Permissions are the unix way of controlling access to resources such as files, directories and devices and may be specified for an owner, group, or all users.

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92
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How to chmod without /usr/bin/chmod?

Today I was told a tale by a Unix trainer, a tale where the root password got leaked to the students, and one of the fellas removed the execute permission from /usr/bin/chmod itself. How do you ...
72
votes
3answers
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Why do directories need the executable (X) permission to be opened?

In my CMS, I noticed that directories need the executable bit (+x) set for the user to open them. Why is the execute permission required to read a directory?
67
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2answers
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Why do we use su - and not just su?

I don't understand why su - is preferred over su to login as root.
58
votes
1answer
60k views

How to set default file permissions for all folders/files in a directory?

I want to set a folder such that anything created within it (directories, files) inherit default permissions and group. Lets call the group "media". And also, the folders/files created within the ...
35
votes
5answers
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Redirecting stdout to a file you don't have write permission on

When you attempt to modify a file without having write permissions on it, you get an error: > touch /tmp/foo && sudo chown root /tmp/foo > echo test > /tmp/foo zsh: permission ...
32
votes
3answers
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How does vim steal root owned files?

Witness the following: sh-3.2$ mkdir testcase sh-3.2$ cd testcase sh-3.2$ sudo touch temp sh-3.2$ ls -al total 0 drwxr-xr-x 3 glen staff 102 19 Dec 12:38 . drwxr-xr-x 12 glen staff 408 19 Dec ...
30
votes
4answers
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Why can't I execute shutdown when the permission is rwxr-xr-x?

I'm in /sbin and I see that shutdown has permissions rwxr-xr-x. Doesn't this mean that anyone can execute it?
30
votes
2answers
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Why can rm remove read-only files?

If I create a file and then change its permissions to 444 (read-only), how come rm can remove it? If I do this: echo test > test.txt chmod 444 test.txt rm test.txt ...rm will ask if I want to ...
28
votes
3answers
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Make all new files in a directory accessible to a group

Suppose I have two users A and B and a group G and a folder foo, both users are members of G (using linux and ext3). If I save as user A a file under foo, the permissions are: -rw-r--r-- A A. ...
27
votes
4answers
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How to recover from a chmod -R 000 /bin?

And now I am unable to chmod it back.. or use any of my other system programs. Luckily this is on a VM I've been toying with, but is there any way to resolve this? The system is Ubuntu Server 12.10. ...
22
votes
9answers
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Why use `chmod 644` instead of `chmod u=rw,go=r,…`?

I've been working on *nix for a few years now, and one of the things I just can't get used to is octal permissions in code. Is there some other reason than line length to prefer chmod 644 ... over ...
22
votes
1answer
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Why can't root execute when executable bits are not set?

root user can write to a file even if its write permissions are not set. root user can read a file even if its read permissions are not set. root user can cd into a directory even if its execute ...
21
votes
4answers
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Is there a reason why 'owner' permissions exist? Aren't group permissions enough?

I think I rather understand how file permissions work in linux. However, I don't really understand why they are split into three levels and not into two. I'd like the following issues answered: Is ...
21
votes
3answers
4k views

Clone ownership and permissions from another file?

Is there a command or flag to clone the user/group ownership and permissions on a file from another file? To make the perms and ownership exactly "like" that of another file?
21
votes
3answers
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Why can't a normal user `chown` a file?

Why is the chown command root-only? Why can't non-root users use chown to give away files they own?
20
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6answers
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Can a script be executable but not readable?

Is it possible to execute a script if there is no permission to read it? In root mode, I made a script and I want the other user to execute this script but not read it. I did chmod to forbid read and ...
20
votes
2answers
3k views

How do I remove a file with no permissions?

A hacker has dropped a file in my tmp dir that is causing issues. Nothing malicious except creating GB's of error_log entries because their script is failing. However, the file they are using to ...
19
votes
2answers
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How do I make a file NOT modifiable?

While logged in, I can do the following: mkdir foo touch foo/bar chmod 400 foo/bar chmod 500 foo Then I can open vim (not as root), edit bar, force a write with w!, and the file is modified! How ...
18
votes
2answers
881 views

Is there any difference between mode value 0777 and 777

I saw a code change at work, where the mode values were changed from 777 to 0777 to make nfs setattr work. What is the difference in the 2 values?
17
votes
3answers
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What are common rights for /tmp ? I unintentionnally set it all public recursively

I have abused sudo. I have created a really really short life temporary directory that I wanted to share between some users for a few hours... and I named this directory /some/path/tmp Unfortunately ...
17
votes
2answers
9k views

Do the parent directory's permissions matter when accessing a subdirectory?

If I have a root folder with some restrictive permission, let's say 600, and if the child folders/files have 777 permission will everybody be able to read/write/execute the child file even though the ...
15
votes
1answer
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How to skip “permission denied” errors when running find in Linux? [duplicate]

Possible Duplicate: How do I remove “permission denied” printout statements from the find program? When I run this command in Linux (SuSE): find / -name ant I get many error ...
14
votes
3answers
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Is there any way to prevent deletion of certain files from user owned directory?

Let's say user has Directory1 and it contains File1 File2 CantBeDeletedFile How to make so the user would never be allowed to delete the CantBeDeletedFile? If I change the ownership of Directory1 and ...
13
votes
6answers
2k views

Can root/superuser read my read-protected files?

On shared unix hosting, if I have a file sensitive-data.txt and I issue: chmod 600 sensitive-data.txt Can root user still read my file? Specifically I'm wondering if it's safe to store my password ...
13
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3answers
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How to “jail” a process without being root?

Were I root, I could simply create a dummy user/group, set file permissions accordingly and execute the process as that user. However I am not, so is there any way to achieve this without being root?
13
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7answers
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What to use to backup files, preserving ACLs?

