I have two small python files, the first reads a line using input and then prints another line
a = input()
print('complete')
The second attempts to run this as a subprocess
import subprocess
proc = subprocess.Popen('./simp.py',
stdout=subprocess.PIPE,
stdin=subprocess.PIPE,
bufsize=1)
print('writing')
proc.stdin.write(b'hey\n')
print('reading')
proc.stdout.readline()
The above script will print "writing" then "reading" but then hang. At first I thought this was a stdout buffering issue, so I changed bufsize=1 to bufsize=0, and this does fix the problem. However, it seems it's the stdin that's causing the problem.
With bufsize=1, if I add proc.stdin.flush() below the write, the process continues. Both of these approaches seem clumsy since (1) unbuffered streams are slow (2) adding flushes everywhere is error-prone. Why does the above write not flush on a newline? The docs say that bufsize is used when creating stdin, stdout, and stderr stream for the subprocess, so what's causing the write to not flush on the newline?
From the docs: "1 means line buffered (only usable if universal_newlines=True i.e., in a text mode)". This works:
import subprocess
proc = subprocess.Popen('./simp.py',
stdout=subprocess.PIPE,
stdin=subprocess.PIPE,
bufsize=1,
universal_newlines=True)
print('writing')
proc.stdin.write('hey\n')
print('reading')
proc.stdout.readline()
Related
I'm using Python's subprocess.communicate() to read stdout from a process that runs for about a minute.
How can I print out each line of that process's stdout in a streaming fashion, so that I can see the output as it's generated, but still block on the process terminating before continuing?
subprocess.communicate() appears to give all the output at once.
To get subprocess' output line by line as soon as the subprocess flushes its stdout buffer:
#!/usr/bin/env python2
from subprocess import Popen, PIPE
p = Popen(["cmd", "arg1"], stdout=PIPE, bufsize=1)
with p.stdout:
for line in iter(p.stdout.readline, b''):
print line,
p.wait() # wait for the subprocess to exit
iter() is used to read lines as soon as they are written to workaround the read-ahead bug in Python 2.
If subprocess' stdout uses a block buffering instead of a line buffering in non-interactive mode (that leads to a delay in the output until the child's buffer is full or flushed explicitly by the child) then you could try to force an unbuffered output using pexpect, pty modules or unbuffer, stdbuf, script utilities, see Q: Why not just use a pipe (popen())?
Here's Python 3 code:
#!/usr/bin/env python3
from subprocess import Popen, PIPE
with Popen(["cmd", "arg1"], stdout=PIPE, bufsize=1,
universal_newlines=True) as p:
for line in p.stdout:
print(line, end='')
Note: Unlike Python 2 that outputs subprocess' bytestrings as is; Python 3 uses text mode (cmd's output is decoded using locale.getpreferredencoding(False) encoding).
Please note, I think J.F. Sebastian's method (below) is better.
Here is an simple example (with no checking for errors):
import subprocess
proc = subprocess.Popen('ls',
shell=True,
stdout=subprocess.PIPE,
)
while proc.poll() is None:
output = proc.stdout.readline()
print output,
If ls ends too fast, then the while loop may end before you've read all the data.
You can catch the remainder in stdout this way:
output = proc.communicate()[0]
print output,
I believe the simplest way to collect output from a process in a streaming fashion is like this:
import sys
from subprocess import *
proc = Popen('ls', shell=True, stdout=PIPE)
while True:
data = proc.stdout.readline() # Alternatively proc.stdout.read(1024)
if len(data) == 0:
break
sys.stdout.write(data) # sys.stdout.buffer.write(data) on Python 3.x
The readline() or read() function should only return an empty string on EOF, after the process has terminated - otherwise it will block if there is nothing to read (readline() includes the newline, so on empty lines, it returns "\n"). This avoids the need for an awkward final communicate() call after the loop.
On files with very long lines read() may be preferable to reduce maximum memory usage - the number passed to it is arbitrary, but excluding it results in reading the entire pipe output at once which is probably not desirable.
If you want a non-blocking approach, don't use process.communicate(). If you set the subprocess.Popen() argument stdout to PIPE, you can read from process.stdout and check if the process still runs using process.poll().
