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Next: , Previous: update, Up: Advanced tar


4.2.4 Combining Archives with --concatenate

Sometimes it may be convenient to add a second archive onto the end of an archive rather than adding individual files to the archive. To add one or more archives to the end of another archive, you should use the --concatenate (--catenate, -A) operation.

To use --concatenate, give the first archive with --file option and name the rest of archives to be concatenated on the command line. The members, and their member names, will be copied verbatim from those archives to the first one.

The new, concatenated archive will be called by the same name as the one given with the --file option. As usual, if you omit --file, tar will use the value of the environment variable TAPE, or, if this has not been set, the default archive name.

To demonstrate how --concatenate works, create two small archives called bluesrock.tar and folkjazz.tar, using the relevant files from practice:

     $ tar -cvf bluesrock.tar blues rock
     blues
     classical
     $ tar -cvf folkjazz.tar folk jazz
     folk
     jazz

If you like, You can run ‘tar --list’ to make sure the archives contain what they are supposed to:

     $ tar -tvf bluesrock.tar
     -rw-r--r-- melissa user    105 1997-01-21 19:42 blues
     -rw-r--r-- melissa user     33 1997-01-20 15:34 rock
     $ tar -tvf folkjazz.tar
     -rw-r--r-- melissa user     20 1996-09-23 16:44 folk
     -rw-r--r-- melissa user     65 1997-01-30 14:15 jazz

We can concatenate these two archives with tar:

     $ cd ..
     $ tar --concatenate --file=bluesrock.tar jazzfolk.tar

If you now list the contents of the bluesclass.tar, you will see that now it also contains the archive members of jazzfolk.tar:

     $ tar --list --file=bluesrock.tar
     blues
     rock
     jazz
     folk

When you use --concatenate, the source and target archives must already exist and must have been created using compatible format parameters. Notice, that tar does not check whether the archives it concatenates have compatible formats, it does not even check if the files are really tar archives.

Like --append (-r), this operation cannot be performed on some tape drives, due to deficiencies in the formats those tape drives use.

It may seem more intuitive to you to want or try to use cat to concatenate two archives instead of using the --concatenate operation; after all, cat is the utility for combining files.

However, tar archives incorporate an end-of-file marker which must be removed if the concatenated archives are to be read properly as one archive. --concatenate removes the end-of-archive marker from the target archive before each new archive is appended. If you use cat to combine the archives, the result will not be a valid tar format archive. If you need to retrieve files from an archive that was added to using the cat utility, use the --ignore-zeros (-i) option. See Ignore Zeros, for further information on dealing with archives improperly combined using the cat shell utility.

 
 
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