How to check and change Linux permissions (permissions) (chmod) (I want to be saved from Permiss on denied)


Until now, when I executed the command and the characters `Pemission denied``` were displayed, I knew that I should add sudo``` for the time being (not good). I think that it is not good as it is, and since I learned about access authority this time, I would like to write down my memorandum.

How to check file permissions

All files handled by Linux have an owner. You can check it with the ls -l command. The ls command is a command to list files and directories, but by adding the -l``` option, you can display not only the file name but also detailed information. I will.

$ ls -l sample.txt
-rw-r--r--  1 morimorimokenpi  staff  0  4 19 22:24 sample.txt

File type

The leading `-` indicates the file type. There are three types of files, and in this case it is `` `-```, so it represents a normal file.

symbol meaning
- Normal file
d directory
l Symbolic link

File mode

The following `rw-r--r--``` is called file mode and represents the file permissions. ```rw-``` ```r--``` ```r--```And 3 are separated, each is an "owner","group",Represents the permissions of "other users". Only r``` and `` w are mentioned here, but there is another `` `x as a symbol for permission.

Permission symbol and meaning

symbol meaning
r reading(read)
w writing(write)
x Run(execute)

The permissions of sample.txt this time are as follows.

User type reading writing Run
Owner Permit Permit Ban
group Permit Ban Ban
Other users Permit Ban Ban

How to change permissions

Now that you know what the file type and file mode are, how do you change the permissions? Use the chmod (short for change mode) command to change permissions. There are two ways to specify the chmod command, so I will introduce them in order.

Change by symbol mode

$ chmod [ugoa] [+-=] [rwx]file name

[ugoa]Represents which user to target. This user specification can be omitted, but if omitted, it is considered that a is specified.

symbol meaning
u Owner
g group
o Other users
a ugo all

[+-=]Specifies the addition or prohibition of privileges.

symbol meaning
+ Add permissions
- Prohibit authority
= Make the specified authority

[rwx]Represents the meaning of permissions, as explained above.

Here, as an example, let's add write permission for the group to sample.txt.

$ chmod g+w sample.txt

Before execution

$ ls -l sample.txt 
-rw-r--r--  1 morimorimokenpi  staff  0  4 19 22:45 sample.txt

After execution

$ ls -l sample.txt 
-rw-rw-r--  1 morimorimokenpi  staff  0  4 19 22:47 sample.txt

Added write permission for the group.

It is also possible to set the privileges of multiple users at once. Let's add execute permission to the owner and other users.

$ chmod uo+x sample.txt
$ ls -l sample.txt 
-rwxr--r-x  1 morimorimokenpi  staff  0  4 19 22:50 sample.txt

Execute permission has been added to the owner and other users.

Change by numerical mode

In numeric mode, the permissions granted in `` `rwx``` are replaced with the numbers in the table below and added together to represent the permissions numerically.

$chmod octadecimal number filename
Numbers meaning
4 reading(read)
2 writing(write)
1 Run(execute)

It looks like this in the figure (I'm sorry it's dirty lol) 5QuIWXyCRbuVBSR1%OZU8w_thumb_21f8.jpg

Let's actually change the permissions of sample.txt.

$ chmod 755 sample.txt

Before execution

$ ls -l sample.txt
-rw-r--r--  1 morimorimokenpi  staff  0  4 19 23:14 sample.txt

After execution

$ ls -l sample.txt
-rwxr-xr-x  1 morimorimokenpi  staff  0  4 19 23:15 sample.txt

In this way, the numeric mode is an absolute specification that changes the value to the new permission regardless of the original permission. On the other hand, the symbol mode is a relative specification that does not change the permissions other than those specified. It may be better to use it properly according to the changed part.


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