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Launching scripts from bash and directing outputs

I have a question about syntax of bash regarding launching scripts from within bash script.
My questions are:
I've seen the following syntax:
#!/bin/bash
python do_something.py > /dev/null 2>&1 &
Can you please explain what is directed to /dev/null, and what is the meaning of 2>&1 if before already mentioned /dev/null?
In addition if I have a line defined like:
python do_something.py > 2>&1 &
how is that different?
If I have the same python file in many paths, how can I differentiate between each process after launching ps -ef |grep python.
When I'm doing so, I get a list of processes which are all called do_something.py, it would be nice if I could have the full execution path string of each pid; how can I do that?
NOTE: The python file launched is writing its own log files.

Ok, diclaimer: I don't have access to a bash right now, so I might be wrong.
Let's break your command: python do_something.py > /dev/null 2>&1 &
python do_something.py will run your command
> /dev/null will redirect stdout to /dev/null
2>&1 will redirect stderr to stdout
& will fork your process and run in background
So your command will ignore stdout/stderr and be run in background which
is equivalent to the command python do_something.py >& /dev/null & [1][2]
python do_something.py > 2>&1 &:
> 2 will redirect stdout to a file named 2
>&1 will redirect stdout to stdout (yes stdout to stdout)
& will fork your process and run in background
So this command is almost equivalent to python do_something.py >2 &,
it will redirect the output to a file named 2 (eg: echo 'yes' > 2>&1)
Note: the behavior of >&1 is probably unspecified.
Since you have run your command using &, your command will be fork and
run in background, therefore I'm not aware of any way to do it in that
case. You can still lookup the /proc directory [3] to see from
which directory your command have been run thought.
[1]: What is the difference between &> and >& in bash?
[2]: In the shell, what does “ 2>&1 ” mean?
[3]: ls -l /proc/$PROCCESSID/cwd

1) stdout (Standard Output) is redirected to /dev/null and stderr (error messages) is redirected to standard output i.e console.
1>filename : Redirect stdout to file "filename."
1>>filename: Redirect and append stdout to file "filename."
2>filename : Redirect stderr to file "filename."
2>>filename: Redirect and append stderr to file "filename."
&>filename : Redirect both stdout and stderr to file "filename."
3) Using the ps auxww flags, you will see the full path to output in both your terminal window and from shell scripts. "ps manual":
-w Wide output. Use this option twice for unlimited width.

Answers:
1, 2. > redirects whatever that is printed in stdout as result of executing the command (in your case python do_something.py) to a file called /dev/null. The /dev/null is kind of a black hole. Whatever you write to it disappers.
2>&1 redirects the output of stderr (which has fd as 2) to stdout (whose fd is 1).
Refer I/O redirection for more info about redirections.
Refer this link for more info about /dev/null

Script run from crontab leaves an empty file

I've tried to run a script from crontab on my Linux system which is leaving an empty file. When I run the script logged on the terminal, it works fine.
Cron line:
#reboot sleep 1m; /bin/bash /root/start_reader_services
The script "start_reader_services" calls a Python script as below:
/root/java/tag_output >> $TAGS_PATH/tags_$DATE_FILE.log
tag_output basically prints out a series of IDs. The same mechanics used to work when I was sending the stdout to my serial port (tag_output > /dev/ttyO0), but now, writing it to the file from cron, the file is created, but is empty.
As I mentioned, running start_reader_services or any piece of that on command line, it works as expected.
Have done:
- Set bash as cron shell
- Set java environments on cron
As requested:
ls -l /root/java/tag_output
-rwxr-xr-x 1 root root 1981 Aug 6 12:06 /root/java/tag_output
First line of tag output:
#!/usr/bin/python
Any help?

Is it possible that tag_output is writing to stderr instead of stdout? Try redirecting stderr too:
/root/java/tag_output >> $TAGS_PATH/tags_$DATE_FILE.log 2>&1
As an aside, you might also want to quote and use braces with the shell variable expansion; that makes it easier to read and probably a little safer:
/root/java/tag_output >> "${TAGS_PATH}/tags_${DATE_FILE}.log" 2>&1

After initialising and updating the crontab using crontab python and before issuing
cron.write(), give the command,
cron.__reduce__()[2]['lines'] = filter(None,
cron.__reduce__()[2]['lines'])
Doing this will remove the empty lines being printed in Linux crontab.

