Scheduling task with crontab
I think schduling a job with crontab should be easy for me, as I am a veteran
linux user, the truth is I don’t know crontab, totally.
Each cronjob has the following syntax:
# +---------------- minute (0 - 59)
# | +------------- hour (0 - 23)
# | | +---------- day of month (1 - 31)
# | | | +------- month (1 - 12)
# | | | | +---- day of week (0 - 6) (Sunday=0 or 7)
# | | | | |
* * * * * command to be executed
Adding an entry to crontab is as easy as:
(crontab -l 2>/dev/null; echo "*/5 * * * * /tmp/test.sh") | crontab -
And confirmed it was added correctly with:
crontab -l
# */5 * * * * /tmp/test.sh
So I waited for 1 minute expecting my test script being executed, but I cannot find where is the output of the script, the only log I can find is crontab log in syslog:
# tail -f /var/log/syslog
Jul 27 07:41:01 fdbai-desktop CRON[6859]: (fdbai) CMD (/tmp/test.sh)
I found a solution in askubuntu, here is the new crontab task:
crontab -r # remove task
(crontab -l 2>/dev/null; echo "*/5 * * * * /tmp/test.sh >> /tmp/test.log 2>&1") | crontab -
UPDATE:
The output of crontab jobs are logged into mail:
tail -f /var/mail/fdbai
The log shows the script was not executed correctly, it reports:
/tmp/test.sh: line 3: aapt: command not found
Here is the simplified version of test script:
#!/bin/bash
apkname=$(aapt dump badging Contacts.apk | sed -n "s/.*package: name='\([^']*\).*/\1/p")
if [ "$apkname" == "com.android.contacts" ]; then
echo "do something"
fi
I installed android sdk in /opt/sdk/build-tools/28.0.3/ and I am pretty sure
have $PATH variable set correctly as the script can be executed manually without
any problem.
echo $PATH in test script shows that, there is only two items in $PATH, they are:
/usr/bin:/bin
As top rated answer to question Why crontab scripts are not working? said, crontab using different environment, it passes a minimal set of environment variables to the scheduling tasks.
So there are two solutions for this issue, first set $PATH variable in the script,
and the second is use absolute path instead:
export PATH=/usr/bin:/bin:/opt/sdk/build-tools/28.0.3
Finally, the script may run more than 5 minutes, so I wanna exit script directly
if the previous job has not finished, flock can be used for this purpose, lock
file at the beginning of the script will do the trick:
exec 100>/var/tmp/apk.lock || exit 1
flock -n 100 || exit 1
# do some stuff