Find files with Specific Extension Bash

Today i am going to show you how can we make a deep scanner by which we can scan whole directory including subdirectories and fetch all files of given extension.

Here is Code :)

Explanation :

In Line 3, i declared a array database which will save our fetched files name.
In Line 5, introduced a new function with the name of scanner which expects two values as arguments i.e PATH to be searched & EXTENSION of required files.
Line 27, take input from user (path & extension)
Line 29, function scanner called with required arguments
Line 6 to 8, inside scanner function, two local variables are declared & initialised. One is path_to_be_scanned which is initialised to $1 (path to be scanned) and Second is file initialised to empty String. Now third one is ext initialised to EXTENSION of file.
Line 10, cd into given directory.
Line 12, so now we are looping through all files & directories in current directory. (* is just like a universal set which consists all elements )

if you are more curious guy then just search regex on google

Line 13, so now check if file is real file or directory.

# -d returns true if given argument is directory otherwise false
if [ -d DIR ]; then
 echo "DIR is a directory & exists"
else
 echo "DIR is a not a directory or not exists"
fi

Line 15, i am skipping one folder for scanning to save our time but you can scan it too if you want.
Line 16, ok so if folder name is not Android then call scanner again with current value of file and ext. Always cd back for each recursive call.
Line 21, now i am checking if file contains given ext or not. If yes then append it to database array.

if you never heard about recursion then you should keep reading this post :)

Example For Better Understanding

Content of directory $HOME/bash ( See Attached Screenshot )

After running script with required inputs i got output like below

As you can see i got my all .sh files within given directory (including subdirectories test1, test2, test3, test4). But how ?

Behind The Scene

So after taking input, scanner called with $HOME/bash as path and sh as extension of file to be searched.
Now after cd into $HOME/bash, magic begins with maggy masala magic ;)
Ok so first i got a list of all things within current directory i.e $HOME/bash by * a.k.a asterisk and feed that list to for loop.
```
# for loop now looks like
for file in debug.sh sort.sh test1 test2 test3 test4 wget.sh; do
...
done
```

Now i checked if $file is a file or a directory. So in first run of loop $file is debug.sh.

# Offcourse debug.sh is not a directory
if [ -d debug.sh ];  then
... # will not run.
fi

Now i am checking if extension of debug.sh is sh or not by simply trimming $file name using ${file##*.}. It will return anything after . from debug.sh i.e sh
```
if [ sh = sh ]; then
  database+=(debug.sh)
fi
```
Now in second run of loop, $file is sort.sh and the same process will be done.
In third run of loop when $file is test1 which is a folder, [ -d test1 ] will return 0 or true. Then if its not a folder with name of Android then call again scanner with current value of file variable i.e test1.
Now inside scanner function path_to_be_scanned assigned to test1 & ext is same as before i.e sh.
After cd into test1 folder, for loop looks like
```
for file in t1 t2 t3 t4; do
  ...
done
```
They all are files, but not with sh extension, so nothing happens. Neither [ -d t1 ] nor [ * = sh ] will be execute.
Now after looping four times (t1,t2,t3,t4) function ends and will return to it’s previous state where execution was stopped. (See below diagram)

So program was on line 16 when scanner was called second time. And one more thing, first called scanner function comes in paused state while executing scanner second time. And when second scanner will do all his work then current scanner function will be wiped out from memory ( stack ) and then our paused scanner will resume from Line 17

yes your Operating system ( more specifically CPU ) maintains a stack where it stores recursive functions along with data like local variables. You should learn about stack if you are confused :)
We are on Line 17 now, it’s time to cd back from test1 folder and then check if test1 has sh extension but it does not make sense because we know that test1 is a folder not file, so it’s your homework to not check folders for extension. Well so if part will not execute.
Now fourth run of for loop, $file is test2, which is a folder and same thing will happen now ( recursive call)
Fifth run of loop, $file is test3, which is also a folder or directory, ok so it’s time to call our hero again scanner test3 sh and now inside scanner, for loop looks like
```
for file in my.sh; do
...
done
```
my.sh is a file so no recursive call takes place and we shifted to second if condition
```
if [ sh = sh ]; then
  database+=(my.sh)
done
```
Now for loop ends and there is nothing to execute so second scanner wiped out again and control returns to our first initial called function scanner.
At last when initial called scanner ends ( means there is nothing to do ) , this also wiped out from stack and stack is empty and we are ready to go for next instruction in script which is echo ${database[@]}

Ok so this was my best ( i think 😬). Thank you for reading.