Multiple line shell commands using Python commands module

Question

I'm trying to write a Python function that transforms a given coordinate system to another using gdal. Problem is that I'm trying to execute the command as one string, but in shell, I have to press enter before entering the coordinates.

x = 1815421
y = 557301

ret = []

tmp = commands.getoutput( 'gdaltransform -s_srs \'+proj=lcc +lat_1=34.03333333333333 
+lat_2=35.46666666666667 +lat_0=33.5 +lon_0=-118 +x_0=2000000 +y_0=500000 +ellps=GRS80 
+units=m +no_defs\' -t_srs epsg:4326 \n' + str(x) + ' ' + str(y) )

I tried it using '\n', but that doesn't work.

Is there any reason why you dont call osr.CoordinateTransformation() from within Python? — Rutger Kassies
– Rutger Kassies, Commented Apr 9, 2013 at 6:30

jfs · Accepted Answer · 2013-04-08 16:29:14Z

3

My guess is that you run gdaltransform by pressing Enter and the coordinates are read by the program itself from its stdin, not the shell:

from subprocess import Popen, PIPE

p = Popen(['gdaltransform', '-s_srs', ('+proj=lcc ' 
    '+lat_1=34.03333333333333 ' 
    '+lat_2=35.46666666666667 '
    '+lat_0=33.5 '
    '+lon_0=-118 +x_0=2000000 +y_0=500000 +ellps=GRS80 '
    '+units=m +no_defs'), '-t_srs', 'epsg:4326'],
    stdin=PIPE, stdout=PIPE, universal_newlines=True) # run the program
output = p.communicate("%s %s\n" % (x, y))[0] # pass coordinates

answered Apr 8, 2013 at 16:29

jfs

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Community · Accepted Answer · 2017-05-23 11:59:55Z

from subprocess import *

c = 'command 1 && command 2 && command 3'
# for instance: c = 'dir && cd C:\\ && dir'

handle = Popen(c, stdin=PIPE, stderr=PIPE, stdout=PIPE, shell=True)
print handle.stdout.read()
handle.flush()

If i'm not mistaken, the commands will be executed over a "session" and thus keeping whatever niformation you need in between the commands.

More correctly, using shell=True (from what i've been tought) is that it's supposed to be used if given a string of commands rather than a list. If you'd like to use a list suggestions are to do as follows:

import shlex
c = shlex.split("program -w ith -a 'quoted argument'")

handle = Popen(c, stdout=PIPE, stderr=PIPE, stdin=PIPE)
print handle.stdout.read()

And then catch the output, Or you could work with a open stream and use handle.stdin.write() but it's a bit tricky.

Unless you only want to execute, read and die, .communicate() is perfect, or just .check_output(<cmd>)

Good information n how Popen works can be found here (altho different topic): python subprocess stdin.write a string error 22 invalid argument

Solution

Anyway, this should work (you have to redirect STDIN and STDOUT):

from subprocess import *

c = 'gdaltransform -s_srs \'+proj=lcc +lat_1=34.03333333333333 +lat_2=35.46666666666667 +lat_0=33.5 +lon_0=-118 +x_0=2000000 +y_0=500000 +ellps=GRS80 +units=m +no_defs\' -t_srs epsg:4326 \n' + str(x) + ' ' + str(y) + '\n'

handle = Popen(c, stdin=PIPE, stderr=PIPE, stdout=PIPE, shell=True)
print handle.stdout.read()
handle.flush()

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