If you're simply trying to pass the output through in realtime, it's hard to get simpler than this:
import subprocess
# This will raise a CalledProcessError if the program return a nonzero code.
# You can use call() instead if you don't care about that case.
subprocess.check_call(['ls', '-l'])
See the docs for subprocess.check_call().
If you need to process the output, sure, loop on it. But if you don't, just keep it simple.
Edit: J.F. Sebastian points out both that the defaults for the stdout and stderr parameters pass through to sys.stdout and sys.stderr, and that this will fail if sys.stdout and sys.stderr have been replaced (say, for capturing output in tests).
myCommand="ls -l"
cmd=myCommand.split()
# "universal newline support" This will cause to interpret \n, \r\n and \r equally, each as a newline.
p = subprocess.Popen(cmd, stderr=subprocess.PIPE, universal_newlines=True)
while True:
print(p.stderr.readline().rstrip('\r\n'))
Adding another python3 solution with a few small changes:
Allows you to catch the exit code of the shell process (I have been unable to get the exit code while using the with construct)
Also pipes stderr out in real time
import subprocess
import sys
def subcall_stream(cmd, fail_on_error=True):
# Run a shell command, streaming output to STDOUT in real time
# Expects a list style command, e.g. `["docker", "pull", "ubuntu"]`
p = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, bufsize=1, universal_newlines=True)
for line in p.stdout:
sys.stdout.write(line)
p.wait()
exit_code = p.returncode
if exit_code != 0 and fail_on_error:
raise RuntimeError(f"Shell command failed with exit code {exit_code}. Command: `{cmd}`")
return(exit_code)
I'm using Python's subprocess.communicate() to read stdout from a process that runs for about a minute.
How can I print out each line of that process's stdout in a streaming fashion, so that I can see the output as it's generated, but still block on the process terminating before continuing?
subprocess.communicate() appears to give all the output at once.
To get subprocess' output line by line as soon as the subprocess flushes its stdout buffer:
#!/usr/bin/env python2
from subprocess import Popen, PIPE
p = Popen(["cmd", "arg1"], stdout=PIPE, bufsize=1)
with p.stdout:
for line in iter(p.stdout.readline, b''):
print line,
p.wait() # wait for the subprocess to exit
iter() is used to read lines as soon as they are written to workaround the read-ahead bug in Python 2.
If subprocess' stdout uses a block buffering instead of a line buffering in non-interactive mode (that leads to a delay in the output until the child's buffer is full or flushed explicitly by the child) then you could try to force an unbuffered output using pexpect, pty modules or unbuffer, stdbuf, script utilities, see Q: Why not just use a pipe (popen())?
Here's Python 3 code:
#!/usr/bin/env python3
from subprocess import Popen, PIPE
with Popen(["cmd", "arg1"], stdout=PIPE, bufsize=1,
universal_newlines=True) as p:
for line in p.stdout:
print(line, end='')
Note: Unlike Python 2 that outputs subprocess' bytestrings as is; Python 3 uses text mode (cmd's output is decoded using locale.getpreferredencoding(False) encoding).
Please note, I think J.F. Sebastian's method (below) is better.
Here is an simple example (with no checking for errors):
import subprocess
proc = subprocess.Popen('ls',
shell=True,
stdout=subprocess.PIPE,
)
while proc.poll() is None:
output = proc.stdout.readline()
print output,
If ls ends too fast, then the while loop may end before you've read all the data.
You can catch the remainder in stdout this way:
output = proc.communicate()[0]
print output,
I believe the simplest way to collect output from a process in a streaming fashion is like this:
import sys
from subprocess import *
proc = Popen('ls', shell=True, stdout=PIPE)
while True:
data = proc.stdout.readline() # Alternatively proc.stdout.read(1024)
if len(data) == 0:
break
sys.stdout.write(data) # sys.stdout.buffer.write(data) on Python 3.x
The readline() or read() function should only return an empty string on EOF, after the process has terminated - otherwise it will block if there is nothing to read (readline() includes the newline, so on empty lines, it returns "\n"). This avoids the need for an awkward final communicate() call after the loop.
On files with very long lines read() may be preferable to reduce maximum memory usage - the number passed to it is arbitrary, but excluding it results in reading the entire pipe output at once which is probably not desirable.
If you want a non-blocking approach, don't use process.communicate(). If you set the subprocess.Popen() argument stdout to PIPE, you can read from process.stdout and check if the process still runs using process.poll().
