this post was submitted on 27 Jul 2024
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I interview developers and information security people all the time. I always ask lots of questions about Linux. As far as I'm concerned:
So yeah: Get good with Linux. Especially permissions! Holy shit the amount of people I interview that don't know basic Linux permissions (or even about file permissions in general) is unreal.
Like, dude: Have you just been
chmod 777everything all this time? WTF! Immediate red flag this guy cannot be trusted with anything.chmod 777do?Quick and dirty: the basic permissions are read, write, and execute, and are applied to the owner, the group, and everyone else. They're applied to all files and directories individually.
It's represented by a 3 digit number (in octal, which is base 8, so 0 to 7). The first number is the permission given to the file's owner, the second to the file's group owner, and the third to everyone else. So, the owner of the file is the one user account that owns it, the group applies to all members of that group. User and group ownership are also applied to each file and directory individually.
Read, write, and execute are represented by the numbers 4, 2, and 1, respectively, and you add them together to get the permission, so 0 would be nothing, 1 would be execute but not read or write, 2 would be write but not read or execute (and yes there are uses for that), 3 would be write and execute but not read, 4 is read only, etc through to 7 which is basically full control.
This will take a little bit to make sense for most people.
chmod (change modifier, I think) is the program you use to set permissions, which you can do explicitly by the number (there are other modes but learn the numbers first), so chmod 777 basically means everyone has full control of the file or directory. Which is bad to do with everything for what I hope are obvious reasons.
chown (change owner) is the program you use to set the owner (and optionally the group) of a file or directory, and chgrp (change group) changes the group only.
It gets deeper with things like setuid bits and sticky bits, and when you get to SELinux it really gets granular and complex, but if you understand the octal 3 digit permissions, you'll have the basics that will be enough for quite a lot of use cases.
(Additionally to the 3 digit number, permissions can be represented a bit friendlier where it just lists letters and dashes, so 750 (full control user, read and execute group) could be shown as rwxr-x---, where r=read, w=write, and x=execute, and what they're applied to can be represented by the letters u for user (aka owner), g for group, and o for other)
This goes into more detail of those basics: https://opensource.com/article/19/6/understanding-linux-permissions
Maybe I'm the weird one here but this seems like a counter intuitive way to remeber/explain it. Each octal digit in the three digit number is actually just 3 binary digits ( 3 bit flags) in order of rwx. For example read and execute would be 101 -> 5.
While that's literally what it is, that's not really how it's represented and requires also understanding binary numbers.
Even knowing that, I've always found it easiest to get to the permissions the way I described, which when you think about it is actually the same as what you'd do to translate binary into decimal/octal if you don't have them memorized: look at the values of each position that's set to 1 and add them together. So, 101 in binary would be 4+0+1, or 5, which is the same as saying read is 4 and execute is 1 and add them together, the latter of which I think is easier to learn (and is how I've always seen it taught, though clearly YMMV)
Both get you to the same place though
That's a fair point, I guess I used binary numbers so much i uni that I just know the small ones by heart and that's why I find it easier. Following the example, I never convert 101 as 4+0+1, I just see it and know it's 5.
Thanks. Bookmarking for future reference.
Juwt remember "man chmod".
Also, they didn't mention it but you can always just do this (the easy way, thanks to GNU):
chmod a+x somefileto give it execute bits. It works intuitively like that forwandrpermissions too.It's just quicker to type out
chmod 775than it is to do it the other way 🤷"chmod 777 is opening your front door with a sledgehammer"