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:

-------------------------------------

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

-------------------------------------

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?

Linux - Backtik Operator - Return the result of a command as a parameter for another.

Linux has many useful features which make work easier. While managing packages I often find myself trying to run commands like yum remove but can't remember the exact version numbers, character case, or exact spelling. Usually the solution is to run rpm -qa | grep -i . For example:

$ rpm -qa | grep -i virtualbox

This would yield:

VirtualBox-4.1-4.1.0_73009_fedora14-1.x86_64

Try remembering how to type that package name every time you want to upgrade an RPM with yum.

Wouldn't it be nice to be able to combine both "yum remove" and "rpm -qa..." Linux offers this capability through the back-tick operator. ``. What it does is simply returns the output of the command executed within it. This enables the output to be passed as a parameter to some other command.

# yum remove `rpm -qa | grep -i virtualbox`

Yum will then nicely ask if you wish to remove the above mentioned program. Imagine the possibilities.  

WARNING: Be careful if you use this to remove packages from your system.  I only have one package called virtualbox installed on the system, but if you type in some other package name like "pl" you could end up in a lot of trouble.  Double check what your system is trying to remove.

For example:

# yum remove `rpm -qa | grep -i pl`

My system tells me it will remove 693 packages using 4.5GB of space, and is asking me if I want to continue.  That's nearly all my supporting packages.

Basically, use the backtick operator to your heart's content, but be very wary when cross scripting with commands such as 'yum remove'.