Understanding tar Commands and Finding Better Alternatives

bashlinuxtar

I am going over some documentation pertaining to creating a distro repo. However, I cannot understand exactly what these commands perform:

# cd /mnt/
# ls
centos-1  centos-2  centos-3
# for i in 1 2 3 
> do
> cd /mnt/centos-$i
> tar cpf - . | ( cd /var/ftp/ks/centos/; tar xpf - )
> done

My (obviously incorrect) rundown of it is: Loop through every mounted CD (/mnt/centos-<index>) and create a tar of the folder preserving the permissions. Then, extract that tar file to another folder, again, preserving permissions.

If that is the case, why do it like this? Is there really not a better way? Or perhaps I'm missing the whole idea.

Best Answer

The reason why tar (or cpio) is recommended over cp for this procedure is because of how the tools operate.

cp operates on each file object in turn, reading it from disk and then writing it in its new location. Since the locations of the source and destination may not be close on the disk, this results in a lot of seeking between the locations.

tar and cpio read as much as possible in one go, and then write it into the archive. This means that the source files will be read one after another and the destination files will be written one after another (allowing for process switching, of course), resulting in much less seeking and hence less time taken.

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Linux Bash – What Do These Commands Mean?

Others have already mentioned that $# is the number of command-line arguments, so I'll ignore that part of your question.

Given cmd1 && cmd2, the shell first executes cmd1. If cmd1 fails, the entire expression fails. If cmd1 succeeds, the shell proceeds to execute cmd2. The entire expression succeeds if both cmd1 and cmd2 succeed. So, the && operator is the short-circuiting boolean AND applied to shell commands. Uses of this operator can be replaced with

if cmd1
then
    cmd2
fi

Given cmd1 || cmd2, the shell first executes cmd1. If it succeeds, the entire expression succeeds. Otherwise, the shell executes cmd2 and the success or failure of the expression is determined by cmd2. In short, this is the short-circuiting boolean OR applied to shell commands. You can replace this form with

if ! cmd1
then
    cmd2
fi

I've carefully used the phrases succeeded / succeeds and failed / fails above. Success, for shell commands, is defined as a process exit status of zero. Failure is any non-zero status. However, attempting to understand the && and || operators in terms of exit statuses is, at least to me, confusing since you end up with a weird sort of inverted logic.

Bash – difference between these two commands

In terms of functionality I think they are the same.

If I had to venture a guess about which is faster, I would say the cp command because its purpose is to do file operations just for copy purposes, so it would be optimized for that.

cat, by contrast, is meant concatenate files, meaning joining several files into a series. If no file is specified, it will display a file to the console (thanks to @bahamat for reminding us). In this example the output is redirected to another file. I think this indirection would be less efficient than a direct cp.

I don't know if the difference would be noticeable for regular sized files, though it would be interesting to time these on very large files. I guess one could do repeated trials with /usr/bin/time and see if one is consistently faster/slower than the other.

Do you have any particular reason to ask about this, or is it just simple curiosity (nothing wrong with that at all of course)

Best Answer

Related Solutions

Linux Bash – What Do These Commands Mean?

Bash – difference between these two commands

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