Daily Archives: 8 June 2017

How to get the pid of the last executed command that was sent to the background in a bash shell

Recently we came to the need of writing a bash script that needed periodically to check if a specific process, that was started by the script, had ended and restart it (something like watchdog but with not so many features).

To achieve this, we used the one of the shell special parameters, the $!. Like all other special parameters $! may only be referenced and the user cannot make an assignment to it.

($!) Expands to the process ID of the job most recently placed into the background, whether executed as an asynchronous command or using the bg builtin command.

From GNU.org: https://www.gnu.org/software/bash/manual/html_node/Special-Parameters.html#index-_0021-1

Example of Usage

In this example we wanted to get the PID of the application called server to be used later on in the script.

server &
echo $!; #This will print the process ID of the 'server' application

C: Implicit declaration of function ‘read’ and ‘write’ 6

While working on an socket-based application, we received the following warnings from the compiler:

implicit declaration of function 'read'
implicit declaration of function 'write'

read and write functions are declared in unistd.h which we forgot to include in our code.

Adding the directive

#include <unistd.h>

to the source file that used read and/or write removed the warnings.

How to set a static IP Address from the Command Line in GNU/Linux using ip addr and ip route

Assuming you want to make the following changes to the network device eth0

  1. Change the IP to the static value
  2. Set the Subnet Mask to
  3. Set the Default Gateway for the device to be

and you want to avoid using ifconfig and route that are obsolete you can perform these changes using the following two commands

sudo ip addr add dev eth0;
sudo ip route add default via dev eth0;

Please note that the netmask is given in CIDR notation (it is the /24 right after the IP of the device in the ip addr command).

A subnet mask (netmask) is a bitmask that encodes the prefix length in quad-dotted notation: 32 bits, starting with a number of 1 bits equal to the prefix length, ending with 0 bits, and encoded in four-part dotted-decimal format: A subnet mask encodes the same information as a prefix length, but predates the advent of CIDR. In CIDR notation, the prefix bits are always contiguous, whereas subnet masks may specify non-contiguous bits.

From Wikipedia: https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing