Posts Tagged ‘bash’

Send TCP / UDP strings and commands to Local and Remote Applications without netcat with Bash

Friday, July 24th, 2020

Reading Time: 2minutes


Did you ever needed to send TCP / UDP packets manually to send commands to local or remote applications, having a fully functional BASH Shell but not having the luxury to have NC (Netcat Swiss Army Knife of Networking) tool?
This happens if you have some Linux based embeded device as Arduino or a Linux server with a high security PCI requirement which can't affort to have Netcat in place or another portable hardware with a Linux kernel, that needs to communicate in UDP for any reason but you don't want to waste additional 28KB or physically you have access to a Linux device that doesn't have netcat but you want to be able to send UDP externally …

SInce some time in newer GNU Bash's releases support for TCP / UDP data sending is described in Bash's Manual and should be working it is not as good as you might expect but for a small things it could save you the day.

The syntax to use it is:


To open new socket connection to a UDP / TCP protocol with bash you have to simply open a new Shell handler (lets say 3) to:






1. Get GOOGLE HTML Source with simple BASH / Getting URL Index with bash sockets

If you happen to have access to a machine where no network downloader tool or a text browser such as curl, wget, lynx, links is available but you want to dump the content of a index.html or any other URL with simply bash you can do it like so:

exec 3<>/dev/tcp/
echo -e "GET / HTTP/1.1\r\n\r\n" >&3 

cat <&3

If you need to open a connection to a Internet Domain with bash and store the output into a separate .html file:

exec 3<>/dev/tcp/
echo -e "GET / HTTP/1.1\r\n\r\n" >&3 

cat <&3 | tee -a output.html

Note that this will work only if you're logged into into an interactive shell.
If you want instead do it from a shell script (and omit usage) of wget etc. use something like basic script :

exec 3<>/dev/tcp/
printf "GET /\r\n\r\n" >&3
while IFS= read -r -u3 -t2 line || [[ -n “$line” ]]; do echo "$line"; done
exec 3>&-
exec 3<&-


2. Sending UDP protocol data via bash socket

To send test  variables or commands data to localhost ( UDP listening service:

echo 'TEST COMMAND' > /dev/udp/


echo "Any UDP data" > /dev/udp/

If you happent to have netcat or running on a bash shell that doesn't properly support TCP / UDP sending you can always do it netcat way:

echo "Command" | nc -u -w0 3000

Of course this little hack is useful just for simple things and eventually for more comlex stuff and scripting you would like to use a fully functional HTML reader ( W3C compliant Web Browser )
still  for a quick dirty stuffBash socketing from the console rocks pretty much ! 🙂


How to disown a process once it is running on Linux – old but useful trick

Thursday, December 20th, 2018

Reading Time: 2minutes


There is one very old but  gold useful UNIX / Linux trick, I remembered which will be interesting to share it's called  it is called disowning.

Lets say you run execution of a job an rsync job or a simple copy job of a very large file, but in the middle of the copy you remembered you need to do something else and thus want to switch back to shell (without opening a new ssh if on remote server) or a new console if on a local machine.
Then how can you background the copy process and move the process to the rest of long running process system list e.g. "disown" it from yourself so the process continues its job in the background just like of the rest of the backgrounded running processes on the system.

Here is the basic syntax of the disown command:

help disown
disown: disown [-h] [-ar] [jobspec …]
    By default, removes each JOBSPEC argument from the table of active jobs.
    If the -h option is given, the job is not removed from the table, but is
    marked so that SIGHUP is not sent to the job if the shell receives a
    SIGHUP.  The -a option, when JOBSPEC is not supplied, means to remove all
    jobs from the job table; the -r option means to remove only running jobs.


Here is a live example of what I meant by above lines and actual situation where disown comes super useful.

The 'disown' command/builtin (this is in bash), which will disassociate the process from the shell and not send the HUP signal to the process on exit.

root@linux:~# cp -rpf SomeReallyLargeFile1 SomeReallylargeFile2

[1]+  Stopped                 cp -i -r SomeReallyLargeFile SomeReallylargeFile2
root@linux:~#  bg %1
[1]+ cp -i -r SomeReallyLargeFile SomeReallylargeFile2 &
root@linux:~#  jobs
[1]+  Running                 cp -i -r testLargeFile largeFile2 &
root@linux:~# disown -h %1
root@linux:~# ps -ef |grep largeFile2
root      5790  5577  1 10:04 pts/3    00:00:00 cp -i -rpf SomeReallyLargeFile SomeReallylargeFile2
root      5824  5577  0 10:05 pts/3    00:00:00 grep largeFile2

Of course you can always use something like GNU screen (VT100/ ANSI Terminal screen manager) or tmux (terminal multiplexer) to detach the process but you will have to have run the screen  / tmux session in advance which you might haven't  yet as well as it is  required one of the 2 to be present on a servers and on many servers in complex client environments this might be missing and hard to install (such as server is behind a firewall DMZ-ed (Demilitirezed Zoned) network and no way to install extra packages), the disown command makes sense.

