cannot create directory : Permission denied

I had created a directory mkdir xyz with user centos

The following is the output of ll command:
drw——-. 2 centos centos 6 Oct 6 06:11 xyz

When I executed cd xyz I received the following error:
bash: cd: xyz/: Permission denied

I could not access the directory in any case
mkdir xyz/abc
touch xyz/test.txt
All returned the same error.

The solution for this is to add +x permission on the directory.
chmod 700 xyz/
Then I could cd into the directory and create files and folders.

The logic behind this is as belows:

Read bit = If set, you can read this list. So, for example, if you have a directory named poems:

  • You can ls poems and you’ll get a list of items living within (-l won’t reveal any details!).
  • You can use command-line completion i.e. touch poems/so <TAB> poems/somefile.
  • You cannot make poems your working directory (i.e. cd into it).

Write bit = If set, you can modify this list i.e. you can {add,rename,delete} names on it. But! You can actually do it only if the execute bit is set too.

Execute bit = Make this directory your working directory i.e. cd into it. You need this permission if you want to:

  • access (read, write, execute) items living within.
  • modify the list itself i.e. add, rename, delete names on it (of course the write bit must be set on the directory).

Interesting case 1: If you have write + execute permissions on a directory, you can {delete,rename} items living within even if you don’t have write perimission on those items. (use sticky bit to prevent this)

Interesting case 2: If you have execute (but not write) permission on a directory AND you have write permission on a file living within, you cannot delete the file (because it involves removing it from the list). However, you can erase its contents e.g. if it’s a text file you can use vi to open it and delete everything. The file will still be there, but it will be empty.

So the solution is to add +x to the directory.
You will be able to cd into the directory even if you don’t have +x if you are the root user.

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Pyinotify – Monitor Filesystem Changes in Real-Time in Linux

Pyinotify is a simple yet useful Python module for monitoring filesystems changes in real-time in Linux.

As a System administrator, you can use it to monitor changes happening to a directory of interest such as web directory or application data storage directory and beyond.

It depends on inotify (a Linux kernel feature incorporated in kernel 2.6.13), which is an event-driven notifier, its notifications are exported from kernel space to user space via three system calls.

The purpose of pyinotify is to bind the three system calls, and support an implementation on top of them providing a common and abstract means to manipulate those functionalities.

In this article, we will show you how to install and use pyinotify in Linux to monitor filesystem changes or modifications in real-time.

Dependencies

In order to use pyinotify, your system must be running:

  1. Linux kernel 2.6.13 or higher
  2. Python 2.4 or higher

How to Install Pyinotify in Linux

First start by checking the kernel and Python versions installed on your system as follows:

# uname -r 
# python -V

Once dependencies are met, we will use pip to install pynotify. In most Linux distributions, Pip is already installed if you’re using Python 2 >=2.7.9 or Python 3 >=3.4 binaries downloaded from python.org, otherwise, install it as follows:

# yum install python-pip      [On CentOS based Distros]
# apt-get install python-pip  [On Debian based Distros]
# dnf install python-pip      [On Fedora 22+]

Now, install pyinotify like so:

# pip install pyinotify

It will install available version from the default repository, if you are looking to have a latest stable version of pyinotify, consider cloning it’s git repository as shown.

# git clone https://github.com/seb-m/pyinotify.git
# cd pyinotify/
# ls
# python setup.py install

How to Use pyinotify in Linux

In the example below, I am monitoring any changes to the user tecmint’s home (/home/tecmint) directory as root user (logged in via ssh) as shown in the screenshot:

# python -m pyinotify -v /home/tecmint

Monitor Directory Changes

Next, we will keep a watch for any changes to the web directory (/var/www/html/tecmint.com):

# python -m pyinotify -v /var/www/html/tecmint.com

To exit the program, simply hit [Ctrl+C].

Note: When you run pyinotify without specifying any directory to monitor, the /tmp directory is considered by default.