tar -f?

chris_crunch · 02-24-2016, 07:24 AM

Hi, this might be a stupid question, but I'm trying to find out exactly what the tar command means.

I looked up

Code:

tar -xf

I know the x stands for extracting files, but the f?

Apparently it means "use file archive or device archive".

Can anyone elaborate on that?

pan64 · 02-24-2016, 07:40 AM

-f requires an additional string to specify what do you want to tar/untar
-f <filename> or -f <device>

rtmistler · 02-24-2016, 07:42 AM

Not very helpful, but from the tar(1) manual page:

Code:

     -f, --file ARCHIVE
           use archive file or device ARCHIVE

I believe the proper, original naming of the command represents "Tape ARchive".

What tar does, and was originally intended to do, was to concatenate files to put them into one file and then place them onto a physical tape archive ... yeah, "way back when".

Experiment with tar, using no compression and just using, "different, but small, text files". And see what it does. An example:

Code:

~/testcode$ echo ABC > a.txt
~/testcode$ echo 123 > b.txt
~/testcode$ tar -cvf new.tar a.txt b.txt
a.txt
b.txt
~/testcode$ cat new.tar
a.txt0000644000175000017510000000000412663330656011223 0ustar  <mylogin><mygroup>ABC
b.txt0000644000175000017510000000000412663330662011221 0ustar  <mylogin><mygroup>123
~/testcode$

Rather inefficient I'll grant when the files are tiny, but there you have it. And whatever all the junk is between the filename and file contents is beyond me, but some of it appears able to be decoded, such as the 644 appears to be file privileges.

chris_crunch · 02-24-2016, 07:48 AM

Quote:

Originally Posted by pan64

-f requires an additional string to specify what do you want to tar/untar
-f <filename> or -f <device>

But if I just said

Code:

tar -x <filename>

wouldn't that work?

pan64 · 02-24-2016, 07:55 AM

have you tried it? See also the man page of tar:

Code:

ENVIRONMENT
     TAPE    Device or file to use for the archive if --file is not specified.  If this environment variable is unset, use stdin or stdout instead.

rtmistler · 02-24-2016, 07:59 AM

Quote:

Originally Posted by chris_crunch

But if I just said

Code:

tar -x <filename>

wouldn't that work?

No it would not. The -f specifies the file, otherwise I believe it will be looking at stdin, but could be mistaken.

As offered by pan64, suggest you try these things. Make a test tar file and just try things out.

rknichols · 02-24-2016, 08:06 AM

Quote:

Originally Posted by chris_crunch

But if I just said

Code:

tar -x <filename>

wouldn't that work?

It's easy enough to try it, but no, that says to extract something named <filename> from the default device, which, depending on what version of tar you have, is either stdin or the (probably nonexistent) tape drive. Starting with version 1.11.5, GNU tar uses standard input and standard output as the default device. Prior versions looked for a TAPE environment variable, or in the absence of that used a compiled-in default device, typically "/dev/nst0".

JockVSJock · 02-24-2016, 08:31 AM

Quote:

Originally Posted by chris_crunch

Hi, this might be a stupid question, but I'm trying to find out exactly what the tar command means.

I looked up

Code:

tar -tf

I know the x stands for extracting files, but the f?

Apparently it means "use file archive or device archive".

Can anyone elaborate on that?

So I an example that I use with -f is looking at the content of tar file

Code:

tar -tf example.tar 


./file1/ 
./file2/
./file3/
./file4/
./file5/

jpollard · 02-24-2016, 02:15 PM

Quote:

Originally Posted by rtmistler

Not very helpful, but from the tar(1) manual page:

Code:

     -f, --file ARCHIVE
           use archive file or device ARCHIVE

I believe the proper, original naming of the command represents "Tape ARchive".

What tar does, and was originally intended to do, was to concatenate files to put them into one file and then place them onto a physical tape archive ... yeah, "way back when".

Experiment with tar, using no compression and just using, "different, but small, text files". And see what it does. An example:

Code:

~/testcode$ echo ABC > a.txt
~/testcode$ echo 123 > b.txt
~/testcode$ tar -cvf new.tar a.txt b.txt
a.txt
b.txt
~/testcode$ cat new.tar
a.txt0000644000175000017510000000000412663330656011223 0ustar  <mylogin><mygroup>ABC
b.txt0000644000175000017510000000000412663330662011221 0ustar  <mylogin><mygroup>123
~/testcode$

Rather inefficient I'll grant when the files are tiny, but there you have it. And whatever all the junk is between the filename and file contents is beyond me, but some of it appears able to be decoded, such as the 644 appears to be file privileges.

They are the user ownership, group assigned, access mask, and dates taken from the inode when the
archive was created. These can be used to restore the file to its original condition. In addition to the minimum, tar has been extended to also include ACLs, and security labels as well.

tar is a backup tool - as such it has to keep many things about the file (the path used when making the
backup is one - so the file name shown is short).