I recently set up the ability to deploy to my remote server via the git push command, but it failed on my first attempt with the error, remote unpack failed: unable to create temporary object directory. As the error message suggests, git push was attempting to create a directory on my remote server, but my user did not have permission to do this. After researching file permissions and how to modify them, I was able to solve the problem.

Everything in Linux and Unix is a file (including directories), and every file on a system has permissions that allow certain users to view, edit, or execute them. The root user is able to access any file on the system.

To view the permissions on a file or directory, use ls -l filename:

user@host:~$ ls -l /etc/hosts
-rw-r--r-- 1 root root 617 Jul 10 18:43 /etc/hosts

Let’s break down this result into fields.

User Permissions	Owner	Group	Size in Bytes	Date of Last Modification	File Name
`-rw-r--r--`	`root`	`root`	`617`	`Jul 10 18:43`	`/etc/hosts`

User Permissions

The very first character in -rw-r--r-- represents the special permissions on /etc/hosts. Here, we see that the special permissions are empty (represented by -), which tells us that /etc/hosts is a normal file. If we were inspecting a directory or a symlink instead, we would see a d or an l, respectively.

The remaining rw-r--r-- tells us which permissions are available for various users. The first three bits represent the owner’s permissions, the following three are the permissions of the group, and the final three are those of all other users.

User	Permissions	Translation
owner	`rw-`	read and write only
group	`r--`	read only
other	`r--`	read only

/etc/hosts can be read from by any user, but only the owner can edit (write) the file. Some files can be executed and have the “execute” permission represented with an x. For example, a file that could be read from, written to, and executed by only the owner would have the permissions -rwx------.

Note that these permissions are not inheritable—e.g. the owner of the file is not affected by the permissions set for the group or for all other users.

These permissions behave differently if the file in question is a directory. Being able to read from a directory allows the user to view the directory’s contents (such as using the ls command), while being able to write to it allows the user to create and delete files in the directory. A word of caution: even if a user does not have write access to a file itself, they could still delete it if they have write access to the file’s directory. Finally, having execution permission allows a user to change into the directory (i.e. using the cd command).

Owner

Unless otherwise specified during touch, the owner of the file is the user who created it. The owner of a file can be changed with the chown command.

user@host:~# ls -l demo.txt
-rw-r--r-- 1 root root 0 Jul 17 16:13 demo.txt
user@host:~# chown deploy demo.txt
user@host:~# ls -l demo.txt
-rw-r--r-- 1 deploy root 0 Jul 17 16:13 demo.txt

Group

The group of a file can also be changed with the chown command, which will accept the owner and the group separated by a colon, like [OWNER]:[GROUP]. If the colon is present but the owner is missing, chown will still change the group accordingly.

user@host:~# ls -l demo.txt
-rw-r--r-- 1 deploy root 0 Jul 17 16:13 demo.txt
user@host:~# chown :deploy demo.txt
user@host:~# ls -l demo.txt
-rw-r--r-- 1 deploy deploy 0 Jul 17 16:13 demo.txt

Changing Permissions

We change the permissions on a file with the chmod command, using either numbers or letters. I find the letters easier to understand, so let’s use them.

The usage for the chmod command is: chmod {users}{action}{permissions} filename, and here are all of the available options.

User Options	Definition
`u`	owner
`g`	group
`o`	other
`a`	all users (same as `ugo`)

Action Options	Definition
`+`	add permission
`-`	remove permission
`=`	set permission

Permission Options	Definition
`r`	read
`w`	write
`x`	execute

Let’s change the permissions of demo.txt so the owner can execute it.

user@host:~# ls -l demo.txt
-rw-r--r-- 1 deploy deploy 0 Jul 17 16:13 demo.txt
user@host:~# chmod u+x demo.txt
user@host:~# ls -l demo.txt
-rwxr--r-- 1 deploy deploy 0 Jul 17 16:13 demo.txt

You can change permissions for multiple users at once by simply listing them (e.g. ug+w), just as you can add or remove multiple permissions at the same time (e.g. o-wx).

I hope you now have a better understanding of file permissions!