If you're simply trying to pass the output through in realtime, it's hard to get simpler than this:
import subprocess
# This will raise a CalledProcessError if the program return a nonzero code.
# You can use call() instead if you don't care about that case.
subprocess.check_call(['ls', '-l'])
See the docs for subprocess.check_call().
If you need to process the output, sure, loop on it. But if you don't, just keep it simple.
Edit: J.F. Sebastian points out both that the defaults for the stdout and stderr parameters pass through to sys.stdout and sys.stderr, and that this will fail if sys.stdout and sys.stderr have been replaced (say, for capturing output in tests).
myCommand="ls -l"
cmd=myCommand.split()
# "universal newline support" This will cause to interpret \n, \r\n and \r equally, each as a newline.
p = subprocess.Popen(cmd, stderr=subprocess.PIPE, universal_newlines=True)
while True:
print(p.stderr.readline().rstrip('\r\n'))
Adding another python3 solution with a few small changes:
Allows you to catch the exit code of the shell process (I have been unable to get the exit code while using the with construct)
Also pipes stderr out in real time
import subprocess
import sys
def subcall_stream(cmd, fail_on_error=True):
# Run a shell command, streaming output to STDOUT in real time
# Expects a list style command, e.g. `["docker", "pull", "ubuntu"]`
p = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, bufsize=1, universal_newlines=True)
for line in p.stdout:
sys.stdout.write(line)
p.wait()
exit_code = p.returncode
if exit_code != 0 and fail_on_error:
raise RuntimeError(f"Shell command failed with exit code {exit_code}. Command: `{cmd}`")
return(exit_code)
In python 2.7, this works and returns the expected string it works!
process = subprocess.Popen(['/bin/bash'], shell=False, stdin=subprocess.PIPE, stdout=subprocess.PIPE)
process.stdin.write('echo it works!\n')
print process.stdout.readline()
When I know try this in python 3.4 it gets stuck at the readline command
process = subprocess.Popen(['/bin/bash'], shell=False, stdin=subprocess.PIPE, stdout=subprocess.PIPE)
process.stdin.write(bytes('echo it works!\n','UTF-8'))
print(process.stdout.readline().decode('UTF-8'))
The hint about buffering is helpfull. The following information can be found in the Subprocess library module documentation:
bufsize will be supplied as the corresponding argument to the open()
function when creating the stdin/stdout/stderr pipe file objects:
0 means unbuffered (read and write are one system call and can return short)
1 means line buffered (only usable if universal_newlines=True i.e., in a text mode)
and
If universal_newlines is False the file objects stdin, stdout and
stderr will be opened as binary streams, and no line ending conversion
is done.
If universal_newlines is True, these file objects will be opened as
text streams in universal newlines mode using the encoding returned by locale.getpreferredencoding(False)
Putting it all together gives the following Python3 code:
import subprocess
process = subprocess.Popen(['/bin/bash'], shell=False, stdin=subprocess.PIPE, stdout=subprocess.PIPE, bufsize=1, universal_newlines=True)
process.stdin.write('echo it works!\n')
print(process.stdout.readline())
I've created a subprocess using
subprocess.Popen(shlex.split(command), stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
The command it calls will print various information, and then wait for a \n before printing more information. Eventually, the process will end when \n has been pressed enough times. I need to be able to programatically simulate the pressing of \n until the process ends, as well as capturing all output. I do not want the output to be printed to the Terminal. I would like it to be returned and set to a variable.
How would I do this?
If you just have to write to stdin once, you could use
proc = subprocess.Popen(..., stdin = subprocess.PIPE)
proc.stdin.write('\n')
However, if you need to wait for a prompt or interact with the subprocess in a more complicated way, then use pexpect.
(pexpect works with any POSIX system, or Windows with Cygwin.)
Try this way
from subprocess import Popen, PIPE
import shlex
with open (filename, 'w') as fileHandle
proc = Popen(shlex.split(command), stdin = PIPE, stdout = PIPE, stderr = PIPE)
out, err = proc.communicate(input = '\n')
print out, err
I'm using Python's subprocess.communicate() to read stdout from a process that runs for about a minute.
