Patternmatching with variable part, which is the same in different filenames

Question

I have a lot of files in one directory named: world_map_1.png, world_map_2.png, world_country_1.png, world_country_2.png, ...

I want to use ImageMagick to append world_map_2.png to world_map_1.png and store the output in world_map.png. I want to do this for all files starting with world and ending with 1 and 2, so my approach was:

convert world_*_1.png world_*_2.png +append world_*.png

The problem is that * must be the same in all 3 names, so I need a kind of variable for this, which I could reuse in the 2nd and 3rd name.

Answer 1

I'd use a command to detect uniq "infixes":

for image in world_*_*.png
do 
    tmp="${image/world_/}"
    echo "${tmp/_*/}"
done | sort -u

Then, loop over them

for token in $(for a in world_*_*.png; do tmp="${a/world_/}"; echo "${tmp/_*/}"; done | sort -u)
do 
    convert "world_$token"_*.png +append "world_$token.png"
done

Creating some fake image files:

touch world_{map,country,something,else}_{01..04}.png

The command runs all of the following lines:

convert world_country_01.png world_country_02.png world_country_03.png world_country_04.png +append world_country.png
convert world_else_01.png world_else_02.png world_else_03.png world_else_04.png +append world_else.png
convert world_map_01.png world_map_02.png world_map_03.png world_map_04.png +append world_map.png
convert world_something_01.png world_something_02.png world_something_03.png world_something_04.png +append world_something.png