Loop through all the files with a specific extension

Question

No fancy tricks needed:

for i in *.java; do
    [ -f "$i" ] || break
    ...
done

The guard ensures that if there are no matching files, the loop will exit without trying to process a non-existent file name *.java. In bash (or shells supporting something similar), you can use the nullglob option to simply ignore a failed match and not enter the body of the loop.

shopt -s nullglob
for i in *.java; do
    ...
done

Recursively add subfolders,

for i in `find . -name "*.java" -type f`; do
    echo "$i"
done

the correct answer is @chepner's

EXT=java
for i in *.${EXT}; do
    ...
done

however, here's a small trick to check whether a filename has a given extensions:

EXT=java
for i in *; do
    if [ "${i}" != "${i%.${EXT}}" ];then
        echo "I do something with the file $i"
    fi
done

Loop through all files ending with: .img, .bin, .txt suffix, and print the file name:

for i in *.img *.bin *.txt;
do
  echo "$i"
done

Or in a recursive manner (find also in all subdirectories):

for i in `find . -type f -name "*.img" -o -name "*.bin" -o -name "*.txt"`;
do
  echo "$i"
done

as @chepner says in his comment you are comparing $i to a fixed string.

To expand and rectify the situation you should use [[ ]] with the regex operator =~

eg:

for i in $(ls);do
    if [[ $i =~ .*\.java$ ]];then
        echo "I want to do something with the file $i"
    fi
done

the regex to the right of =~ is tested against the value of the left hand operator and should not be quoted, ( quoted will not error but will compare against a fixed string and so will most likely fail"

but @chepner 's answer above using glob is a much more efficient mechanism.

I agree withe the other answers regarding the correct way to loop through the files. However the OP asked:

The code above doesn't work, do you know why?

Yes!

An excellent article What is the difference between test, [ and [[ ?] explains in detail that among other differences, you cannot use expression matching or pattern matching within the test command (which is shorthand for [ )

Feature            new test [[    old test [           Example

Pattern matching    = (or ==)    (not available)    [[ $name = a* ]] || echo "name does not start with an 'a': $name"

Regular Expression     =~        (not available)    [[ $(date) =~ ^Fri\ ...\ 13 ]] && echo "It's Friday the 13th!"
matching

So this is the reason your script fails. If the OP is interested in an answer with the [[ syntax (which has the disadvantage of not being supported on as many platforms as the [ command), I would be happy to edit my answer to include it.

EDIT: Any protips for how to format the data in the answer as a table would be helpful!