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rexecsync: a script for efficiently saving the output of a remote command into a local file

This script runs a command remotely and saves its output to a local file. The data transfer is done using an rsync-like difference-based, algorithm to reduce communication on subsequent updates.

Usage

rexecsync [-b] [-c] [-v] [-a ...] [-r ...] 'rshell_cmd' 'remote_cmd' file

where rshell_cmd is the prefix command used to execute a remote shell commands, remote_cmd is the remote command whose output is to be saved (must be a legal bash command with no single quotes), and file is the local file to be written,

Options:

  • -b: create backup of old file with "~" suffix; overwrite if already exists.
  • -c: check whether the exit value of remote_cmd is zero; abort otherwise.
  • -v: verbose output and progress indicators (progress percentage is estimated according to the original file size).
  • -a arg: add arg as command-line arguments to the local rdiff (e.g., "rexecsync -a '--block-size=16384 --sum-size=4' ...").
  • -r cmd: use cmd instead of "$REMOTE_RDIFF" for invoking rdiff on the remote machine (must be a legal bash command with no single-quotes).
  • -d: do a dumb transfer of the whole data, without rdiff (for debugging, or in absence of a remote rdiff).

Examples

  • Full list of files on remote host:
rexecsync -v 'ssh user@remote.host' 'ls -lR ~/' file-list
  • Backup of directory on remote host into a local tar.gz file:
rexecsync -v 'ssh root@remote.host' 'tar czf - /home/remote-dir/' backup.tar.gz
  • Same, but more efficient by using an difference-friendly compression:
rexecsync -v 'ssh root@remote.host' 'tar cf - /home/remote-dir/ | gzip -c --rsyncable' backup.tar.gz
  • Full backup of root filesystem on remote host:
rexecsync -v 'ssh root@remote.host' 'tar cf - --atime-preserve / --exclude=/proc --exclude=/sys --exclude=/dev --exclude=/tmp | gzip -c --rsyncable' backup.tar.gz

Notes

Dependencies: rdiff (from librsync) and pv.

The script blindly invokes complex commands on arbitrary remote systems, which may cause disasterous results. Always test first in non-critical setting. Patches to improve error checking and handling are very welcome.

Large files (greater than 4GB) are supported, assuming librsync is version 0.9.7 (relesed in 2004). Earlier versions of librsync will silently corrupt files larger than 4GB.