Find all files that need updating - bash

Question

I use the script below to convert all pdf files in a directory to png files, and I want to run it only over the files that have yet to be converted.
#!/bin/bash
# Convert pdf to png
for f in *.pdf
do
echo "Converting $f"
gs -dNOPAUSE -dBATCH -sDEVICE=png256 -r480 -q -sOutputFile="png/$f.png" "$f"
done
rename -f 's/\.pdf\.png/\.png/' png/*.pdf.png
How do I modify the loop so that it is restricted to files where the corresponding png file either does not exist or is older than the pdf file?

Answer 1

A simple modification to your script:
#!/bin/bash
# Convert pdf to png
for f in *.pdf
do
png="png/${f%pdf}png"
if [ -e "$png" -a "$f" -nt "$png" ]; then
continue
fi
echo "Converting $f"
gs -dNOPAUSE -dBATCH -sDEVICE=png256 -r480 -q -sOutputFile="$png" "$f"
done
Firstly, we create a $png variable using in place editing. Basically the %pdf tells bash to remove the last occurrence of pdf, which is the extension. Then we can place this into a string that prefixes it with png/ and adds the png extension. This saves you the last rename command.
Now we have an if statement that continues the loop if a certain condition is met. Continue means go to the next iteration of the loop, without executing anything else for this iteration. The condition is "$png" exists (-e "$png") and (-a) "$f" is newer than "$png" ("$f" -nt "$png").
But I would suggest writing a Makefile:
PDFS := $(wildcard *.pdf)
PNGS := $(addprefix png/,$(PDFS:.pdf=.png))
all: $(PNGS)
png/%.png: %.pdf
gs -dNOPAUSE -dBATCH -sDEVICE=png256 -r480 -q -sOutputFile="$#" "$<"
And run it with make any time you want to generate newer PNGs.