Bash scripting, checking for errors, logging

Question

Here\'s one for the bash-fu wizards. No, actually, I\'m just kidding, you\'ll all probably know this except for me..

I\'m trying to create a backup shell script. The ide

Answer 1

For logging, you can wrap sections of your script in curly braces and redirect the stdout to a log file:

{
    script_command_1
    script_command_2
    script_command_3
} >> /path/to/log_file

Answer 2

GNU tar has --remove-files that will remove files once they've been added to the archive. v will cause it to list out files as it adds them. z will pipe the tar through gzip on the fly.

Your find solution is racy; a file may fit the criteria in between invocations, thus getting deleted but not backed up.

Answer 3

For logging, you can arrange for all output written on standard output and/or standard error to go to a file. That way, you don't need to redirect the output of each command:

# Save standard output and standard error
exec 3>&1 4>&2
# Redirect standard output to a log file
exec 1>/tmp/stdout.log
# Redirect standard error to a log file
exec 2>/tmp/stderr.log

# Now the output of all commands goes to the log files
echo "This goes to /tmp/stdout.log"
echo "This goes to /tmp/stderr.log" 1>&2
...

# Print a message to the original standard output (e.g. terminal)
echo "This goes to the original stdout" 1>&3

# Restore original stdout/stderr
exec 1>&3 2>&4
# Close the unused descriptors
exec 3>&- 4>&-

# Now the output of all commands goes to the original standard output & error
...

To execute a command only if a previous one succeeds, you can chain them with conditionals:

# Execute command2 only if command1 succeeds, and command3 only if both succeed:
command1 && command2 && command3

# Execute command2 only if command1 fails
command1 || command2

so you can do things like

{ find . -mtime +7 -type f -print0 | xargs -0 tar -cf "${TAR}" &&
  gzip ${TAR} && 
  find . -mtime +7 -type f -print0 | xargs -0 rm -f } || 
    { echo "Something failed" 1>&2; exit 1 }

or provide details in the log output:

find . -mtime +7 -type f -print0 | xargs -0 tar -cf "${TAR}" || 
  { echo "find failed!!" 1>&2; exit 1 }
gzip ${TAR} || 
  { echo "gzip failed!!" 1>&2; exit 1 }
find . -mtime +7 -type f -print0 | xargs -0 rm -f || 
  { echo "cleanup failed!!" 1>&2; exit 1}

Answer 4

The easy way out, but with no explicit error message add -e to the shebang, i.e. #!/bin/sh -e which will cause the shell to exit if a command fails..

Cron should give you an error message through the mail I guess though.

If you want to go full blown back-up scheme though, I'd suggest you use something that has already been made. There are a bunch out there, and most work very well.