Uly.me

cloud engineer

Linux

Linux Fuser

Who is accessing a particular directory? There’s a Linux command called fuser which will tell you who’s running processes in that directory.

fuser /etc/apache/
fuser /home/john/

Verbose.

fuser /data/exports/ -v

Check out the fuser man page.

Ansible Run Commands

Typically you will run Ansible using playbooks, just like the playbook that I ran from the previous post to patch a bunch of servers all at once. In addition to running playbooks, you can also run single ad hoc commands to a server or to a group of servers. Below are a few examples of that.

First, here’s my /etc/ansible/hosts file.

[servers]
server1
server2
[webservers]
server3
server4
[appservers]
server5
server6
[dbservers]
server7
server8

Here are the commands.

ansible servers -m ping
ansible webservers -m command -a "df -h"
ansible appservers -m command -a "crontab -l"
ansible dbservers -m command -a "cat /etc/hosts"
ansible all -m command -a "cat /etc/passwd"
ansible webservers -m command -a "systemctl status apache2"
ansible appservers -m command -a 'uptime'

The first command runs ping on server1 and server2. The second command displays disk information of server3 and server4. The third command displays the crontab of server5 and server6, and so on and on. You get the drift.

Ansible Update Playbook

I have an Ansible playbook that will patch all Ansible clients defined in the Ansible hosts file. The following are contents of my hosts file, and the update playbook.

File: /etc/ansible/hosts

[all:vars]
ansible_user='ubuntu'
ansible_become=yes
ansible_become_method=sudo
ansible_python_interpreter='/usr/bin/env python3'
[servers]
server1
server2
server3
[servers:vars]
ansible_python_interpreter=/usr/bin/python3

File: /etc/ansible/update.yml

---
- hosts: servers
  become: true
  become_user: root
  tasks:
    - name: Update apt repo and cache on all Debian/Ubuntu boxes
      apt: update_cache=yes force_apt_get=yes cache_valid_time=3600

    - name: Upgrade all packages on servers
      apt: upgrade=dist force_apt_get=yes

    - name: Check if a reboot is needed on all servers
      register: reboot_required_file
      stat: path=/var/run/reboot-required get_md5=no

    - name: Reboot the box if kernel updated
      reboot:
        msg: "Reboot initiated by Ansible for kernel updates"
        connect_timeout: 5
        reboot_timeout: 300
        pre_reboot_delay: 0
        post_reboot_delay: 30
        test_command: uptime
      when: reboot_required_file.stat.exist

Here’s how I run my Ansible update playbook.

ansible-playbook -i /etc/ansible/hosts /etc/ansible/update.yml

The advantage of using Ansible is, I can run a single playbook to update dozens of servers. It’s also a great tool for rolling out software as well as executing commands to a group of servers.

Change Default Editor

Ubuntu and Linux Mint use nano as their default editor. You’ll see it if you try to edit crontab or visudo. Now, there’s nothing wrong with nano, but I prefer using vim instead. Here’s how to change the default editor.

sudo update-alternatives --config editor

You’ll be prompted to change the default editor.

There are 4 choices for the alternative editor (providing /usr/bin/editor).
 
  Selection    Path                Priority   Status
------------------------------------------------------------
  0            /bin/nano            40        auto mode
  1            /bin/ed             -100       manual mode
  2            /bin/nano            40        manual mode
* 3            /usr/bin/vim.basic   30        manual mode
  4            /usr/bin/vim.tiny    15        manual mode
 
Press <enter> to keep the current choice[*], or type selection number:

Type 3 to choose vim basic.

Remove Sudo Password

Removing sudo password is typically not a good practice. If you’re a seasoned system administrator, or you’re the only one using your own desktop, being prompted for a sudo password can get really annoying. Here’s how to stop getting prompted every time you run sudo.

Run visudo. 

sudo visudo

Edit the sudo line and set to below.

%sudo ALL=(ALL) NOPASSWD: ALL

Save file.

Remove Quotes From Strings

Here’s the command to remove quotes from a string in Bash by using the tr -d command. I’m using jq tool to extract the access key from a json file. Bash returns a string surrounded by quotes. To remove the quotes, just pipe the string to the tr -d command.

jq .AccessKey.AccessKeyId key.json

Result:

"ASDFSDFSDFASDFSDFSDFGHGDF"

Using tr -d.

jq .AccessKey.AccessKeyId key.json | tr -d \"

Result:

ASDFSDFSDFASDFSDFSDFGHGDF