Crontab not restarting process

I am trying to setup a crontab to run 6 python data scrapers. I am tired of having to restart them manually when one of them fails. When running the following:
> ps -ef | grep python
ubuntu 31537 1 0 13:09 ? 00:00:03 python /home/ubuntu/scrapers/datascraper1.py
etc... I get a list of the datascrapers 1-6 all in the same folder.
I edited my crontab like this:
sudo crontab -e
# m h dom mon dow command
* * * * * pgrep -f /home/ubuntu/scrapers/datascraper1.py || python /home/ubuntu/scrapers/datascraper1.py > test.out
Then I hit control+X to exit and hit yes to save as /tmp/crontab.M6sSxL/crontab .
However it does not work in restarting or even starting datascraper1.py whether I kill the process manually or if the process fails on its own. Next, I tried reloading cron but it still didn't work:
sudo cron reload
Finally I tried removing nohup from the cron statement and that also did not work.
How can I check if a cron.allow or cron.deny file exists?
Also, do I need to add a username before pgrep? I am also not sure what the "> test.out" is doing at the end of the cron statement.
After running
grep CRON /var/log/syslog
to check to see if cron ran at all, I get this output:
ubuntu#ip-172-31-29-12:~$ grep CRON /var/log/syslog
Jan 5 07:01:01 ip-172-31-29-12 CRON[31101]: (root) CMD (pgrep -f datascraper1.py ||
python /home/ubuntu/scrapers/datascraper1.py > test.out)
Jan 5 07:01:01 ip-172-31-29-12 CRON[31100]: (CRON) info (No MTA installed, discarding output)
Jan 5 07:17:01 ip-172-31-29-12 CRON[31115]: (root) CMD ( cd / && run-parts --report /etc/cron.hourly)
Jan 5 08:01:01 ip-172-31-29-12 CRON[31140]: (root) CMD (pgrep -f datascraper1.py || python /home/ubuntu/scrapers/datascraper1.py > test.out)
Since there is evidence of Cron executing the command, there must be something wrong with this command, (note: I added the path to python):
pgrep -f datascraper1.py || /usr/bin/python /home/ubuntu/scrapers/datascraper1.py > test.out
Which is supposed to check to see if datascaper1.py is running, if not then restart it.
Since Cron is literally executing this statement:
(root) CMD (pgrep -f datascraper1.py || python /home/ubuntu/scrapers/datascraper1.py > test.out)
aka
root pgrep -f datascraper1.py
Running the above root command gives me:
The program 'root' is currently not installed. You can install it by typing:
sudo apt-get install root-system-bin
Is there a problem with Cron running commands from root?
Thanks for your help.

First of all, you need to see if cron is working at all.
Add this to your cron file (and ideally delete the python statement for now, to have a clear state)
* * * * * echo `date` >>/home/your_username/hello_cron
This will output the date in the file "hello_cron" every minute. Try this, and if this works, ie you see output every minute, write here and we can troubleshoot further.
You can also look in your system logs to see if cron has ran your command, like so:
grep CRON /var/log/syslog
Btw the >test.out part would redirect the output of the python program to the file test.out. I am not sure why you need the nohup part - this would let the python programs run even if you are logged out - is this what you want?
EDIT: After troubleshooting cron:
The message about no MTA installed means that cron is trying to send you an e-mail with the output of the job but cannot because you dont have an email programm installed:
Maybe this will fix it:
sudo apt-get install postfix
The line invoking the python program in cron is producing some output (an error) so it's in your best interests to see what happens. Look at this tutorial to see how to set your email address: http://www.cyberciti.biz/faq/linux-unix-crontab-change-mailto-settings/
Just in case the tutorial becomes unavailable:
MAILTO:youremail#example.com

You need to add python home to your path at the start of the job, however you have python set up. When you're running it yourself and you type python, it checks where you are, then one level down, then your $PATH. So, python home (where the python binary is) needs to be globally exported for the user that owns the cron (so, put it in a rc script in /etc/rc.d/) or, you need to append the python home to path at the start of the cron job. So,
export PATH=$PATH:<path to python>
Or, write the cron entry as
/usr/bin/python /home/ubuntu/etc/etc
to call it directly. It might not be /usr/bin, run the command
'which python'
to find out.
The 'No MTA' message means you are getting STDERR, which would normally get mailed to the user, but can't because you have no Mail Transfer Agent set up, like mailx, or mutt, so no mail for the user can get delivered from cron, so it is discarded. If you'd like STDERR to go into the log also, at the end, instead of
"command" > test.out
write
"command" 2>&1 > test.out
to redirect STDERR into STDOUT, then redirect both to test.out.

Direct output of a niced command to a text file

I'm trying to run a python script with the nice level set.
nice -n 5 python3 blah.py
runs as expected and sends text output to the screen. However, I would like to pipe the output to a text file and run this all in the background so I can go and check on the progress remotely.
However,
nice -n 5 python3 blah.py > log.txt &
creates the log file log.txt but doesn't write anything to the text file so I'm not sure where the standard output is being sent to or how to direct it to my text file.

I eventually solved this using the command
nice -n 5 python3 -u blah.py >log.txt &
-u forces the binary I/O layers of stdin, stdout and stderr to be unbuffered. This allows the output of the python script to be written to the text file whilst the process is running.

I'm guessing you're running the command via ssh and want to log out between running and checking the log. To do this run:
nohup nice -n 5 python3 blah.py > log.txt &
This will prevent killing the program on logout. As well nohup redirects stderr to stdout, which also might be what's causing an empty log.txt file.

cron job to execute script and email errors

I have a python script in /usr/share/myscript.py
I want to execute this script from a cron job, which if the script produces any errors, emails these errors to a specific user (and does not inform the root account).
I do not want to over-ride any of the cron settings - other cron jobs should still notify root.
Currently I am using a shell wrapper, which should pipe errors to a log file and then email it to me. The cron job then executes this .sh file rather than the python script directly.
#!/bin/sh
python /usr/share/scripts/myscript.py 2>&1 > /home/me/logs/myscript.log
test -s /home/me/logs/myscript.log && cat /home/me/logs/myscript.log | mail -s "myscript errors" bob#myplace.com
In production, if nothing goes wrong, then the script executes correctly and nobody is emailed. However, if there is an error in the execution of the python script, then this is still being emailed to the root user from cron.
How should I change the .sh script to suppress this and report to me instead?

This command does the redirection of err output not in the order you want:
python /usr/share/scripts/myscript.py 2>&1 > /home/me/logs/myscript.log
instead you need to redirect stdin first, and stderr second, like so:
python /usr/share/scripts/myscript.py > /home/me/logs/myscript.log 2>&1
Also, have you appended >/dev/null 2>&1 to the end of the wrapped script call in crontab?