Why didn't the cron job succeed?

It's a question I've spent much of my adult life pondering.

• The cron background job wasn't running. On Arch Linux, installing cron does not automatically start the background job. You need to run sudo systemctl enable cronie.service && sudo systemctl start cronie.service.
• The command used Bash syntax. Cron executes jobs with /bin/sh by default, which doesn't support all Bash syntax. Notably, the &>> operator for redirecting standard output and error together won't work. You can change the default shell by declaring, e.g., SHELL=/bin/bash at the top of your crontab.
• The program required an environment variable that wasn't set. It's common for cron to run scripts in a limited environment, perhaps only defining SHELL, PATH, LOGNAME, and USER. This is true even if you use Bash, since Bash only reads ~/.bashrc when invoked interactively.
• The executable wasn't on the PATH. A special case of the above: cron will run your jobs with PATH set to a limited number of locations. You can set PATH in the crontab, or spell out the full path to the executable.
• The program's output went into oblivion. On Debian, the output of my cron jobs gets discarded because I haven't configured cron to send email. Annoyingly, the journalctl logs don't even tell me if the command returned an error code or not. Arch Linux's cron is much more helpful: both standard output and standard error are in sudo journalctl -u cronie.service.
• The redirection syntax was wrong. mycmd 2>&1 > cron.log seems logical, but it doesn't work: the right order is mycmd > cron.log 2>&1. (To be fair, this one isn't cron's fault.)
• The command contained a percent sign. According to crontab(5): "A '%' character in the command, unless escaped with a backslash (\), will be changed into newline characters, and all data after the first % will be sent to the command as standard input." This is true even if the percent sign is in a quoted string!

Further reading

• Cron best practices by Tom Ryder (2016)
• Systemd/Timers on the Arch Linux wiki – an alternative to cron for scheduling regular jobs ∎