Here it goes: Need to create a bash script which asks for a word from user, say either “one”, “two” or “three” and then check in single if statement (no else if section) that if its not “one” or “two” or “three” then print Not OK otherwise print OK.

Further, here is a snippet, check and figure out why its not working as expected:

#!/bin/bash
echo -n Enter a word:
read myword
 
if [[ $myword != "one" || $myword != "two" || $myword != "three" ]]; then
 echo Not OK
else
 echo OK
fi

Please put your suggestion/solution script in comments.

1. Using Cron
   If script needs to be executed every x minute or a frequency which is more than a minute, then it should run through cron only. You can also place it in cron when the script itself contains code to execute repeatedly. Make sure to redirect all its logs/errors to some log file for checking in case some issue occurs.
2. Using Daemon
   If script requires running on frequency less than a minute which is not supported by cron, then a wrapper script acting as daemon can be created to control the execution of our script. This wrapper shell script will run continuously and execute our script at desired internal after verifying that it should not be running already. Means no two instances should run at a time. What if our wrapper script get’s terminated somehow? Put it in inittab which will keep it running always. Let’s take an example.Suppose we need to execute /usr/local/myscripts/example.php every second. Here are the steps you should take to create its wrapper script and start execution:
1. Create a wrapper shell script with same name as actual script (but with .sh extension), let’s say /root/example.sh:
   $ cat example.sh
#!/bin/bash
while [ true ]; do
sleep 1 ## frequency at which script will be executed
if [ -z "`ps ax | grep example.php| grep -v grep`"]; then ## do not run more than one instance of script
`nohup /usr/bin/php /usr/local/myscripts/example.php`
fi
done

2. Put wrapper script entry in inittab to keep it running:
   $ vim /etc/inittab
# Run xdm in runlevel 5
..
dm:234:respawn:/root/example.sh
..

3. Finally, re-read inittab to start daemon which in result will start our script:
   
$ init q

   You can verify by checking processlist that daemon and our scripts are running.
3. Using hatool
   This is another way to run scripts. We will make use of two external tools: halockrun and hatimerun. halockrun will keep script running and keep a lock file to check avoid multiple instances while with hatimerun we can specify after how much time script can be terminated (because sometimes scripts become stale/defunct).
   Let’s take an example, here’s the example cron entry using these tools:
   * * * * hatimerun -t 300 halockrun -nc /tmp/example.lock /usr/bin/php /usr/local/myscripts/example.php
   In above example, hatimerun will terminate script after 300 ms and halockrun will again execute it keeping lock file to avoid multiple instances.

Though there’s not much choice available as far as apps for Linux admin are concerned, but still there are few useful. I tried hereby compiling list of 5 such apps which can be helpful for us but its definitely possible that I didn’t come through some awesome app which you are aware about, in such case, requesting you to please put a comment about it.

1. ConnectBot

This one is my favorite, may be because I use this frequently. ConnectBot is a powerful open-source Secure Shell (SSH) client developed by Kenny Root and Jeffrey Sharkey. It can manage simultaneous SSH sessions, create secure tunnels, and copy/paste between other applications. This client allows you to connect to Secure Shell servers that typically run on UNIX-based servers. This of course can’t replace you the experience of using your PC/Laptop but in case where you are on the move and needs to get inside your server fast, this is must have. You can get more info about it from here.

2. VNC® Viewer

VNC® Viewer control a computer anywhere in the world from your Android device! I have used it few times and it worked flawlessly. You can run applications, change settings, and access data exactly as you would be permitted to do were you sitting in front of it, and using the keyboard and mouse.

3. ServerUp

A good to have monitoring app for any Sys Admin. ServerUp allows you to check the status of your network and web servers. ServerUp is configurable to check the status as often as you would like. Looks like Android market right now lacks good monitoring apps using which you can keep eye on your network/servers and even at this moment, the apps which are there are not fully tested and kind of bit unstable.

4. DropBox

Who don’t know about this? Though not a specific app for admins out there but can be helpful to keep your important notes/files in sync. I usually keep server related notes, info etc in text files and dropbox does the duty of keep it up to date everywhere, be it PC, web or android phone. After you install Dropbox on your computer, any file you save to your Dropbox will automatically save to all your computers, your Android device, and even the Dropbox website! With the Dropbox app, you can take everything that matters to you on the go. Another worthy app in this space is ES File Explorer.

5. AndFTP

AndFTP is a FTP, FTPS, SCP, SFTP client. It can manage several FTP configurations. It comes with both device and FTP file browser. It provides download, upload, synchronization and share features with resume support. Though I used it once when I needed to upload a file to a remote server through FTP urgently and fortunately file was there in my phone. AndFTP worked as expected.

Do you know or aware about any other app which is useful for Admin community? Please put a comment about it.

1. Install and run vsftpd without login shell

Most obvious thing while running a ftp server is to run it under non login shell. This is quite easy in Redhat based distro but here in Ubuntu, you need to do something extra also to enable users accessing ftp running under non login shell.

Install vsftpd server and start it:

$ apt-get install vsftpd
$ /etc/init.d/vsftpd start

Add user who needs ftp access without shell access:

$ useradd -s /usr/sbin/nologin ftpuser

Make sure that /usr/sbin/nologin is listed in /etc/shells file otherwise while trying to access ftp, you may encounter following issue:

$ ftp ftpuser@serverip
Connected to serverip.
220 (vsFTPd 2.2.2)
331 Please specify the password.
Password: 
530 Login incorrect.
ftp: Login failed

Do not misinterpret the message here that the password is wrong or something, its actually due to shell.

Add /usr/sbin/nologin into /etc/shells file:

$ vim /etc/shells
/usr/sbin/nologin

Now try to access ftp server again and you should be able to do it smoothly.

