Alexander Leidinger

Just another weblog

Sep
21

Speed traps with chmod

I have the habit to chmod with the rel­a­tive nota­tion (e.g. g+w or a+r or go-w or sim­i­lar) instead of the absolute one (e.g. 0640 or u=rw,g=r,o=). Recently I had to chmod a lot of files. As usual I was using the rel­a­tive nota­tion. With a lot of files, this took a lot of time. Time was not really an issue, so I did not stop it to restart with a bet­ter per­form­ing com­mand (e.g. find /path –type f –print0 | xargs –0 chmod 0644; find /path –type d –print0 | xargs –0 chmod 0755), but I thought a lit­tle tips&tricks post­ing may be in order, as not every­one knows the difference.

The rel­a­tive notation

When you spec­ify g+w, it means to remove the write access for the group, but keep every­thing else like it is. Nat­u­rally this means that chmod first has to lookup the cur­rent access rights. So for each async write request, there has to be a read-request first.

The absolute notation

The absolute nota­tion is what most peo­ple are used to (at least the numeric one). It does not need to read the access rights before chang­ing them, so there is less I/O to be done to get what you want. The draw­back is that it is not so nice for recur­sive changes. You do not want to have the x-bit for data files, but you need it for direc­to­ries. If you only have a tree with data files where you want to have an uni­form access, the exam­ple above via find is prob­a­bly faster (for sure if the direc­tory meta-data is still in RAM).

If you have a mix of bina­ries and data, it is a lit­tle bit more tricky to come up with a way which is faster. If the data has a name-pattern, you could use it in the find.

And if you have a non-uniform access for the group bits and want to make sure the owner has write access to every­thing, it may be faster to use the rel­a­tive nota­tion than to find a replace­ment command-sequence with the absolute notation.

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