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cow-shell

NAME

cow-shell - Start a copy-on-write session and invoke a shell.

SYNOPSIS

cow-shell [commandline]

DESCRIPTION

cow-shell Executes specified command with cowdancer or a shell if no command-line option is given.

Inside the cowdancer session, files under the current directory are protected in a way so that a write operation to an i-node which existed at the point of running cow-shell is not modified.

Useful for managing hard-linked source-trees and scratch-filesystems.

EXAMPLES

cp -al cowdancer/ cowdancer.new && cd cowdancer.new && cow-shell

Create a hardlinked tree, and cd into the newly created tree. After invoking cow-shell inside that tree, a write operation against a file that is hardlinked with the original tree will be copied to a new file before write.

cp -al cowdancer/ cowdancer.new && cd cowdancer.new && cow-shell debuild

Tries to run dpkg-buildpackage on the new tree, to avoid damage to the original tree.

Configure debuild to ignore .ilist files, through -i or -I option.

FILES

.ilist

Generated dynamically in the current directory. The file contains the list of i-node numbers that should be protected from write operation.

/usr/lib/cowdancer/libcowdancer.so

The shared library used internally that overrides some functions that potentially write to files. This library implements the COW feature and is preloaded with LD_PRELOAD.

/bin/cp

Used for copying. Requires GNU option -a to be functional.

ENVIRONMENT

COWDANCER_ILISTFILE

The path to the current .ilist file.

COWDANCER_IGNORE

Used internally to work around infinite loops. It will be unset.

LD_PRELOAD

Utilized to preload libcowdancer.so

COWDANCER_DEBUG

Enables debug messages. cowdancer will give debug messages in standard error output.

COWDANCER_REUSE=yes

Enables reuse of cowdancer .ilist file found for consecutive runs of cow-shell.

RETURN VALUE

The functions that are overridden with cowdancer will give an errno value of ENOMEM when there is a problem.

System isn't really out of memory, but this error code is chosen because this error is usually handled gracefully by applications.

RESTRICTIONS

Can only support directories that are scanned by cow-shell command.

The operation is not semantically correct when files are originally hardlinked, hardlinks are always broken.

Does not support anything when LD_PRELOAD trick does not work.

Does not work unless hardlinks are supported on the filesystem.

Not very user-friendly.

Leaves .ilist file around.

AUTHOR

Junichi Uekawa (dancer@debian.org)

Upstream page is available at http://www.netfort.gr.jp/~dancer/software/cowdancer.html

SEE ALSO

cowdancer-ilistcreate (1)