Loop through all the files with a specific extension

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

I want to loop through each file in the current folder and check if it matches a specific extension. The code above doesn't work, do you know why?

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

---

Some more detail on the break-vs-continue discussion in the comments. I consider it somewhat out of scope whether you use break or continue, because what the first loop is trying to do is distinguish between two cases:

1. *.java had no matches, and so is treated as literal text.
2. *.java had at least one match, and that match might have included an entry named *.java.

In case #1, break is fine, because there are no other values of $i forthcoming, and break and continue would be equivalent (though I find break more explicit; you're exiting the loop, not just waiting for the loop to exit passively).

In case #2, you still have to do whatever filtering is necessary on any possible matches. As such, the choice of break or continue is less relevant than which test (-f, -d, -e, etc) you apply to $i, which IMO is the wrong way to determine if you entered the loop "incorrectly" in the first place.

That is, I don't want to be in the position of examining the value of $i at all in case #1, and in case #2 what you do with the value has more to do with your business logic for each file, rather than the logic of selecting files to process in the first place. I would prefer to leave that logic to the individual user, rather than express one choice or the other in the question.

---

As an aside, zsh provides a way to do this kind of filtering in the glob itself. You can match only regular files ending with .java (and disable the default behavior of treating unmatched patterns as an error, rather than as literal text) with

for f in *.java(.N); do
  ...
done

With the above, you are guaranteed that if you reach the body of the loop, then $f expands to the name of a regular file. The . makes *.java match only regular files, and the N causes a failed match to expand to nothing instead of producing an error.

There are also other such glob qualifiers for doing all sorts of filtering on filename expansions. (I like to joke that zsh's glob expansion replaces the need to use find at all.)

Recursively add subfolders,

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

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

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 http://mywiki.wooledge.org/BashFAQ/031"> 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!

I found this solution to be quite handy. It uses the -or option in find:

find . -name \*.tex -or -name "*.png" -or -name "*.pdf"

It will find the files with extension tex, png, and pdf.