Cannot run escaped Windows command line on python

Question

When I run the following command on a Windows command prompt, no problem.

java -jar "C:\Program Files (x86)\SnapBackup\app\snapbackup.jar" main

I tried running the command in a python script. It fails. My python script is like this;

import os
command_line = 'java -jar "C:\Program Files (x86)\SnapBackup\app\snapbackup.jar" main'
os.system(command_line)

The error message received is

Error: Unable to access jarfile C:\Program Files (x86)\SnapBackuppp\snapbackup.jar

Can someone help? Thanks.

I am using python 2.7.9

Did you notice that the error message has 3 letters 'p' at the end of SnapBackup? Is this a typo in the question or in the python file? — anneb
– anneb, Commented Sep 8, 2015 at 2:03
Nothing wrong with the python code. Just checked again. I copied and pasted the error message from pycharm IDE. I really don't know why there are 3 p. Strange. — guagay_wk
– guagay_wk, Commented Sep 8, 2015 at 2:06
backslashes! Use forward slashes or escape the backslash like \\ — anneb
– anneb, Commented Sep 8, 2015 at 2:09
Hey! That's the answer. Thanks! Answer is command_line = 'java -jar "C:/Program Files (x86)/SnapBackup/app/snapbackup.jar" main'. Please copy it down, post it as answer and I will give you the votes. You deserve it! — guagay_wk
– guagay_wk, Commented Sep 8, 2015 at 2:11

jknotek · Accepted Answer · 2015-09-08 02:27:58Z

Try this:

command_line = 'java -jar "C:\\Program Files (x86)\\SnapBackup\\app\\snapbackup.jar" main'

Explanation

Windows uses backslashes ("\") instead of forward slashes ("/"). This isn't an issue when using the Windows command line itself, but causes problems when using other tools that process escape sequences. These escape sequences each start with a backslash. Typically, strings in Python process them.

https://docs.python.org/2/reference/lexical_analysis.html#string-literals

If you look closely at your error...

Error: Unable to access jarfile C:\Program Files (x86)\SnapBackuppp\snapbackup.jar

...you'll notice that it says SnapBackuppp, instead of SnapBackup\app

What happened?

The "\a" was being processed as the ASCII Bell (BEL) escape sequence (see link above). The other characters with a backslash in front of them weren't affected, because they don't correspond to an escape sequence. (That is, the \P in C:\Program Files (x86), etc.)

To avoid processing the \a as the Bell character, we need to escape the backslash itself, by typing \\a. That way, the second backslash is treated as a "normal" backslash, and the "a" gets read as expected.

You can also tell Python to use a raw (unprocessed) string by doing this:

command_line = r'java -jar "C:\Program Files (x86)\SnapBackup\app\snapbackup.jar" main'

(Note the r just before the first quote character.

Alternatively, you could also use forward slashes.

command_line = 'java -jar "C:/Program Files (x86)/SnapBackup/app/snapbackup.jar" main'

However, this may not work in all cases.

anneb · Accepted Answer · 2015-09-08 02:13:30Z

2

You are using backslashes in the command line. In python better use forward slashes or escape the backslashes with a backslash

answered Sep 8, 2015 at 2:13

anneb

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Comments

notorious.no · Accepted Answer · 2015-09-08 02:14:32Z

2

You should probably use subprocess instead of os.system https://docs.python.org/2/library/subprocess.html. Also, like others before me have noted, you have unescaped backslashes. Use os.path.join to remedy the situation:

import subprocess, os
jarPath = os.path.join('c:', 'Program Files (x86)', 'SnapBackup', 'app', 'snapbackup.jar')
command_line = ['java', '-jar', jarPath, 'main']
subprocess.call(command_line)

edited Sep 8, 2015 at 2:14

answered Sep 8, 2015 at 2:04

notorious.no

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2 Comments

guagay_wk Over a year ago

Strange. The same error appears. But thanks. Upvoted.

notorious.no Over a year ago

Just noticed the comments under the question. Other people have faster fingers than I do :). I've updated my answer to provide a cross platform way of doing it. Using os.path.join will make it so you don't have to worry about what kind of slash is appropriate to use.

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