One real important part of programming is to control the flow of the program, you need to be able to repeat some part of the code, jump to another part, etc, depending on the estate of some variables, there are a lot of commands to achieve that, and one of them is
It can be used directly at the command line like this example using basename
for i in *.jpg; do echo $i $(basename $i .jpg); done
This command will print all the filenames which ends with .jpg with and without the .jpg suffix.
This line has three parts, divided by the semicolon.
The first part for i in *.jpg assigns the names of the files ending in .jpg to the variable i (one at a time, not like a matrix), then the second part do echo $i $(basename $i .jpg) prints to the screen the value of the variable i, and then the variable i without the trailing .jpg, so you may have an output like this.
100_0055.jpg 100_0055 100_0056.jpg 100_0056 100_0057.jpg 100_0057 100_0058.jpg 100_0058 100_0059.jpg 100_0059 100_0060.jpg 100_0060 100_0061.jpg 100_0061 100_0062.jpg 100_0062 100_0063.jpg 100_0063 100_0064.jpg 100_0064 100_0065.jpg 100_0065 100_0066.jpg 100_0066 100_0067.jpg 100_0067 100_0068.jpg 100_0068 100_0069.jpg 100_0069
Which is the filename itself $i and the file name without the trailing .jpg $(basename $i .jpg).
Finally the last part done ends the sequence when there is no more file with .jpg at the end of its name, in the working directory.
As you can see this is really useful to work with lots of files in batches