Another useful old tip, that new Linux users might not konw is the nohup command (which runs a command immune to hangups with output to a non-tty), nohup's main use is if you want to run process in background with (ampersand) from bash / zsh / tcsh etc. and keep the backgrounded process running even once you've exited the active shell, to do so run the proc background as follows:

$ nohup command-to-exec &


Hope this helps someone, Enjoy!


Automatic network restart and reboot Linux server script if ping timeout to gateway is not responding as a way to reduce connectivity downtimes

Monday, December 10th, 2018

Reading Time: 4minutes


Inability of server to come back online server automaticallyafter electricity / network outage

These days my home server  is experiencing a lot of issues due to Electricity Power Outages, a construction dig operations to fix / change waterpipe tubes near my home are in action and perhaps the power cables got ruptered by the digger machine.
The effect of all this was that my server networking accessability was affected and as I didn't have network I couldn't access it remotely anymore at a certain point the electricity was restored (and the UPS charge could keep the server up), however the server accessibility did not due restore until I asked a relative to restart it or under a more complicated cases where Tech aquanted guy has to help – Alexander (Alex) a close friend from school years check his old site here – helps a restart the machine physically either run a quick restoration commands on root TTY terminal or generally do check whether default router is reachable.

This kind of downtime issues over the last month become too frequent (the machine was down about 5 times for 2 to 5 hours and this was too much (and weirdly enough it was not accessible from the internet even after electricity network was restored and the only solution to that was a physical server restart (from the Power Button).

To decrease the number of cases in which known relatives or friends has to  physically go to the server and restart it, each time after network or electricity outage I wrote a small script to check accessibility towards Default defined Network Gateway for my server with few ICMP packages sentwith good old PING command
and trigger a network restart and system reboot
(in case if the network restart does fail) in a row.

1. Create reboot-if-nwork-is-downsh script under /usr/sbin or other dir

Here is the script itself:


# Script checks with ping 5 ICMP pings 10 times to DEF GW and if so
# triggers networking restart /etc/inid.d/networking restart
# Then does another 5 x 10 PINGS and if ping command returns errors,
# Reboots machine
# This script is useful if you run home router with Linux and you have
# electricity outages and machine doesn't go up if not rebooted in that case


run_ping () {
for i in $(seq 1 10); do
    ping -c 5 $GATEWAY_HOST


reboot_f () {
if [ $? -eq 0 ]; then
        echo "$(date "+%Y-%m-%d %H:%M:%S") Ping to $GATEWAY_HOST OK" >> /var/log/reboot.log
    /etc/init.d/networking restart
        echo "$(date "+%Y-%m-%d %H:%M:%S") Restarted Network Interfaces:" >> /tmp/rebooted.txt
    for i in $(seq 1 10); do ping -c 5 $GATEWAY_HOST; done
    if [ $? -eq 0 ] && [ $(cat /tmp/rebooted.txt) -lt ‘5’ ]; then
         echo "$(date "+%Y-%m-%d %H:%M:%S") Ping to $GATEWAY_HOST FAILED !!! REBOOTING." >> /var/log/reboot.log

    # increment 5 times until stop
    [[ -f /tmp/rebooted.txt ]] || echo 0 > /tmp/rebooted.txt
    n=$(< /tmp/rebooted.txt)
        echo $(( n + 1 )) > /tmp/rebooted.txt
    # if 5 times rebooted sleep 30 mins and reset counter
    if [ $(cat /tmprebooted.txt) -eq ‘5’ ]; then
    sleep 1800
        cat /dev/null > /tmp/rebooted.txt


You can download a copy of script here.

As you see in script successful runs  as well as its failures are logged on server in /var/log/reboot.log with respective timestamp.
Also a counter to 5 is kept in /tmp/rebooted.txt, incremented on each and every script run (rebooting) if, the 5 times increment is matched

a sleep is executed for 30 minutes and the counter is being restarted.
The counter check to 5 guarantees the server will not get restarted if access to Gateway is not continuing for a long time to prevent the system is not being restarted like crazy all time.

2. Create a cron job to run every 15 minutes or so 

I've set the script to re-run in a scheduled (root user) cron job every 15 minutes with following  job:

To add the script to the existing cron rules without rewriting my old cron jobs and without tempering to use cronta -u root -e (e.g. do the cron job add in a non-interactive mode with a single bash script one liner had to run following command:


{ crontab -l; echo "*/15 * * * * /usr/sbin/ 2>&1 >/dev/null; } | crontab –

I know restarting a server to restore accessibility is a stupid practice but for home-use or small client servers with unguaranteed networks with a cheap Uninterruptable Power Supply (UPS) devices it is useful.


Time will show how efficient such a  "self-healing script practice is.
Even though I'm pretty sure that even in a Corporate businesses and large Public / Private Hybrid Clouds where access to remote mounted NFS / XFS / ZFS filesystems are failing a modifications of the script could save you a lot of nerves and troubles and unhappy customers / managers screaming at you on the phone 🙂

I'll be interested to hear from others who have a better  ideas to restore ( resurrect ) access to inessible Linux server after an outage.?