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
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)
I have searched and experimented for over an hour on this and there doesn't seem to be a way to both do a 'here document' and get the output line by line as it occurs:
python = '''var="some character text"
print(var)
print(var)
exit()
'''
from subprocess import Popen, PIPE, STDOUT
import shlex
def run_process(command):
p = Popen(shlex.split(command), stdin=PIPE, stdout=PIPE, stderr=STDOUT)
p.stdin.write(python)
while True:
output = p.stdout.readline()
if output == '' and p.poll() is not None:
break
if output:
print output.strip()
rc=p.poll()
return rc
run_process("/usr/bin/python")
The above code hangs indefinitely. Yes, it's a snake eating its tail, but it was just to prove the concept.
The problem is my subprocess takes a LONG time to run and I need to be able to see the output without waiting hours to figure out if anything is wrong. Any hints? Thanks.
The Python interpreter behaves differently when run in interactive vs. non-interactive mode. From the python(1) manual page:
In non-interactive mode, the entire input is parsed before it is executed.
Of course, “entire input” is delimited by EOF, and your program never sends an EOF, which is why it hangs.
Python runs in interactive mode if its stdin is a tty. You can use the Ptyprocess library to spawn a process with a tty as stdin. Or use the Pexpect library (based on Ptyprocess), which even includes ready-made REPL wrappers for Python and other programs.
But if you replace Python with sed — which of course doesn’t have an interactive mode — the program still doesn’t work:
sed = '''this is a foo!\n
another foo!\n
'''
from subprocess import Popen, PIPE, STDOUT
import shlex
def run_process(command):
p = Popen(shlex.split(command), stdin=PIPE, stdout=PIPE, stderr=STDOUT)
p.stdin.write(sed)
while True:
output = p.stdout.readline()
if output == '' and p.poll() is not None:
break
if output:
print output.strip()
rc=p.poll()
return rc
run_process("/bin/sed -e 's/foo/bar/g'")
This is caused by a different problem: output buffering in sed. Some programs have options to disable buffering. In particular, both sed and Python have a -u option, which solves this problem:
run_process("/bin/sed -ue 's/foo/bar/g'")
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()
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)