On reading documentation when programs don't work as expected

No matter how often you use UNIX tools, once in a while you get caught out trying to put everything together. This happened this morning while I was setting up a cronjob and it didn't work as expected.

This happened to me earlier this week when I tried to setup a quick cron script to dump the contents of an SQL table every day. I had set it up as follows:

37 3 * * * /usr/lib/postgresql/8.2/bin/pg_dump db -U backup -t table | bzip2 -  > /backups/table-monitoring/`date +%Y%m%d`.table.sql.bz2

I checked the next day that things were working and found the directory empty. Interesting.

After a quick check of my email I found the culprit:

/bin/sh: -c: line 0: unexpected EOF while looking for matching `"'
/bin/sh: -c: line 1: syntax error: unexpected end of file

This is where things got complicated. Had I checked crontab(5) this story would probably have ended here. I didn't.

After a couple of attempts at escaping this command line to make things work I ended up with the following:

backup:~% ls /backups/table-monitoring | head -n 3
`date \+%Y%m%d`.table.sql.bz2
date +%Y%m%d.table.sql.bz2

Even more interesting...

Turns out the answer is quite simple. The man page has this to say about it:

The ‘‘sixth’’ field (the rest of the line) specifies the command to be run. The entire command portion of the line, up to a newline or % character, will be executed by /bin/sh or by the
shell specified in the SHELL variable of the crontab file. Percent-signs (%) in the command, unless escaped with backslash (\), will be changed into newline characters, and all data after
the first % will be sent to the command as standard input. There is no way to split a single command line onto multiple lines, like the shell’s trailing "\".

Ooops. A quick change later, I end up with the following snippet:

37 3 * * * /usr/lib/postgresql/8.2/bin/pg_dump db -U backup -t table | bzip2 -  > /backups/table-monitoring/`date +\%Y\%m\%d`.table.sql.bz2

I'm still wondering about the usefulness of the % to send data to the standard input as opposed to using standard pipes though.