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Python: Making '.bak' Files/Directories Without Clobbering Existing Ones
drks — Mon, 2009-07-20 19:11
Inevitably we all need to read/write new files and modify existing files. When scripting for different projects, I almost always need to update a configuration file here or drop something into a system file there (like /etc/hosts or similar). The first thing you want to do however is make a safe backup of whatever file you are modifying before you alter it.
To safely backup a file isn't a complicated task, but sometimes it gets overlooked. Sure, I can rename a file or directory with one line of code... but what if the 'backup' path already exists? Then you'd be overwriting a previous backup. Maybe one that another sysadmin made... and needs. And what if, there is a backup of that backup? You can see that there are any number of things to consider if you really care about data integrity and not borking someone else's backups (if your on a system with multiple administrators). That said, adding a simple function to your scripts can save you a lot of time and headache by just ensuring you don't overwrite existing data.
The following function will take a path, and make a copy of it to '.bak' or '.bak.X' where X is a safe extension to avoid overwriting another similar file. Alternatively, the 'keep_original=False' will make the function move ithe file out of place rather than copying it.
import os import shutil def safe_backup(path, keep_original=True): """ Rename a file or directory safely without overwriting an existing backup of the same name. """ count = -1 new_path = None while True: if os.path.exists(path): if count == -1: new_path = "%s.bak" % (path) else: new_path = "%s.bak.%s" % (path, count) if os.path.exists(new_path): count += 1 continue else: if keep_original: if os.path.isfile(path): shutil.copy(path, new_path) elif os.path.isdir(path): shutil.copytree(path, new_path) else: shutil.move(path, new_path) break else: break return new_path
The following script shows it in action:
for i in xrange(10): path='./johnny' bak_path = safe_backup(path) if bak_path: print '%s safely backed up as %s' % (path, bak_path) else: print '%s does not exist, nothing to backup' % path f = open(path, 'w') f.write('data for file %s' % i) f.close()
And the output:
wdierkes@slice01 [~] $ python safe_backup_example.py ./johnny does not exist, nothing to backup ./johnny safely backed up as ./johnny.bak ./johnny safely backed up as ./johnny.bak.0 ./johnny safely backed up as ./johnny.bak.1 ./johnny safely backed up as ./johnny.bak.2 ./johnny safely backed up as ./johnny.bak.3 ./johnny safely backed up as ./johnny.bak.4 ./johnny safely backed up as ./johnny.bak.5 ./johnny safely backed up as ./johnny.bak.6 ./johnny safely backed up as ./johnny.bak.7
Keep your scripts safe... keep your sanity... everyone's happy.