Question

I saw the following interesting usage of tar in a co-worker’s Bash scripts:

`tar cf - * | (cd <dest> ; tar xf - )` 

Apparently it works much like rsync -av does, but faster. The question arises, how?

-m

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EDIT: Can anyone explain why should this solution be preferable over the following?

cp -rfp * dest 

Is the former faster?

Answer 1

On the difference between cp and tar to copy the directory hierarchies, a simple experiment can be conducted to show the difference:

alastair box:~/hack/cptest [1134]% mkdir src alastair box:~/hack/cptest [1135]% cd src alastair box:~/hack/cptest/src [1136]% touch foo alastair box:~/hack/cptest/src [1137]% ln -s foo foo-s alastair box:~/hack/cptest/src [1138]% ln foo foo-h alastair box:~/hack/cptest/src [1139]% ls -a total 0 -rw-r--r--  2 alastair alastair    0 Nov 25 14:59 foo -rw-r--r--  2 alastair alastair    0 Nov 25 14:59 foo-h lrwxrwxrwx  1 alastair alastair    3 Nov 25 14:59 foo-s -> foo alastair box:~/hack/cptest/src [1142]% mkdir ../cpdest alastair box:~/hack/cptest/src [1143]% cp -rfp * ../cpdest alastair box:~/hack/cptest/src [1144]% mkdir ../tardest alastair box:~/hack/cptest/src [1145]% tar cf - * | (cd ../tardest ; tar xf - ) alastair box:~/hack/cptest/src [1146]% cd .. alastair box:~/hack/cptest [1147]% ls -l cpdest total 0 -rw-r--r--  1 alastair alastair    0 Nov 25 14:59 foo -rw-r--r--  1 alastair alastair    0 Nov 25 14:59 foo-h lrwxrwxrwx  1 alastair alastair    3 Nov 25 15:00 foo-s -> foo alastair box:~/hack/cptest [1148]% ls -l tardest total 0 -rw-r--r--  2 alastair alastair    0 Nov 25 14:59 foo -rw-r--r--  2 alastair alastair    0 Nov 25 14:59 foo-h lrwxrwxrwx  1 alastair alastair    3 Nov 25 15:00 foo-s -> foo 

The difference is in the hard-linked files. Notice how the hard-linked files are copied individually with cp and together with tar. To make the difference more obvious, have a look at the inodes for each:

alastair box:~/hack/cptest [1149]% ls -i cpdest 24690722 foo  24690723 foo-h  24690724 foo-s alastair box:~/hack/cptest [1150]% ls -i tardest 24690801 foo  24690801 foo-h  24690802 foo-s 

There are probably other reasons to prefer tar, but this is one big one, at least if you have extensively hard-linked files.