Undo Chown and Chmod

Undo chown and chmod

I think there is no way undo chown and chmod. But you can see default permission of these folder in any other machine which has fresh installation or you can install lampp again in different folder. Then change chown and chmod permissions of /opt/lampp/htdocs to default.

Undoing erroneous chmod

Thanks for the comment. I ended up doing a clean reinstall of Unbuntu. All fixed now! Just that it was really laborious to manually transfer all my existing files since wholesale copy+pasting didn't work (seemed like the erroneous permissions got copied over too).

How to restore the permissions of files and directories within git if they have been modified?

Git keeps track of filepermission and exposes permission changes when creating patches using git diff -p. So all we need is:

1. create a reverse patch
2. include only the permission changes
3. apply the patch to our working copy

As a one-liner:

git diff -p -R --no-ext-diff --no-color \
    | grep -E "^(diff|(old|new) mode)" --color=never  \
    | git apply

you can also add it as an alias to your git config...

git config --global --add alias.permission-reset '!git diff -p -R --no-ext-diff --no-color | grep -E "^(diff|(old|new) mode)" --color=never | git apply'

...and you can invoke it via:

git permission-reset

Note, if you shell is bash, make sure to use ' instead of " quotes around the !git, otherwise it gets substituted with the last git command you ran.

Thx to @Mixologic for pointing out that by simply using -R on git diff, the cumbersome sed command is no longer required.

Change all files and folders permissions of a directory to 644/755

One approach could be using find:

for directories

find /desired_location -type d -print0 | xargs -0 chmod 0755

for files

find /desired_location -type f -print0 | xargs -0 chmod 0644

How to remove setgid (linux/unix)?

Change the + for adding a permission into a - to remove it:

sudo chmod g-s filename

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If you want to do this programatically, you’ll need to use some bitwise operators. Normally it’s

mode_without_suid = bitwise_and(existing_mode, bitwise_not(S_ISUID))

where S_ISUID is 0o4000, a constant that uses mode bits above the typical rwx ones of something like 0644.

For example, in python

import os
import stat

def mode_details(m):
    return f"mode={oct(m)} = {stat.filemode(m)}"

mode = os.stat('foo').st_mode
print("old mode", mode_details(mode))

new_mode = mode & ~stat.S_ISUID

os.chmod('foo', new_mode)
print("new mode", mode_details(new_mode))

which prints

old mode mode=0o104654 = -rwSr-xr--
new mode mode=0o100654 = -rw-r-xr--

Changing File Permissions Linux

chmod

The chmod command is used to change the permissions of a file or directory. To use it, you specify the desired permission settings and the file or files that you wish to modify. There are two ways to specify the permissions, but I am only going to teach one way.

It is easy to think of the permission settings as a series of bits (which is how the computer thinks about them). Here's how it works:

rwx rwx rwx = 111 111 111

rw- rw- rw- = 110 110 110

rwx --- --- = 111 000 000

and so on...

rwx = 111 in binary = 7

rw- = 110 in binary = 6

r-x = 101 in binary = 5

r-- = 100 in binary = 4

777

(rwxrwxrwx) No restrictions on permissions. Anybody may do anything. Generally not a desirable setting.

755

(rwxr-xr-x) The file's owner may read, write, and execute the file. All others may read and execute the file. This setting is common for programs that are used by all users.

700

(rwx------) The file's owner may read, write, and execute the file. Nobody else has any rights. This setting is useful for programs that only the owner may use and must be kept private from others.

666

(rw-rw-rw-) All users may read and write the file.

644

(rw-r--r--) The owner may read and write a file, while all others may only read the file. A common setting for data files that everybody may read, but only the owner may change.

600

(rw-------) The owner may read and write a file. All others have no rights. A common setting for data files that the owner wants to keep private.

Directory permissions

The chmod command can also be used to control the access permissions for directories. In most ways, the permissions scheme for directories works the same way as they do with files. However, the execution permission is used in a different way. It provides control for access to file listing and other things. Here are some useful settings for directories:

777

(rwxrwxrwx) No restrictions on permissions. Anybody may list files, create new files in the directory and delete files in the directory. Generally not a good setting.

755

(rwxr-xr-x) The directory owner has full access. All others may list the directory, but cannot create files nor delete them. This setting is common for directories that you wish to share with other users.

700

(rwx------) The directory owner has full access. Nobody else has any rights. This setting is useful for directories that only the owner may use and must be kept private from others.

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