How can I print out each line of that process's stdout in a streaming fashion, so that I can see the output as it's generated, but still block on the process terminating before continuing?
subprocess.communicate() appears to give all the output at once.
To get subprocess' output line by line as soon as the subprocess flushes its stdout buffer:
#!/usr/bin/env python2
from subprocess import Popen, PIPE
p = Popen(["cmd", "arg1"], stdout=PIPE, bufsize=1)
with p.stdout:
for line in iter(p.stdout.readline, b''):
print line,
p.wait() # wait for the subprocess to exit
iter() is used to read lines as soon as they are written to workaround the read-ahead bug in Python 2.
If subprocess' stdout uses a block buffering instead of a line buffering in non-interactive mode (that leads to a delay in the output until the child's buffer is full or flushed explicitly by the child) then you could try to force an unbuffered output using pexpect, pty modules or unbuffer, stdbuf, script utilities, see Q: Why not just use a pipe (popen())?
Here's Python 3 code:
#!/usr/bin/env python3
from subprocess import Popen, PIPE
with Popen(["cmd", "arg1"], stdout=PIPE, bufsize=1,
universal_newlines=True) as p:
for line in p.stdout:
print(line, end='')
Note: Unlike Python 2 that outputs subprocess' bytestrings as is; Python 3 uses text mode (cmd's output is decoded using locale.getpreferredencoding(False) encoding).
Please note, I think J.F. Sebastian's method (below) is better.
Here is an simple example (with no checking for errors):
import subprocess
proc = subprocess.Popen('ls',
shell=True,
stdout=subprocess.PIPE,
)
while proc.poll() is None:
output = proc.stdout.readline()
print output,
If ls ends too fast, then the while loop may end before you've read all the data.
You can catch the remainder in stdout this way:
output = proc.communicate()[0]
print output,
I believe the simplest way to collect output from a process in a streaming fashion is like this:
import sys
from subprocess import *
proc = Popen('ls', shell=True, stdout=PIPE)
while True:
data = proc.stdout.readline() # Alternatively proc.stdout.read(1024)
if len(data) == 0:
break
sys.stdout.write(data) # sys.stdout.buffer.write(data) on Python 3.x
The readline() or read() function should only return an empty string on EOF, after the process has terminated - otherwise it will block if there is nothing to read (readline() includes the newline, so on empty lines, it returns "\n"). This avoids the need for an awkward final communicate() call after the loop.
On files with very long lines read() may be preferable to reduce maximum memory usage - the number passed to it is arbitrary, but excluding it results in reading the entire pipe output at once which is probably not desirable.
If you want a non-blocking approach, don't use process.communicate(). If you set the subprocess.Popen() argument stdout to PIPE, you can read from process.stdout and check if the process still runs using process.poll().
If you're simply trying to pass the output through in realtime, it's hard to get simpler than this:
import subprocess
# This will raise a CalledProcessError if the program return a nonzero code.
# You can use call() instead if you don't care about that case.
subprocess.check_call(['ls', '-l'])
See the docs for subprocess.check_call().
If you need to process the output, sure, loop on it. But if you don't, just keep it simple.
Edit: J.F. Sebastian points out both that the defaults for the stdout and stderr parameters pass through to sys.stdout and sys.stderr, and that this will fail if sys.stdout and sys.stderr have been replaced (say, for capturing output in tests).
myCommand="ls -l"
cmd=myCommand.split()
# "universal newline support" This will cause to interpret \n, \r\n and \r equally, each as a newline.
p = subprocess.Popen(cmd, stderr=subprocess.PIPE, universal_newlines=True)
while True:
print(p.stderr.readline().rstrip('\r\n'))
Adding another python3 solution with a few small changes:
Allows you to catch the exit code of the shell process (I have been unable to get the exit code while using the with construct)
Also pipes stderr out in real time
import subprocess
import sys
def subcall_stream(cmd, fail_on_error=True):
# Run a shell command, streaming output to STDOUT in real time
# Expects a list style command, e.g. `["docker", "pull", "ubuntu"]`
p = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, bufsize=1, universal_newlines=True)
for line in p.stdout:
sys.stdout.write(line)
p.wait()
exit_code = p.returncode
if exit_code != 0 and fail_on_error:
raise RuntimeError(f"Shell command failed with exit code {exit_code}. Command: `{cmd}`")
return(exit_code)