Groovy script to modify text file line by line - file

Question

I have a input file from which a Groovy file reads input. Once a particular input is processed, Groovy script should be able to comment the input line it used and then move on.
File content:
1
2
3
When it processes line 1 and line 2, the input file will look as below:
'1
'2
3
By this way, if I re-run the Groovy, I would like to start from the line it stopped last time. If a input was used and it failed, that particular line shall not be commented (') so that a retry can be attempted.
Appreciate if you can help to draft a Groovy script.
Thanks

Answer 1

AFAIK in Groovy you can only append text at the end of the file.
Hence to add ' on each line when it is processed you need to rewrite the entire file.
You can use the follow approach but I only recommend you to use for a small files since you're loading all the lines in memory. In summary an approach for your question could be:
// open the file
def file = new File('/path/to/sample.txt')
// get all lines
def lines = file.readLines()
try{
// for each line
lines.eachWithIndex { line,index ->
// if line not starts with your comment "'"
if(!line.startsWith("'")){
// call your process and make your logic...
// but if it fails you've to throw an exception since
// you can not use 'break' within a closure
if(!yourProcess(line)) throw new Exception()
// line is processed so add the "'"
// to the current line
lines.set(index,"'${line}")
}
}
}catch(Exception e){
// you've to catch the exception in order
// to save the progress in the file
}
// join the lines and rewrite the file
file.text = lines.join(System.properties.'line.separator')
// define your process...
def yourProcess(line){
// I make a simple condition only to test...
return line.size() != 3
}
An optimal approach to avoid load all lines in memory for a large files is to use a reader to read the file contents, and a temporary file with a writer to write the result, and optimized version could be:
// open the file
def file = new File('/path/to/sample.txt')
// create the "processed" file
def resultFile = new File('/path/to/sampleProcessed.txt')
try{
// use a writer to write a result
resultFile.withWriter { writer ->
// read the file using a reader
file.withReader{ reader ->
while (line = reader.readLine()) {
// if line not starts with your comment "'"
if(!line.startsWith("'")){
// call your process and make your logic...
// but if it fails you've to throw an exception since
// you can not use 'break' within a closure
if(!yourProcess(line)) throw new Exception()
// line is processed so add the "'"
// to the current line, and writeit in the result file
writer << "'${line}" << System.properties.'line.separator'
}
}
}
}
}catch(Exception e){
// you've to catch the exception in order
// to save the progress in the file
}
// define your process...
def yourProcess(line){
// I make a simple condition only to test...
return line.size() != 3
}