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Server Availability cannot always be checked by using ping, since often a secured subnet will not allow traffic back.  That common practice of dropping packets is an attempt to hide servers and provide additional security, but it can make troubleshooting more difficult. Since ssh is typcially running and responding on any linux servers that need to be managed, check_ssh seems to be more practical than the default check_ping as the check_command for each host definition in Nagios. This simple shell script uses a Nagios plugin /usr/local/nagios/libexec/check_ssh to quickly check the status of a server.  One thing to note is that a Nagios installation is not required, just the plugins have to be compiled so the binary can be available to the wrapper script. #!/bin/sh # checkssh.sh # # Author: Scott McClelland # 2017-03-29 # # checkssh, uses Nagios plugin check_ssh # /usr/local/nagios/libexec/check_ssh # # nagios-plugins are downloadable from: # https://www.nagios.org...

Here are a few notes to add to the previous article on ping. This time, we look at some bash one-liner tips and tricks. Combine multiple commands Return Values Ping Multiple Hosts Using Bash Nmap and Fping Ping, and Command Line Variables Start from the beginning. We want to see what happens when attempting to ping a host that resolves an IP address from DNS, but is not reachable from our network at the moment. This is to show how we might build a simple monitoring tool from scratch, and also to see what kind of fun we can have with the command line. To ping a host only one time use the count option, which in Windows is -n, and in Linux is -c. Windows C:\>set host=google.com C:\>ping -n 1 %host% Pinging google.com [216.58.219.46] with 32 bytes of data: Request timed out. Ping statistics for 216.58.219.46: Packets: Sent = 1, Received = 0, Lost = 1 (100% loss), Bash: Linux, Mac, Unix Note that in bash, we don't use the set command to assign a varia...

Simple ways to loop in bash shell scripts, or one-liners. Note that the variable in bash is declared without a "$", and called with it, but in perl the variable is declared with and called with a "$" at the beginning of the name. The Bash "for" Loop Loop a number of times for i in {1..10}; do echo -n "$i ";done;echo Output: 1 2 3 4 5 6 7 8 9 10 For short lists, the items can be entered manually. for num in 1 2 3;do echo -n $num;done;echo Output: 1 2 3 Loop Through Items on a List for host in `cat all.txt `;do ping -c 1 $host; done See also: How to Run a Bash Command on All Items in a List and: Ping Multiple Hosts Using Bash Nmap and Fping Loop using Perl This syntax should be familiar to C programmers. In Perl. the scalar variable does not have to be declared as it would in C (int $counter). Note that the variable name starts with "$" to indicate a scalar. #!/usr/bin/perl for($counter = 1; $counter <= 10; $counter++){ print ...

I wrote some shell scripts to generate Nagios configuration files, and I finally decided to publish them on Freshmeat . The initial version is crude, and has some hard-coded information, but it is still better than trying to edit the .cfg files by hand. Here is a brief description nagios-config-build is a group of shell scripts that generate Nagios configuration files from a list of hostnames. It automatically resolves IP addresses from DNS, and provides a quick way to manage a large number of hosts without having to set up a complicated system. Nagios is more than just a server monitoring tool. From the Nagios website: " Nagios is a powerful monitoring system that enables organizations to identify and resolve IT infrastructure problems before they affect critical business processes." Systemnotes.org download: http://www.systemnotes.org/download/shell/nagios-config-build.html Freshmeat project: http://freshmeat.net/projects/nagios-config-build Please feel free to use thi...

As I was writing a script, I noticed that I pressed Shift-PageUp without even thinking about it. Then I realized that there are a lot of shortcuts we take for granted as Sysadmins. I thought it would be helpful to share some of them. This is not a comprehensive list, but a few shortcuts I find myself using all the time. Bash Shell shortcuts Shift-PageUp / PageDown See more of the screen Ctrl-a, Ctrl-e Move to beginning / end of line up & down arrows See bash history !! / !$ Execute the previous command / parameter !n Execute the nth command in history Ctrl-D Logout, or exit Ctrl-u, Ctrl-h, Ctrl-b Delete previous char, or entire line Alt-F1 to Alt-F7 Switch to another console —see also: using-bash-history , ...

An alias is a built-in way of modifying the way a command works, or it is used as a shortcut for another command. For example, you may find that you often type "ls -l", or "ls -ltr", and think it would be nice if you could type less characters. You can create an alias, so that all you would have to type is "ll", instead of "ls -l." To add an alias to your .bashrc file: $ vi ~/.bashrc Insert these lines: alias ls='ls --color=tty' alias ll='ls -l' In order for the new alias to take effect, you can open a new console, login again, or simply source your .bashrc file. You also have the option of running bash again to open a new shell, or just running the alias command. What do I mean by source the file? You can either run a script, or have the shell read the variables in a script without actually running it. This is called sourcing a file, and is accomplished by typing either "source", or "." followed by th...