2. Allow user to upload/overwrite files using ftp

Next thing to remember is that the default behavior of vsftpd under Ubuntu is to deny write operation. If you try to upload/overwrite files, you may encounter follow error:

Requested action not taken (e.g., file or directory not found, no access).

You need to specify explicitly that local users should be able to use ftp and upload/overwrite files by updating configuration:

$ vim /etc/vsftpd.conf
## uncomment local_umask option, it should look like below
local_umask=022
 
## uncomment write_enable option, it should look like below
write_enable=YES

Save and close the config file and restart vsftpd service to apply changes.

$ /etc/init.d/vsftpd restart
vsftpd start/running, process 13340

You should be able to use ftp service smoothly now.

3. Allow only secure access to ftp or allow only sftp

FTP is inheriently insecure as it transfer data using plain text. To make sure that user can only access ftp service through a secure channel, replace shell in /etc/passwd to /usr/lib/openssh/sftp-server for the user.

let’s say you want to make sure that the user ‘ftpuser’ can only access ftp through secure channel, then update its entry in /etc/passwd file to look like below:

$ grep ftpuser /etc/passwd
ftpuser:x:1000:1000::/var/path/to/work:/usr/lib/openssh/sftp-server

Do you have any good tips/tricks for ftp users? Please share in comments.

Other related and helpful article you may like to read:
* 5 Steps to secure your Linux Server
* 5 highly recommended books for aspiring Linux Admins
* FTP error – could not write to socket broken pipe
* FTP error – 500 oops vsf_sysutil_recv_peek while connecting to vsftpd
* How to install and configure ftp server in Amazon EC2 instance

1. Linux Administration: A Beginner’s Guide
If you are familiar with Linux or Unix and want to start to dig into services, this book is a solid introduction. Though title says, this book is for beginner’s, but the author explains things well for intermediate to advance users. He has done a nice job of selecting basic tasks, and for each one lays out the commands, file locations, and basic configurations for the files. You can opt for this for basic and intermediate details about essential services of Linux/Unix. The collection of services which author explained in the book almost covers all essential working of Linux in general. Covered with good syntax highlighting and examples, the book is helpful and both beginner and intermediate users. Other books either relied on GUI utilities, or used twice as many pages going into too much detail on some sections and not enough on others. This book is just what it says, a beginner’s guide to help you get started with Linux servers and/or integrating Linux into an existing other networks like a Windows one.

2. Linux Troubleshooting for System Administrators and Power Users
Highly recommended for aspiring Sysadmins, this book provide many solutions and method overviews on how to troubleshoot recurring problems. It differs from other troubleshooting books in that it delivers solutions which help you maintain sound, smooth running system configurations within your company network and allows you to see the bigger picture. Very informative and explore technical aspects in well manner. Another aspect of the book is to explore advance topic in easy to understand manners. A delight for advance users who many times just turn pages to search for something cool. Friends who often find themselves in mid of crisis or troubleshooting, will find this piece quite helpful, interesting and insightful.

RELATED: Top 5 most useful tools/commands for Linux Admins

3. O’Reilly Linux Server Hacks
This book is for intermediate to advance users. After having overview of Linux, this book provides you some advance cool hacks related to various services. You will have complete fun while implementing such good practices. Linux Server Hacks is a collection of 100 industrial-strength hacks, providing tips and tools that solve practical problems for Linux system administrators. Every hack can be read in just a few minutes but will save hours of searching for the right answer. Some of the hacks are subtle, many of them are non-obvious, and all of them demonstrate the power and flexibility of a Linux system. You’ll find hacks devoted to tuning the Linux kernel to make your system run more efficiently, as well as using CVS or RCS to track the revision to system files. You’ll learn alternative ways to do backups, how to use system monitoring tools to track system performance and a variety of secure networking solutions. Linux Server Hacks also helps you manage large-scale Web installations running Apache, MySQL, and other open source tools that are typically part of a Linux system.

RELATED: 5 steps to secure your Linux Server

4. Linux System Administration Handbook
In Linus Torvalds words “As this book shows, Linux systems are just as functional, secure, and reliable as their proprietary counterparts. Thanks to the ongoing efforts of its thousands of developers, Linux is more ready than ever for deployment at the frontlines of the real world. The authors of this book know that terrain well, and I am happy to leave you in their most capable hands”. An essential book for avid Linux Sysadmin. Book gives deep insights into almost all of essential services/servers and explains the necessary details in easy to grasp manner. There are instances where authors in try to explain things in depth get’s elaborate and this sometimes makes text difficult to understand but who says good things are easy to understand? A good book for reader with patience.

5. Web Operations: Keeping the Data On Time
System Administration is not kind of job where you will do everything from start to end. Its kind of collaborative challenge where your efforts results best when other teams also do their best. Whether its development team, storage ops, network ops etc. This book doesn’t explain any tool/command but the best practices of collaborative efforts as how to excel in every aspect. Approaches to essential integration of teams to achieve the best. You are not going to find detailed how-to’s or pages of code samples in this book, but you will find some amazing best-practice knowledge from people who know what they are talking about. Web Operations is still a young discipline, but being armed with the knowledge and experience in this book may be the difference between a lot of sleepless nights or an infrastructure that hums along quietly. A must read book and deserve place in your library.

Are these books, I mentioned above are best in respective topic? well, frankly I do not know but few of these I’ve read personally and heard very good reviews. Do you think there’s book which fascinated you on the topic and that should be included in the list here? Inviting you to please put a comment about it below.

More related and helpful articles:
* 5 steps to secure your Linux Server
* Ensuring secure access to production Linux Servers
* Bash script to backup essential log files in Linux
* Detect directory changes in Linux
* Setup multiple MySQL servers in single host