Thursday, May 19, 2016

Oracle Linux - Clone file permissions with chmod

Every now and then file permissions under Linux can be tricky. in some cases a wrong file permission can make it happen that things do not work they way you would expect them. Also I found that a lot of people find it challenging to set the correct file permissions using the command line under Linux. A way to make life more easy in some cases is to use the option to "clone" file permissions with a single command.

For example, if you have created some addons to a tool running on an Oracle Linux system and you want to the addon file to have the same permissions as another file you can use the --reference option from the chmod command.

As an example we have two .jar files:

[root@localhost ~]# ls -l *.jar
-rwxr-xr-x. 1 root root 88 May 19 10:48 addonExecution.jar
-rw-r--r--. 1 root root 11 May 19 10:48 executionLib.jar

We want to make sure that the addonExecution.jar has exactly the same permissions as the executionLib.jar file. We can do this by specifying the desired stated in a chmod command, we can also use the --reference option as shown below:

[root@localhost ~]# chmod --reference=executionLib.jar addonExecution.jar

This will make sure that the addonExecution.jar file now has exactly the same permissions as the file used as a reference.

[root@localhost ~]# ls -l *.jar
-rw-r--r--. 1 root root 88 May 19 10:48 addonExecution.jar
-rw-r--r--. 1 root root 11 May 19 10:48 executionLib.jar

Another use case example of this is that you can use it in a bash script where you might not be sure what the permissions should be for a certain file and only know that they always need to be the same as a specified other file. By using the --reference option you do not explicitly need to know the permissions during the creation of the bash script, you only need to know which file can be used as a reference. 
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