use shell script to traverse subdirectories and do grep - shell

Question

My directory has the following structure (on Mac)
main_folder/
|_folder1/
|_folder2/
|_folder3/
each sub folder has a file that has the identical name "classFile.trans"
I want to traverse the sub folders and do grep on the file classFile.trans. But I don't know how to save the output new file in the corresponding sub folder. Thanks
#!/bin/bash
find file in ./main_folder/**classFile.trans; do
grep -v "keyword" $file > newClassFile.trans #how do I save the output new file in the corresponding sub folder?
done

Answer 1

Probably easiest to run the grep in each subdirectory:
#!/bin/bash
for d in ./main_folder/*; do
( cd $d;
file=classFile.trans
test -f $file && grep -v "keyword" $file > newClassFile.trans
)
done
The parentheses cause the body of the loop to iterate in a new subshell, so the working directory of the main shell is not changed. However, this makes error messages from grep fairly useless. If any of the classFile.trans is not readable, for example, the error message will not indicate which directory. So it is probably better to do:
for d in ./main_folder/*; do grep -v keyword $d/$file > $d/$newfile; done

Answer 2

I would use
#!/bin/bash
for file in `find ./main_folder/ -name "classFile.trans" `; do
newFile=`dirname $file`/newClassFile.trans;
grep -v "keyword" $file > $newFile
done

Answer 3

The find command has -exec* flags that allow it to run commands for each file matched. For your case you would do:
find ./main_folder -name classFile.trans \
-execdir $SHELL -c "grep -v '$keyword' {} >newClassFile.trans" \;
(\ and linebreak added so the whole command can be read without scrolling)
Breaking the various arguments down:
-name classFile.trans searches for all files named classFile.trans
-execdir runs everything up to the ; character in the directory that contains the matched file
$SHELL -c runs your $SHELL (e.g., /bin/bash) with the -c argument which immediately executes its respective value instead of creating an interactive shell that you can type in
"grep -v '$keyword' {} >newClassFile.trans" runs your grep and output redirection in the file's respective directory thanks to -execdir; note that find turns {} in to the matched file's name
This is necessary so the > redirection runs in the sub-command, not the "current" shell the find command is being run in
\; escapes the ; character so it can be sent to find instead of acting as a command terminator
A test:
# Set up the folders and test files
$ mkdir -p main_folder/{f1,f2,f3}
$ for i in main_folder/f?; do printf 'a\nb\nc\n' >$i/classFile.trans; done
# Contents of one of the test files
$ cat main_folder/f1/classFile.trans
a
b
c
# Set the keyword to the letter 'b'
$ keyword=b
$ find ./main_folder -name classFile.trans -execdir $SHELL -c "grep -v '$keyword' {} >newClassFile.trans" \;
# newClassFile.trans was created sans 'b'
$ cat main_folder/f1/newClassFile.trans
a
c