We Keep Coding

Python subprocess.Popen stdout=subprocess.PIPE blocking execution [duplicate]

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)

python how to read output without EOF from stdout of subprocess [duplicate]

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)

Python: Capture stdout from subprocess.call

I'm trying to do two things when executing a shell cmd with Python:
Capture stdout and print it as it happens
Capture stdout as a whole and process it when the cmd is complete
I looked at subprocess.check_output, but it does not have an stdout param that would allow me to print the output as it happens.
So after reading this question, I realized I may need to try a different approach.
from subprocess import Popen, PIPE
process = Popen(task_cmd, stdout = PIPE)
stdout, stderr = process.communicate()
print(stdout, stderr)
The problem with this approach is that according to the docs, Popen.communicate():
Reads data from stdout and stderr, until end-of-file is reached.
Wait for process to terminate
I still cannot seem to redirect output both to stdout AND to some sort of buffer that can be parsed when the command is complete.
Ideally, I'd like something like:
# captures the process output and dumps it to stdout in realtime
stdout_capture = Something(prints_to_stdout = True)
process = Popen(task_cmd, stdout = stdout_capture)
# prints the entire output of the executed process
print(stdout_capture.complete_capture)
Is there a recommended way to accomplish this?

You were on the right track with using giving Popen stdout=PIPE, but you can't use .communicate() because it returns the values after execution. Instead, I suggest you read from .stdout.
The only guaranteed way to get the output the moment it's generated is to read from the pipe one character at a time. Here is my approach:
def passthrough_and_capture_output(args):
import sys
import subprocess
process = subprocess.Popen(args, stdout=subprocess.PIPE, universal_newlines=True)
# universal_newlines means that the output of the process will be interpreted as text
capture = ""
s = process.stdout.read(1)
while len(s) > 0:
sys.stdout.write(s)
sys.stdout.flush()
capture += s
s = process.stdout.read(1)
return capture
Note that reading one character at a time can incur significant overhead, so if you are alright with lagging behind a bit, I suggest that you replace the 1 in read(1) with a different number of characters to output in batches.

from subprocess import check_output, CalledProcessError
def shell_command(args):
try:
res = check_output(args).decode()
except CalledProcessError as e:
res = e.output.decode()
for r in ['\r', '\n\n']:
res = res.replace(r, '')
return res.strip()

Using stdin instead of a file in a python call to a perl script

I'm running a perl script that accepts a file as input from Python using subprocess.Popen(). I now need the input to the script to accept input from the standard input and not a file. If I run the perl scrip from the shell like this:
perl thescript.perl --in /dev/stdin --other_args other_values
It works perfectly. However, in python, nothing happens using the following commands:
mytext = "hi there"
args = ["perl", "myscript.perl", "--in", "/dev/stdin", "--other_args", other_values]
pipe = subprocess.Popen(args, stdin=subprocess.PIPE, stdout=subprocess.PIPE)
result = pipe.communicate(input=mytext.encode("utf8"))[0]`
result always returns empty (I've also tried using pipe.stdin.write(mytext") and result=pipe.stdout.read())
Please let me know what I'm doing wrong.

Thanks to the comments by #J.F.Sebastian above, I managed to solve this problem with echo and pipes.
args = ["perl", "myscript.perl", "--in", "/dev/stdin", "other_args", other_vals]
pipe1 = subprocess.Popen(["echo", mytext], stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
pipe2 = subprocess.Popen(args, stdin=pipe1.stdout, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
pipe1.stdout.close()
result = pipe2.communicate()[0]
Which returns the expected output. Still not sure why the original (posted in the question) didn't work (using communicate to send the text to the stdin)

/dev/stdin should work (if it works in the shell on your system):
>>> from subprocess import Popen, PIPE
>>> import sys
>>> p = Popen([sys.executable, '-c', 'print(open("/dev/stdin").read()[::-1])'],
... stdin=PIPE, stdout=PIPE)
>>> p.communicate(b'ab')[0]
'ba\n'
stdin=PIPE creates a pipe and connects it to the child process' stdin. Reading from /dev/stdin is equivalent to reading from the standard input (0 fd) and therefore the child reads from the pipe here as shown in the example.

Read streaming input from subprocess.communicate()

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)