Linux / Unix file system permissions. How to memorize the bits.

A very, very brief note on Linux / Unix file system permission bits.

Numeric Bit Value Table:

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num bit   rep   meaning

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0   000   ---   no permissions

1   001   --x   execute

2   010   -w-   write

3   011   -wx   write, execute

4   100   r--   read

5   101   r-x   read, execute

6   110   rw-   read, write

7   111   rwx   read, write, execute

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Mnemonics:

Using  the above bits is simple, but it's not always obvious remembering which value represents which permissions. Eg: Read and Write is 5 or 6? It's easy if you have the chart in front of you, and if you can visualize the chart in your mind.

Here are 4 simple tricks to keep in mind to successfully memorize permission bits.

1) Always remember the order in which permissions are assigned (RWX). From left to right, "Read, Write, Execute." - Never "Write, Execute, Read" or any other combination.

2) Think of 4 as what cuts the permissions table in half. All read permissions are values 4 or above. Binary works from right to left, but the highest values will have a new position digit on the left side. Hence, think of "read" as in the highest values since it's on the left.

3) All write permissions are two values above 0 and 4. So: 2,3 and 6,7. Write access is probably the most important permission to remember due to it's sensitive nature.

4) Execute permissions have the easiest trick. Any odd bit is executable. How simple is that?