Daily Archives: 26 October 2014

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

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.

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

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.

Bash: Get Filename, File Extension and Path from Full Path

The following commands will allow you extract various information from the full path of a file.

Part of the information is the filename, the file extension, the file base and the directory it is located in.

# Truncate the longest match of */ from the beginning of the string
# Get the sub-string from the start (position 0) to the position where the filename starts
directory="${fullpath:0:${#fullpath} - ${#filename}}";
# Strip shortest match of . plus at least one non-dot char from end of the filename
# Get the sub-string from length of base to end of filename
extension="${filename:${#base} + 1}";
# If we have an extension and no base, it means we do not really have an extension but only a base
if [[ -z "$base" && -n "$extension" ]]; then
echo -e "Original:\t'$fullpath':\n\tdirectory:\t'$directory'\n\tfilename:\t'$filename'\n\tbase name:\t'$base'\n\textension:\t'$extension'"

Bash: Get random number that belongs in a range 1

In case you need to create a random number that is between a certain range, you can do it in a very easy way in bash.

Bash has the $RANDOM internal function that returns a different (pseudo) random integer at each invocation. The nominal range of the results is 0 – 32767 (it is the range of a signed 16-bit integer).

We use the result of $RANDOM to create our number using the following script:

echo "You will generate a random number between $FLOOR and $CEILING (both inclusive). There are $RANGE possible numbers!"
echo "We just generated the random number $RESULT, which might not be in the range we want";
let "RESULT %= $RANGE";

echo "Congratulations! You just generated a random number ($RESULT) the is in between $FLOOR and $CEILING (inclusive)";

We use basic math operations on the variables to create the random number that is in the range we like without need-less loops.

What we do is:

  1. We compute the length of the range of valid numbers (ceiling-floor)
    Here we added the +1 because we want the ceiling to be included in the result set.
  2. We generate a random number that is between 0 and the length of range using modulo to limit the results.
  3. We add to the result the value of the floor and this create our final result which is a number equal or greater to the floor and equal or smaller to the ceiling