Create a new file, but add a number if the filename already exists

Question

Is it possible to achieve this from the command line?

Syntax: I can capture an image with the following command:

raspistill -o image.jpg

But I want to be able to run this command via cron each minute and to get something like this

image1.jpg, image2.jpg, image3.jpg, etc...

Answer 1

If you don't mind the suffix value as long as images are in ascending numerical order you could use the date and time

raspistill -o "$(date --utc +'image%Y%m%d%H%M.jpg')"

Extend with %S for seconds if needed.

If you do plan to add this as a one-liner remember that % is a special character in a crontab entry. Either sidestep the issue and put this in a (very short) script and call the script from cron, or precede each % with a backslash \. See man 5 crontab for the details (search for %).

Answer 2

#!/bin/sh

num=1
outfile="image$num.jpg"

while [ -e "$outfile" ]; do
    num=$(( num + 1 ))
    outfile="image$num.jpg"
done

raspistill -o "$outfile"

The loop in the above script will increment the num variable by one and create a new output filename in outfile using this number, until a filename that is no taken is found.

This potentially fails if multiple copies of the script are run simultaneously, as there is a race condition between testing for the existence of the filename and creating the new file. Running the script with a one minute interval should not be a problem unless it takes a minute or longer for raspistill to create the new file. In this case, add touch "$outname" after the loop to "claim" the filename as soon as possible (simultaneous invocations of the script may still fail though).

By "failing" I mean two or more invocations of the script using the same filename with raspistill.