crontab doesn't seem to recognize `~`

Question

Has anybody noticed that crontab doesnt seem to recognize ~ to mean the home directory?

I tried using a full expansion of /Users/username/filename.txt in crontab, which worked, but using ~/filename.txt results in failure.

Does anybody know why?

Answer 1

The question proposed using ~/username/filename.txt instead of /Users/username/filename.txt, but the proposed syntax should instead be either ~username/filename.txt or ~/filename.txt. For an illustration of a crontab entry which uses ~, see the top rated answer to Get low battery notifications for mouse earlier.

You can also test a simple example with this crontab entry:

* * * * * touch ~/NOT_A_FILE_I_CARE_ABOUT.txt

Answer 2

~ is a symbol shells automatically expand to $HOME, a standard /bin/sh (which cron uses to run the commands) does not need to support ~ expansion. The portable way is to use $HOME instead in your crontab file.

Answer 3

Some shells support ~, and some don't. Perhaps Apple uses dash as the default cron shell in Catalina, and bash in Monterey?; perhaps Apple changes the cron environment depending upon the phase of the moon?

Better than guessing what Apple has (or has not) done on some particular version of macOS, and on a machine that may or may not have user modifications... wouldn't it be better to run some simple tests - ask cron to tell you something about its environment? This would be an objective truth; one that requires no assumptions, and is not dependent upon vagaries in Apple's documentation.

Once a day (or whenever you wish), you can run a cron job that logs its environment and some other useful information from your system to a file. The file is always there, and its never more than 24 hours old (assuming your system was operating at the time you scheduled it).

A job similar to the one below in your crontab would answer your questions "definitively" (however see NOTE 3. below):

0 12 * * * (echo $(date); echo "What is '~'?:" ~; printenv) > /Users/seamus/mycronenv.txt 2>&1

This produces a file named mycronenv.txt in my $HOME directory that looks similar to this:

Sat Jul 30 23:35:00 CDT 2022
What is '~'?: /Users/seamus
SHELL=/bin/sh
MAILTO=
USER=seamus
PATH=/usr/bin:/bin
PWD=/Users/seamus
SHLVL=1
HOME=/Users/seamus
LOGNAME=seamus
_=/usr/bin/printenv

As you can see, my system's (Catalina) cron recognizes ~ as $HOME. To verify this, I added another job to my crontab (see NOTE 1. below):

* * * * * (echo "Write this line to the file ~/testdir/testfile with a timestamp: "; echo $(date)) >> ~/testdir/testfile.txt 2>&1

Which worked as expected, writing the result to $HOME/testdir/testfile.txt.

And since I used the append form of the redirect (>>), it adds a new entry every minute - i.e. remove this job after your test!

Hope this helps clear the confusion - please let us know if the issue persists, or you have other questions.

---

NOTES:

1. If you want to try this on your machine, create the directory testdir first:

   % cd
   % mkdir testdir
   

2. On my system (Catalina), Apple has set the default configuration to send mail to me with a note re my cron job - which is not something I want. Consequently, I have added a line to my crontab to disable that:

   MAILTO=""
   

3. Wrt definitively:

   From man sh it seems that, rather than being definitive, Apple has given themselves some latitude: cron may use any one of three shells (bash, dash or zsh) in service of /bin/sh:

   sh is a POSIX-compliant command interpreter (shell). It is implemented by re-execing as either bash(1), dash(1), or zsh(1) as determined by the symbolic link located at /private/var/select/sh. If /private/var/select/sh does not exist or does not point to a valid shell, sh will use one of the supported shells.