Copying Directory Trees with tar and Pipes (Unix Power Tools, 3rd Edition)

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10.13. Copying Directory Trees with tar and Pipes

The tar (Section 39.2) command isn't just for tape archives. It can copy files from disk to disk, too. And even if your computer has cp -r (Section 10.12), there are advantages to using tar.

The obvious way to copy directories with tar is to write them onto a tape archive with relative pathnames -- then read back the tape and write it somewhere else on the disk. But tar can also write to a Unix pipe -- and read from a pipe. This looks like:

% reading-tar | writing-tar

with one trick: the writing-tar process has a different current directory (Section 24.3, Section 24.4) (the place where you want the copy made) than the reading-tar. To do that, run the writing-tar in a subshell (Section 43.7), or if your tar supports it, use the -C option.

The argument(s) to the reading-tar can be directories or files. Just be sure to use relative pathnames (Section 31.2) that don't start with a slash -- otherwise, the writing-tar may write the copies in the same place from where the originals came!

"How about an example," you ask? Figure 10-1 has one. It copies from the directory /home/jane, with all its files and subdirectories. The copy is made in the directory /work/bkup/jane:

% mkdir /work/bkup/jane
% cd /home/jane
% tar cf - . | (cd /work/bkup/jane && tar xvf -)

Or, if you want to use -C:

% tar cf - . | tar xvf - -C /work/bkup/jane

In the subshell version, the && operator (Section 35.14) tells the shell to start tar xvf - only if the previous command (the cd) succeeded. That prevents tar writing files into the same directory from which it's reading -- if the destination directory isn't accessible or you flub its pathname. Also, don't use the v (verbose) option in both tars unless you want to see doubled output; one or the other is plenty. I usually put it in the writing-tar to see write progress, as that's more interesting to me than how far ahead the system has cached the read for me.

Figure 10-1. Copying /home/jane to /work/bkup with tar

WARNING: At least one tar version has a v (verbose) option that writes the verbose text to standard output instead of standard error! If your tar does that, definitely don't use v on the reading-tar (the tar that feeds the pipe) -- use v on the writing-tar only.

You can use other options that your tar might have -- such as excluding files or directories -- on the reading-tar, too. Some gotchas:

Be aware that symbolic links (Section 10.4) will be copied exactly. If they point to relative pathnames, the copied links might point to locations that don't exist (Section 10.6). You can search for these symbolic links with find -type l.
If your system has rsh (Section 1.21) or ssh, you can run either the reading-tar or the writing-tar on a remote system. For example, to copy a directory to the computer named kumquat:
```
% ssh kumquat mkdir /work/bkup/jane
% tar cf - . | ssh kumquat 'cd /work/bkup/jane && tar xvf -'
```

--JP and DJPH


10.12. Copying Directory Trees with cp -r		11. Comparing Files