Techworld

On reading documentation when programs don't work as expected

Pascal Hakim

Pascal is the Online Director at IDG Australia. With his team, he makes sure that the servers, code and ads that you can see on this page are all doing what they are supposed to. So please let him know if things are broken.

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.

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