Use Chmod Command Selectively

chmodcommand linefind

I want to set 755 permission on all files and sub-directories under a specific directory, but I want to execute chmod 755 only for those components which does not have 755 permission.

find /main_directory/ -exec chmod 755 {} \;

If the find command returns a long list, this will take a lot of time.
I know that I can use the stat command to check the Octal file level permission of each component and then use if-else to toggle the file permission, but is there any single line approach using find and xargs to first check what permission the file/directory has, and then use chmod to change it to 755 if it is set to something else.

Best Answer

If you want to change permissions to 755 on both files and directories, there's no real benefit to using find (from a performance point of view at least), and you could just do

chmod -R 755 /main_directory

If you really want to use find to avoid changing permissions on things that already has 755 permissions (to avoid updating their ctime timestamp), then you should also test for the current permissions on each directory and file:

find /main_directory ! -perm 0755 -exec chmod 755 {} +

The -exec ... {} + will collect as many pathnames as possible that passes the ! -perm 0755 test, and execute chmod on all of them at once.

Usually, one would want to change permissions on files and directories separately, so that not all files are executable:

find /main_directory   -type d ! -perm 0755 -exec chmod 755 {} +
find /main_directory ! -type d ! -perm 0644 -exec chmod 644 {} +

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Bash Find Command – How to Use Two Bash Commands in -exec

As for the find command, you can also just add more -exec commands in a row:

find . -name "*" -exec chgrp -v new_group '{}' \; -exec chmod -v 770 '{}' \;

Note that this command is, in its result, equivalent of using

chgrp -v new_group file && chmod -v 770 file

on each file.

All the find's parameters such as -name, -exec, -size and so on, are actually tests: find will continue to run them one by one as long as the entire chain so far has evaluated to true. So each consecutive -exec command is executed only if the previous ones returned true (i.e. 0 exit status of the commands). But find also understands logic operators such as or (-o) and not (!). Therefore, to use a chain of -exec tests regardless of the previous results, one would need to use something like this:

find . -name "*" \( -exec chgrp -v new_group {} \; -o -true \) -exec chmod -v 770 {} \;

Is it possible to represent the +X symbolic permission with an octal value

The short story: it's not possible, no.

The longer story: octal permissions are states. The [+-][rwxXst] notation represents operations that culminate in changing states. Note, not the =[rwxXst] one, which sets state and is equivalent to the octal modes as Gilles said. The X one is the only conditional operation, the other ones are all unconditional.

When you chmod a file with octal permissions, you supply the final state of the permission bits verbatim. When you use the operations, you choose what you want done to the permission bits.

Your question is tantamount to asking if there's a single number that represents all square roots. The answers is obviously ‘no’: every square root yields a number (pedants: though it could be imaginary/irrational), but without the starting state (the number), you can't tell which.

Best Answer

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Bash Find Command – How to Use Two Bash Commands in -exec

Is it possible to represent the +X symbolic permission with an octal value

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