I mentioned previously that you can reset a shell by typing "reset." There is another problem that can cause a shell or xterm to freeze. If you ssh to a remote host, and then logout, sometimes the session will freeze. In that case, all you need to do is type "Enter", "~", "." (Enter, Tilde, Dot with no spaces) to get the console back. That's the Enter key, immediately followed by tilde "~", and then a period "." [Enter]~. Also, a convenient way to exit a shell, xterm, or remote ssh terminal is to use Ctrl-D rather than taking the time to type the word "exit."

I wrote a script to unlock Firefox on a linux machine. The symptom is when firefox is not running, but it won't let you start because it thinks it's still running. If you have already run "pkill -9 –f firefox", then all you have to do is delete the lock files -- .parentlock, and lock. This is user specific, so you'll have to find your own home directory under ~/.mozilla/firefox/. Mine is dz4bq7je.scottm on this host. $ cat ~/bin/unlock_firefox.sh #!/bin/sh rm ~/.mozilla/firefox/dz4bq7je.scottm/.parentlock rm ~/.mozilla/firefox/dz4bq7je.scottm/lock $ ll ~/.mozilla/firefox/ total 12 drwx------ 9 scottm users 4096 Sep 9 11:25 dz4bq7je.scottm -rw------- 1 scottm users 1264 Jun 23 2008 pluginreg.dat -rw-r--r-- 1 scottm users 162 Nov 15 2005 profiles.ini $ The advantage of this simple script is that you don't have to go looking for directories and files, or remember where firefox puts them. Just remember when it's l...

The idea to write a shell script is often the result of having to do repeated tasks. Imagine having to check hundreds of domain names by typing "whois domain.com" each time. It would take much longer than writing a simple script. Using whois to Check for Available Domain Names Suppose you want to find a good domain name, but you want to see if all of the domain suffixes are available? For example, you want a good .com domain, but you also want the .net, and .org. Not only that, but you might want to use blogspot.com, or wordpress.com for your blog. You can always check a domain availability by using the whois command, e.g. $ whois google.com Obviously, google.com is not available, so whois will display information about the company that registered the domain name. If the domain is not taken, then whois will usually return "No match", or "NOT FOUND". You have to keep trying a domain name until you find one that is available. This can be time-consumi...

This little shell script is a good example of how to simplify a task with a wrapper script. Rather than trying to remember which settings to use for cdrecord, I just run my cdroast.sh script. ----- begin script ----- #!/bin/sh # cdroast.sh # scottm, 2005-07-20 # wrapper for cdrecord INPUT = $1 if [ $INPUT ] then cdrecord -v -sao dev = 0 , 0 , 0 $INPUT else printf " \n Enter name of file \n " printf " e.g. $0 filename.iso \n\n " fi ----- end script ----- Now, with the script in my path, all I have to type is: $ cdroast.sh myfile.iso and the burn process will start. This type of script is often called a wrapper, since it wraps a simple script around a more complicated procedure.

Would you like an easy way to send files to someone outside of your local network, or quickly test a mail server? Well the old "mail" command is still available, and it comes in handy for sending config files, or using within a shell script. This is all you need: $ mail -s test me@company.com That's the shortcut. If you want more details, you can read the rest of the post... Using the mail command 1) Use Mail Interactively to read mail a) type Mail b) enter the number of the message to read, press enter c) press space to page down, n for next message d) ? for help e) q to quit 2) Use mail Interactively to send mail a) type mail command followed by email address b) Enter subject, press enter c) Enter text of message d) press Ctrl-d on a line by itself when finished e) Enter CC: if desired, or press Ctrl-d again $ mail user@company.com Subject: test Here is my test message <Ctrl-d> Cc: <Ctrl-d>$ For a little less work, add the subject to the command prompt. ...

I explained how to get a list of hosts using nmap -- using-nmap-to-generate-host-lists.html , but here is another look at the subject. The question is, how do I ping multiple hosts to find out which ones are down? Sure this could be considered a topic of system monitoring, but maybe you just want to reboot a bunch of machines, and make sure they all come back online. This quick check will tell you whether there is a problem or not. Here are three methods for pinging a list of hosts: 1.) for host in `cat all.txt `;do ping -c 1 $host; done 2.) nmap -sP -R -iL all.txt 3.) sudo fping -u < xyz/all.txt First, we assume that you have a text file named all.txt that contains a list of hostnames, one per line. Obviously, the examples here contain fake hostnames, domain names and IPs, as described in another article about vi: vim-tips-search-and-replace.html . --- all.txt --- xyz-1 xyz-2 xyz-3 xyz-4 xyz-5 xyz-6 xyz-7 xyz-8 --- end all.txt --- 1.) Use a for loop in ...

Did you ever cat a file that caused the terminal to display all kinds of strange characters? Pressing Ctrl-C and Enter, or typing clear doesn't get the console back to normal. The characters are there, but they are unreadable. A quick way to fix this is to use "reset" -- type: $ reset That should bring the console back to normal. Of course, if you just want to clear the screen, you can type "clear" ("cls" in DOS, or a Windows command prompt).

