Ημερήσια αρχεία: 8 Ιουνίου 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

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C: Implicit declaration of function ‘read’ and ‘write’ 5

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 rewrite the most recent commit message of a commit that has not been pushed

Type git commit --amend and press then Enter key.
This will open a text editor to edit the commit message.
After your done, save the file and commit the changes.

Note: Changing the commit message will change the commit ID as well as the commit message is part of the commit itself.


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 192.168.1.2
  2. Set the Subnet Mask to 255.255.255.0
  3. Set the Default Gateway for the device to be 192.168.1.1

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 192.168.1.2/24 dev eth0;
sudo ip route add default via 192.168.1.1 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: 255.255.255.0. 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


File permissions change date

Recently we wanted to check when did the permissions of a specific file changed.
Unfortunately, there exists no such flag and we do not have a 100% working solution for it.

What we did was to check the last modification time of the file status information (ctime) using the ls -lc command.
This command could indicate the last permissions change time but it is not a reliable source as it represents the modification time of other elements as well.

The modification time of the file status information (ctime) gets updated when any inode information regarding the file changes.
This means that the modification time of the file status information (ctime) will get updated when any of the following changes:

  • owner – The numeric user ID (UID) of the file’s owner.
  • group – The numeric group ID (GID) of the file’s group.
  • link count – The number of links to the file.
  • mode – The bit string that indicated the permissions and privileges
  • serial – The serial number of the file.
  • device – The numeric ID of the device containing the file.

Explanation of ls parameters

  • The parameter -c of the ls command when used with the -l will show ctime and sort by name.
  • The parameter -c of the ls command when used with the -l and the -t will show ctime and sort by ctime (newest first).

Example that demonstrates that we get different values in the time column of -l when -c is used

$ ls -lc ~/.ssh/
total 28
-rwx------. 1 george george  225 May 16 17:05 config
-rwx------. 1 george george 1743 Jun  2 13:36 id_rsa
-rwxrwx---. 1 george george  405 May 16 17:05 id_rsa.pub
-rwxrwx---. 1 george george   32 May 16 17:05 Details.txt
-rw-r--r--. 1 george george 9155 May 30 14:32 known_hosts

$ ls -l ~/.ssh/
total 28
-rwx------. 1 george george  225 Mar 22 11:36 config
-rwx------. 1 george george 1743 Jan 25 10:22 id_rsa
-rwxrwx---. 1 george george  405 Jan 25 10:22 id_rsa.pub
-rwxrwx---. 1 george george   32 Jan 25 10:22 Details.txt
-rw-r--r--. 1 george george 9155 May 30 14:32 known_hosts