Code:

	cat /dev/null > $filename

superfluous being that on the last command a ">" is used and not a ">>"?

But then I remembered "Chapter 3. Special Characters", and on p17 under the Variable expansion/substring replacement section there is the following codeblock:

Code:

: > data.xxx
# File "data.xxx" now empty.
# Same effect as
cat /dev/null >data.xxx
# However, this does not fork a new process, since ":" is a builtin.

So it seems the author is merely trying to broaden our command line flexibility by not repeating the same commands too much.

QUESTION:
By the "fork a new process" sentence above, does this mean that the difference between these two commands is that

Code:

cat/dev/null >data.xxx

does fork a new process BUT

Code:

: > data.xxx

does NOT?

hazel · 05-14-2017, 01:11 AM

Yes, cat is a command, an actual program found on the command path. By default, any command invoked in the shell creates a new process. To prevent this, you must use the exec builtin. Shell builtins are just input that the shell interprets.

You can distinguish between commands and builtins by using the type command:

Code:

hazel [ ~ ]$ type cat
cat is hashed (/bin/cat)
hazel [ ~ ]$ type :
: is a shell builtin

Laserbeak · 05-14-2017, 01:12 AM

Running

Code:

cat /dev/null > somefile

truncates that file to zero length.

Code:

cat /dev/null >> somefile

doesn't seem to do anything. Theoretically it would append a null to the end of the file, but the file length doesn't increase on my system.

I'm not really familiar with using ":"

MadeInGermany · 05-14-2017, 01:14 AM

cat is an external command. An external command involves a fork and the dynamic linker, this is more overhead than a builtin command.
BTW to create/truncate a file you can do

Code:

> data.xxx

No command needed.
And yes,no need for an initial truncation if subsequent commands overwrite the file anyway e.g. with >.

Laserbeak · 05-14-2017, 01:49 AM

Also, I wouldn't assume that "tail" always would give you 10 lines. If you want ten lines, you should say:

Code:

tail -10 somefile

or if you wanted 1000 lines

Code:

tail -1000 somefile

Because I don't know if that's a hard and fast rule that "tail" with no options gives 10 lines. In any case, it's better to make it explicit.

andrew.comly · 05-16-2017, 03:34 AM

hazel and MadeInGermany,

Even though I learned the type command, I just never connected it to this real life application.

Code:

$ type cat
cat is /bin/cat

$ type :
: is a shell builtin

Thanks, your reasoning really solved my question.