environment: OSX Yosemite Bash Python3.5.0
I want to get the output of Unix commands on Python.
OS X on Mac is based on UNIX, so I will use it. (ls, touch, cat, etc.)
os.system Standard library ʻos` I can use Unix commands here as well, but I can't get the return value as a string. (Maybe you can get it, but I don't know) (It's just output to Terminal) Normally, if it is a file operation system of commands such as touch, this is enough. If it is a command such as cat or grep, it cannot be obtained as a character string.
subprocess
Click here for the reference link
17.5. subprocess — Subprocess management
Use subprocess.check_output in this.
As a usage, you put a character string in the argument, but there are two ways, first, if you use it by default,
check_output(["cut", "-d", "test.txt"])
will do.
The other is to use shell = True as an argument, which can be executed by passing a string.
check_output("cut -d test.txt", shell=True)
For details, it is written in the URL that I referred to.
On Unix with shell = False (default): In this case, the Popen class uses os.execvp () to execute the child program. If a string is given as an argument, it will be used as the name or path of the program to be executed; however, the program will only work with no arguments.
If shell = True on Unix: If args is a string, specify a command line string to be executed through the shell. The string must exactly match the format you type at the shell prompt. For example, if you have a filename with spaces in the string, you need to quote or backslash escape. If args is a string, the first element is passed to the shell as a string representing the command name and the remaining elements as subsequent arguments. This is equivalent to Popen below.
This is a sample (cut -d, -f2) that creates a file appropriately and acquires the second column. Since it is a 3 system, the value will be returned in bytes. (There seems to be an option (keyword argument) that returns str) In the case of 3 series, bytes are returned, so it is at the end of the code, but if you add ʻuniversal_newlines = True`, it will be returned as str.
subprocess_test.py
>>> open("test.txt", 'w').write('\n'.join(([repr([i, i+1, i+2]).replace(' ', '') for i in range(5)])))
39
>>> open("test.txt").read()
'[0,1,2]\n[1,2,3]\n[2,3,4]\n[3,4,5]\n[4,5,6]'
>>>
>>> import subprocess
>>> subprocess.check_output(["cut", "-d,", "-f2", "test.txt"])
b'1\n2\n3\n4\n5\n'
>>> subprocess.check_output("cut -d, -f2 test.txt", shell=True)
b'1\n2\n3\n4\n5\n'
>>> subprocess.check_output("cut -d, -f2 test.txt", shell=True, universal_newlines=True)
'1\n2\n3\n4\n5\n'
Generally, it is faster to write ShellScript, but for reference, I wrote how to execute Unix commands in Python.
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