want to use the msg generated during running the script or build-log file generated during run plan

[Ankit]

My problem is to use the warning and error msg generated using compiling any projects.
Like using terminal we can directly use "g++ any_file_name.cpp &> logfile.txt", here if any warning or error msg generated then that will be stored in logfile.txt.But if I write this same in the Bamboo script, the logfile.txt is blank and if there was any warning/error then they are just written into the build.log file of Bamboo.
1.) So, is there a way such that I can store them (warning/error msgs created when running anything) directly into a new_file.txt.
For above, Currently, I am downloading the build.log file using Curl command "curl -u<bamboo-user>:<bamboo-password> <bamboo-url>/download/${bamboo.buildKey}/build_logs/${bamboo.buildResultKey}.log > <some-filename>", but here it seems to be that there is a limit like saturation limit, that if there are many things written in build log file before running the above curl command, I get the whole build-log content written before running this command, but if There aren't many things already written before running this command, then the file I get is blank.
2.) So, how can I get the build-log file (whole) written before running this command?