os.MkDir and os.MkDirAll permission value?

You can use octal notation directly:

os.Mkdir("dirname", 0700)

Permission Bits

+-----+---+--------------------------+
| rwx | 7 | Read, write and execute  |
| rw- | 6 | Read, write              |
| r-x | 5 | Read, and execute        |
| r-- | 4 | Read,                    |
| -wx | 3 | Write and execute        |
| -w- | 2 | Write                    |
| --x | 1 | Execute                  |
| --- | 0 | no permissions           |
+------------------------------------+

+------------+------+-------+
| Permission | Octal| Field |
+------------+------+-------+
| rwx------  | 0700 | User  |
| ---rwx---  | 0070 | Group |
| ------rwx  | 0007 | Other |
+------------+------+-------+

A Unix Permission Primer


Common Permission Usages

0755 Commonly used on web servers. The owner can read, write, execute. Everyone else can read and execute but not modify the file.

0777 Everyone can read write and execute. On a web server, it is not advisable to use ‘777’ permission for your files and folders, as it allows anyone to add malicious code to your server.

0644 Only the owner can read and write. Everyone else can only read. No one can execute the file.

0655 Only the owner can read and write, but not execute the file. Everyone else can read and execute, but cannot modify the file.

www.maketecheasier.com/file-permissions-what-does-chmod-777-means/

Directory Permissions on Linux

When applying permissions to directories on Linux, the permission bits have different meanings than on regular files. (source)

Read bit The user can read the file names contained in the directory.
Write bit The user can {add,rename,delete} files names IF the execute bit is set too.
Execute bit The user can enter the directory and access the files inside.

https://unix.stackexchange.com/a/21252

Permissions Calculator

A handy permissions calculator.

@Daniel's statement in his answer is not really correct, and also it talks about a decimal number and then uses an octal one, as @SashaCrofter correctly pointed out in his comment.

In reality, it doesn't matter what form your permission value is in as long as it represents sensible Unix permissions.

Since permission bits on POSIX file systems come in triples of bits — three bits for owner, group and others access, plus three bits of modifiers (such as sticky bits), — it's customary to use octal numbers to represent permissions as each digit in an octal number represents a three-bit value.

Hence, when you use 0700 in Go code, the leading 0 is stripped and is only there to tell the parser it sees an octal number literal, and the following three letters stand for the owner, group and others permissions, in this order. Should you, say, want to also set the group sticky bit as well as making the file system object group-readable and executable, you'd specify 02750 and so on.

Note that the actual permissions the file system object acquires is further modulated by the active umask of the process which creates the object.

To get more grip on these topics, it's best to read the chmod manual pages and general literature on Unix-like operating systems.

You can reset the umask to 0. I would call this as the first thing in my main file

syscall.Umask(0)

Example

_ = os.MkdirAll("/tmp/dirs/1", 0664)
syscall.Umask(0)
_ = os.MkdirAll("/tmp/dirs/2", 0664)

Result

/tmp/dirs$ stat -c '%A %a %n' *
drw-r--r-- 644 1
drw-rw-r-- 664 2