When using the tar utility to store files in backups one loses the extended ACLs. Is there some commonly used and not hackish solution (like: create a script that will recrate the ACLs from scratch) ...
13
votes
3answers
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I accidentally chmod -R +x on a directory. How do I restore the correct permissions?

Well, to be specific, it was chmod -R 755. Now every file is executable, which I don't want. I am thinking that I should look at the first two bytes of each file for the #!, but will this cover ...
12
votes
3answers
3k views

Wrongly set chmod / 777. Problems?

I was trying to run chmod -R 777 ./ but ended up typing chmod -R 777 / and set 777 on my entire machine. What can go wrong? How can I fix it?
12
votes
2answers
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File permission issues with shared folders under Virtual Box (Ubuntu Guest, Windows Host)

I am using Ubuntu on Virtual Box and I have a folder which is shared between the host (Windows) and the VM (Ubuntu). When I open any file in the share folder in Ubuntu, I can not change it as its ...
12
votes
2answers
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How to apply changes of newly added user groups without needing to reboot?

Assume I'm logged in with user takpar: takpar@skyspace:/$ As root, I've added takpar as a member of group webdev using: # usermod -a -G webdev takpar But it seems it has not been applied. because ...
12
votes
1answer
502 views

Can I restrict a user to use special programs?

Is it possible to add a user and restrict him to run special programs? For example after this user logs in, he can only open Firefox to use Internet and no other programs can be run by this user. For ...
11
votes
6answers
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Is there a way to stop having to write 'sudo' for every little thing in Linux?

I'm a newbie when it comes to Linux. I do most of my development in a Windows environment. That said, I'm going to be doing a fair amount of PHP work shortly, and I'm interested in learning RoR, so ...
11
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3answers
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How does the sticky bit work?

SUID The sticky bit applied to executable programs flagging the system to keep an image of the program in memory after the program finished running. But I don't know that what it's stored in ...
10
votes
2answers
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What is the difference between 'su -' and 'su root'? [duplicate]

Possible Duplicate: Why do we use su - and not just su? I understand that root doesn't have to be a superuser. But in the case that it is ... what is the difference between sudo su - and ...
10
votes
3answers
2k views

How to check if a user can access a given file?

*nix user permissions are really simple, but things can get messy when you have to take in account all the parent directory access before reaching a given file. How can I check if the user has enough ...
10
votes
2answers
720 views

Deny flash plugin access to (a particular) webcam

I recently bought an external webcam to play around with OpenCV. It works great out of the box in Debian, but I realized that when I browse any site that contain flash content (youtube, soundcloud), ...
10
votes
1answer
818 views

Permission bits not being enforced on samba share

I have a problem where permission bits are not being enforced on a samba share using a Linux client. I have samba configured on the server to force a certain user, group and permission bits and this ...
9
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4answers
2k views

How to create a file even root user can't delete it

How to create a file even root user can't delete it ?
9
votes
4answers
1k views

Sudoers for one day per week?

I have been volunteered by my boss to be the sysadmin for our production redhat server. He asked me to tighten the security to avoid mishaps like rm -f * that occured not long ago. Right now, we ...
9
votes
5answers
2k views

How do I recursively check permissions in reverse?

There's a command, I think it comes with apache, or is somehow related to it, that checks permissions, all the way down. So if I have /home/foo/bar/baz it will tell me what the permissions are for ...
9
votes
2answers
2k views

What does the 'd' mean in ls -al results and what is that slot called?

In Linux, what does the d mean in the first position of drwxr-xr-x? And what are all of the possible letters that could be there, and what do they mean? I'm trying to learn more about the Linux file ...
9
votes
1answer
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How to (safely) move /tmp to a different volume?

Today the /tmp directory filled up on a machine at work. The problem was, it was on the root partition which wasn't very big. In order to fix this, a co-worker created a /new/tmp directory elsewhere, ...
8
votes
4answers
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Is there a web based converter between rwx and the octal version?

I can never remember what the conversion is from something like rw-r--r-- to 644. Is there a simple web based converter between the 2?
8
votes
2answers
1k views

Permissions/ownership of /usr/local/bin

From what I understand, the right place to put your own scripts is /usr/local/bin (for instance a script I use to back up some files). I notice that this folder is currently (by default) owned by ...
8
votes
4answers
17k views

“Symbolic link not allowed or link target not accessible” / Apache on CentOS 6

I've got a brand new CentOS 6 installation, which has a symlink in the document root to my development files: [root@localhost html]# ls -l total 4 -rwxrwxrwx. 1 root root 0 Sep 18 20:16 index.html ...
8
votes
5answers
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Directory with +x permission, parents without it. When would this be useful?

Say I have a folder called folder in the following path: my_path = /a/b/c/d/e/folder and a file called file in that folder. Then, say I run this command to remove group permissions under /a/ > ...
8
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2answers
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User can't touch -t

When SCP'ing to my Fedora server, a user keeps getting errors about not being able to modify file timestamps ("set time: operation not permitted"). The user is not the owner of the file, but we cannot ...
8
votes
2answers
300 views

How is file ownership affected across different systems?

As I understand Unix file systems, any file on a unix system must belong to a group and a user of the said system. A file cannot belong to a group or user that does not exist on the system. From that ...
7
votes
3answers
1k views

How Do I check Permissions of Root Folder (/ Folder, not /root)?

Is there a way to check the permissions of the root folder, /? I mean the folder's permissions, not its content's (/var, /usr, etc.) permissions? Running ls /.. shows the content's permissions.
7
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4answers
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sudo permission denied but su grants permission

this is the first occurrence where su was required for me. I read an article about changing the value in /sys/devices/virtual/backlight/acpi_video0/brightness to alter my laptop's screen brightness. ...