[SOLVED] why $$ doesn't generate a new random number

Tasker · 12-21-2014, 02:17 PM

I don't need help to generating a random number to add to a file name because there are many ways and I know a few methods of doing so. However, I am particularly interested in the double dollar sign '$$'.

for example, I wanted to make a few short clips from a video and I wanted the filename to have a random number for each filename.

Quote:

ffmpeg -i video.mp4 -t 300 -vcodec copy -acodec copy out-$$.mp4

it created a file called out-28079.mp4

When I ran the code again to generate another file, ffmpeg complain

Quote:

File 'out-28079.mp4' already exists. Overwrite ? [y/N]

I though the double $$ sign always generate a new random number on each execution. Am I using the $$ characters wrong?

For now, I have to use the other methods for a random number.

astrogeek · 12-21-2014, 02:25 PM

Why would you think it should give a random number?

$$ is the process ID of the shell.

See man bash and search for random, but as I recall $RANDOM is what you want.

Tasker · 12-21-2014, 02:43 PM

I though the $$ characters acted like the $RANDOM variable. I was wrong. It's been a long, long time since I last use the $$ characters.

Here is an excerpt from an article which explain why I wanted to use the $$ characters.

Quote:

Creating Good Temporary File Names:

The $TMPDIR variable contains /tmp (or ~/tmp or a similar directory, depending on availability). This should be used for the location of the temporary file.

It is often a good idea to create a temporary directory (inside of $TMPDIR) if there is a utility that does this securely; then you can add as many temporary files of any name to that directory without worry.

It is a good idea to use the name of the program and the process ID (PID) as part of the file name. Use the $$ shell variable as part of the file name. This identifies which process is responsible for the file, and is useful as a debugging aid. The PID never changes for the life of a shell script, and helps make unique names. (Imagine running a script in two windows at the same time.)

To make the name highly unpredictable, append a random number to the file name. Use the $RANDOM shell variable as part of the name. Using these techniques, file names are both easily identifiable and unpredictable.

The following line of code creates the temporary files using these guidelines:

new=$TMPDIR/swatch-new-$$.$RANDOM
old=$TMPDIR/swatch-old-$$.$RANDOM

You're right the $$ variable is for the process ID of the program.

astrogeek · 12-21-2014, 03:18 PM

It is difficult to keep them all straight!

By the way, welcome to LQ!!

Tasker · 12-21-2014, 03:48 PM

Quote:

Originally Posted by astrogeek

It is difficult to keep them all straight!

By the way, welcome to LQ!!

Thanks and a +1

Miati · 12-21-2014, 04:53 PM

If you don't want large random numbers (5 digits long) you can change the parameters of random:
To make it between 0 & 99 -> $((RANDOM%99))

Also, if you are using this in a script I would suggest using a while loop to ensure the filename doesn't exist causing a problem:

Code:

filen=""$RANDOM"_file.mp4"
while [ -e $filen ]
  do
    filen=""$RANDOM"_file.mp4"
done

ffmpeg -i video.mp4 -t 300 -vcodec copy -acodec copy "$filen"

Tasker · 12-21-2014, 06:31 PM

I like the while loop part to check on an existing file and to pick another random number until it finds a non existing number for the filename.

Thanks Miati and a +1 for the extra info.