Change Mode

You may not share my excitement at this, but if you are someone for whom CLI, UNIX and/or binary have been some sort of enigma at any point, then you may at least have some sympathy for me.

In the previous post I went through how I made file executable for the first time, by using chmod command. Taking one step at a time, I did it the boring way as it didn't require additional explanation... Now, I want to look at the more fun version of it which uses binary!

This is the command I looked at last time:

chmod a+x bin/mastermind

If mt current permissions are:

-rw-r--r--  1 Jarkyn  staff  312 12 Mar 15:30 mastermind

Running the above chmod command would change them to the following:

👻  mastermind [master] ⚡ ls -l bin
-rwxr-xr-x  1 Jarkyn    staff    312    12 Mar 15:40   mastermind

Which means that we added ability to execute mastermind file to all user types. Lets rewind this change by running the opposite of "+" command:

chmod a-x bin/mastermind

This would take away ability to execute from all the user types and we are back to:

-rw-r--r--  1 Jarkyn  staff  312 12 Mar 15:50 mastermind

Next, we will put ability to execute back fot all user types, but this time instead of cmod a+x we will use:

chmod 755 <file_path>

Running it would bring us back to

-rwxr-xr-x  1 Jarkyn    staff    312    12 Mar 16:02   mastermind

So this code 755 seems to be doing the magic, lets see how it's doing it. First, let's look at those permissions:

rwx r-x r-x

We can think of those as switches for a specific permission in each group:

on on on    on off on    on off on

And taking it step further we can represent this as binary switches: 1 means on, 0 means off.

111 101 101

Now, if we convert each one to the decimal:

  111 => 7
  101 => 5
  101 => 5

And this is why we can say:

chmod 755 <file_path>

As you imagine there will be a number of permutations representing whether each r/w/x is on or off

--- => 000 => 0 => NO read write or execute

--x => 001 => 1 => Execute (because last bit set to 1)

-w- => 010 => 2 => Write (because middle bit set to 1)

-wx => 011 => 3 => Write,Execute (because last 2 bits set to 1)

r-- => 100 => 4 => Read (because first bit set to 1)

r-x => 101 => 5 => Read,Execute (because first and last bits set to 1)

rw- => 110 => 6 => Read,Write (because first and second bits set to 1)

rwx => 111 => 7 => Read, Write, Execute (because all bits set to 1)

Say you wanted following permissions for a file:

rwx r-- --x

This is how the command would look like:

chmod 741 <file_path>

That's it, no magic and no more mystery! Which brings me to the end of this geek out session, hope you found it useful!