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.
The following report, is based on a global survey of 706 IT and security professionals conducted in the United States, Canada, Germany, United Kingdom, Australia and New Zealand. The goal of the survey was to capture data on current attitudes and trends with mobile devices and IT security. This is the third survey on this topic and this report evaluates differences in responses to similar questions asked over the past two years.
TheInfoPro's unique Market Window uses IT professionals ratings of vendors on 14 separate criteria to calculate scores for Vendor Promise and Vendor Fulfillment, allowing comparison of vendors’ effectiveness at strategy, marketing, delivery and execution. In prior studies, this research appeared in the Vendor Performance report.
- Google's Project Loon close to launching thosands of balloons
- Shape-shifting robot reveals secrets of Fukushima reactor
- FCC faces seventh net neutrality lawsuit
- Next on the Office expansion list: Windows 10 phone apps
- Why investors are so excited about Slack
- First Apple Watch health IT apps bring important messages to the wrist
- Insurance company now offers discounts -- if you let it track your Fitbit
- Apple's 12-inch MacBook can use third-party USB Type-C cables
- Gurucul identifies cloud threats based on identity, behaviors
- Microsoft shows impressively accurate hand-tracking system for VR
- Docker update speeds and organizes container deployments
- Facebook tries to keep it real by killing more 'fake' likes
- Feds lay down social media rules; enterprises should, too
- Should computers determine how our kids learn to talk? Maybe not
- Keep Windows 10 preview up to date -- or face a dead PC