default shell in cron.md

Fixing the shell in cron

In most environments cron executes commands using sh, while many people assume it will use bash.

To test or fix this for a failing command:

1. Try running the command in sh to see if it works
2. Wrap the command in a bash subshell to make sure it gets run in bash: bash -c "mybashcommand"
3. Tell cron to run all commands in bash by setting the shell at the top of your crontab: SHELL=/bin/bash
4. If the command is a script, make sure the script contains a shebang: #!/bin/bash