Crontab coding standards, file layout conventions etc

crontab.conventions

(master version here: https://serverfault.com/questions/351259/is-there-a-good-layout-convention-for-cron-files/1130792#1130792 )

### 0) Header comment

General useful comments that will reduce the chance of jobs not working as expected.

```
## Characters to escape: %, $
```

### 1) Order
Order your entries by time of the day (morning at the top, evening at bottom) 

   * Rationale: it's easier to find the entry you wish to tweak.

### 2) PATH
2) Explicitly set the PATH at the top of the file

   * Rationale: avoid "no such file or directory" errors (particularly if you need GNU versions of tools rather than Mac builtins!)

```
SHELL=/bin/zsh
PATH=/Volumes/apps/homebrew/Cellar/findutils/4.9.0/libexec/gnubin/:/Volumes/apps/homebrew/Cellar/coreutils/9.1/libexec/gnubin/:/Volumes/apps/homebrew/Cellar/findutils/4.9.0/libexec/gnubin/:/opt/local/bin/:
```

### 3) Limit sh /path/to/script.sh

Do not call scripts unless you really have to (it's better to call primitives directly in the crontab file)

   *  Rationale: you can quickly find the entry responsible for certain behaviour

### 4) Backup crontab

Add the following entry (do the same in the root's crontab):
```
@weekly   crontab -l > ~/crontab.`whoami`.`date -I`.txt
```

   * Rationale: whenever you reinstall your OS, you can't easily rebuild your crontab.

### 5) | tee /tmp/cron_*log

Write the output of each entry to a log file on a ramdisk;
```
| tee /tmp/cron_*.log
| tee /tmp/cron_*.err.log
```
Rationale:
* easier to find errors in crons (e.g. `tail /tmp/cron_*.err.log`)
* junk log files don't accumulate beyond their useful life

### 6) tab after cron expression

* Rationale: Some cron expressions will be longer than others, so starting all commands in the same character column will look neater (and help you compare different commands)

## Other

* Start with a blank crontab every year (so that cruft gets weeded out periodically)