The command history is stored in ~/.bash_history. The history typically contains the last 500 commands typed at the shell prompt. You can view the history by using the history command. history | less Then you can search for a command. e.g. what was the name of that directory I created yesterday? Oh, yeah, I can search history for the mkdir command by using the slash. /mkdir You could also use grep, but with less, you can see the surrounding commands, as you often want to see the sequence of what was done. History Shortcuts !! - Executes the last command !902 - Executes command number 902 (from the history list) !service - Executes the command whose string matches the most recent history entry Up / Down - The most common shortcut is to simply use the up and down arrows to scroll through the bash history. Example If you recently restarted the network service, and then wanted to restart it again, you might do something like this; service network restart ping somehost.example.c...

For Linux / Unix Follow as instructed For Windows Download cygwin from http://cygwin.com For instructions, see this article: http://systemnotesorg.blogspot.com/2007/04/use-cygwin-to-run-linux-on-windows.html Two Easy Steps for One-Liners e.g., you have a list of servers, and would like to do something, such as ping each one, or check their IP address. Here is a quick two-step process, with a one-line shell script that can be run from the command line. 1) Create a Text file, with one hostname per line 2) Run a for loop on the file $ cat servers.txt server01 server02 server03 $ for host in `cat servers.txt`;do host $host;done server01.example.com has address 10.10.10.10 server02.example.com has address 10.10.10.11 server03.example.com has address 10.10.10.12 $ If you want different output, you can use awk, but that's another topic. Notice a few things about this one line script. 1) Each command is separated by a semicolon ";" 2) The ba...

This shell script demonstrates how to write to a text file, and then modify the contents. #!/bin/sh # modfile.sh # by ScottM, 04/12/2007 # demonstrates writing text to a file, and then using sed to modify it. TESTFILE=test.txt FRUIT=banana # add some content to the file (note: file will be overwritten) echo "apple" > $TESTFILE # modify the content sed -e "s/apple/& $FRUIT/g" -i $TESTFILE # sed uses the "s" option, which uses regular expressions to search and replace text # "s/apple/" means search for any lines that contain the characters "apple" # "& " means use the results of whatever was found # "/& $FRUIT/g" -- replace "apple" with "apple banana", # the g is for global, or all lines containing the pattern # output: # $ cat test.txt # apple banana # $ Sed One Liner This is really only a one line script, commonly referred to as a on-liner, so we don't really need a...

An easy way to get a list of hosts from a single domain that you are a part of, is to query DNS host -l mydomain.com But that is not always practical. Sometimes you have machines that are in different domains, but they all are part of a network you manage. Rather than trying write a script that pings hosts and reports the output, just use nmap for a very fast scan. To scan all hosts in a list of subnets 1) Create a subnets.dat file with one subnet on each line: $ cat subnets.dat 192.168.0.* 192.168.1.* 2) Run nmap with the subnets.dat file as input $ nmap -sP -R -iL subnets.dat Reading target specifications from FILE: subnets.dat Starting nmap V. 3.00 ( www.insecure.org/nmap/ ) Host (192.168.0.0) appears to be down. Host box1.mydomain.com (192.168.0.1) appears to be up. Host box2.mydomain.com (192.168.0.2) appears to be down. Host box3.mydomain.com (192.168.0.3) appears to be up. Host (192.168.0.4) appears to be up. ... Notice how names are resolved ...

When you're stuck, just remember "I'm a helpless man" Most commands allow the option "--help", which can then be redirected to "less" (or "more") e.g. cat --help | less To see the man page (more detailed help) for a command type "man" followed by the command e.g. man cat What if you don't remember the name of the command, but you know it was something like lvm? Try apropos apropos lvm What if the man page is the right name, but it displays something about c programming? Use the -a option, and press q until you get the the correct man page. man -a cat For more information look for a HOWTO or other documentation at the Linux Documentation Project -- http://www.tldp.org

How do you get the current date to use in a Linux / Unix shell script? Take a look at this simple backup script: ---------------------------------------------------- #!/bin/sh export   FILE_DATE = `date +%Y%m%d` export   backup_file =backup_ ${ FILE_DATE } .tgz [   !   -d  /opt/backups ]   &&  mkdir -p  /opt/backups tar -zcvf  /opt/backups/ ${ backup_file }  /var/named ---------------------------------------------------- What does that FILE_DATE variable do for me? Well, let's try it on the command line. bash-2.05b$ export FILE_DATE=`date +%Y%m%d` bash-2.05b$ echo $FILE_DATE 20070323 Ah, so that's how we get the current date in a usable format. This method makes the files sortable later. Of course, if you need more than one backup per day, you can always add more date options. Notice a few things about this method of scripting. 1) export, is optional, but it makes the environmet variable available to other s...