Bash

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How to: Extract all usernames that are logged in from who

7 November 2014  in Bash / GNU/Linux / HowTos  tagged bash / cut / GNU/Linux / grep / howtos / sort / unique / who by Tux

who | cut -d ' ' -f 1 | sort -u

who: will show who is logged on

cut  -d ‘ ‘ -f 1: will remove all sections from each line except for column 1. It will use the space character as the delimiter for the columns

sort -u: it will sort the usernames and remove duplicate lines. So if a user is logged in multiple times you will get that username only once.
In case you want to filter out root user from this list you can do it as follows:

who | cut -d ' ' -f 1 | sort -u | grep -v 'root'


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Generate Random Password

27 October 2014  in Bash / GNU/Linux / HowTos  tagged base64 / bash / date / GNU/Linux / head / howtos / password / random / sha256sum by Tux

date +%s | sha256sum | base64 | head -c 32 ; echo
date +%s : will print the system date and time in seconds since 1970-01-01 00:00:00 UTC

sha256sum :  will compute  the SHA256 message digest of the time in seconds we produced before

base64 : will encode the previous data and print them to standard output

head -c 32 : will print the first 32 characters of the previous data

; echo : is used to create a new line at the end of the results

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Bash: Echo – Print with out changing line

26 October 2014  in Bash  tagged \n / bash / echo / GNU/Linux / new line by Tux

We use the -n flag which instructs echo not to output the trailing newline.

echo -n ${percentage}

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Bash: Close a range of open sockets by killing the PIDs that are holding them open

26 October 2014  in Bash / GNU/Linux  tagged awk / kill / lsof / pid / port / socket / tail by Tux

Sometimes you want to use a specific port number but some other process(es) is using it. To get the control of the port you need to terminate all other processes and free it.
To find out which process(es) you need to kill, use lsof -i :port. It will return a list of each command and PID that is using the specific port. After that kill those PID using kill -s 9.

The following script will accept a range of ports to free, and for each it will try to kill all processes that are holding them blocked.

low=12345;
high=12350;
for i in `seq $low $high`; do
  lsof -i :$i | tail -n +2 | awk '{system("kill -s 9 " $2)}';
done

Using tail -n +2 we skip the first line of the input which would be the header information.
The system method will invoke a new sh shell and execute the command in it.
Using kill -s 9 we signal the processes that they have to terminate immediately.

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