Linux command-line call not returning what it should from os system

Question

I need to make some command line calls to linux and get the return from this  however doing it as below is just returning 0 when it should return a time value  like 00 08 19  I am testing the exact same call in regular command line and it returns the time value 00 08 19 so I am confused as to what I am doing wrong as I thought this was how to do it in python   import os retvalue   os system  ps -p 2993 -o time --no-headers   print retvalue

User · Answer

Your code returns 0 if the execution of the commands passed is successful and non zero if it fails  The following program works on python2 7  haven checked 3 and versions above  Try this code    gt  gt  gt  import commands  gt  gt  gt  ret   commands getoutput  ps -p 2993 -o time --no-headers    gt  gt  gt  print ret

User · Answer

What gets returned is the return value of executing this command  What you see in while executing it directly is the output of the command in stdout  That 0 is returned means  there was no error in execution   Use popen etc for capturing the output    Some thing along this line   import subprocess as sub p   sub Popen   your command    arg1    arg2        stdout sub PIPE stderr sub PIPE  output  errors   p communicate   print output   or   import os p   os popen  command   r   while 1      line   p readline       if not line  break     print line   ON SO   Popen and python

User · Answer

This is an old thread  but purely using os system  the following s a valid way of accessing the data returned by the ps call  Note  it does use a pipe to write the data to a file on disk  And OP didn t specifically ask for a solution using os system     gt  gt  gt  os system  ps  gt    Documents ps txt   0     system call is processed   gt  gt  gt  os system  cat   Documents ps txt     PID TTY          TIME CMD  9927 pts 0    00 00 00 bash 10063 pts 0    00 00 00 python 12654 pts 0    00 00 00 sh 12655 pts 0    00 00 00 ps 0   accordingly    gt  gt  gt  os system  ps -p 10063 -o time --no-headers  gt    Documents ps txt   0  gt  gt  gt  os system  cat   Documents ps txt   00 00 00 0   No idea why they are all returning zeroes though

User · Answer

For your requirement  Popen function of subprocess python module is the answer  For example   import subprocess    process   subprocess Popen  ps -p 2993 -o time --no-headers   stdout subprocess PIPE  stderr subprocess PIPE  stdout  stderr   process communicate   print stdout

User · Answer

The simplest way is like this   import os retvalue   os popen  ps -p 2993 -o time --no-headers   readlines   print retvalue   This will be returned as a list

User · Answer

Yes it s counter-intuitive and does not seem very pythonic  but it actually just mimics the unix API design  where these calld are C POSIX functions  Check man 3 popen  amp  amp  man 3 system  Somewhat more convenient snippet to replace os system that I use    from subprocess import  PIPE  Popen    def invoke command               Invoke command as a new system process and return its output              return Popen command  stdout PIPE  shell True  stdout read     result   invoke  echo Hi  bash      Result contains standard output  as you expected it in the first place

User · Answer

I can not add a comment to IonicBurger because I do not have  50 reputation  so I will add a new entry   My apologies   os popen   is the best for multiple complicated commands  my opinion  and also for getting the return value in addition to getting stdout like the following more complicated multiple commands   import os out     i strip   for i in os popen r ls   py   grep -i    file  2 gt  dev null  echo       readlines    print       stdout     out  -1  print  returnValue     out -1    This will list all python files that have the word  file  anywhere in their name   The       is a list comprehension to remove  strip  the newline character from each entry   The echo    is a shell command to show the return status of the last command executed which will be the grep command and the last item of the list in this example   the 2  dev null says to print the stderr of the grep command to  dev null so it does not show up in the output   The  r  before the  ls  command is to use the raw string so the shell will not interpret metacharacters like     incorrectly   This works in python 2 7   Here is the sample output         stdout     fileFilter py    fileProcess py    file access  py    myfile py    returnValue   0

User · Answer

okey I believe the fastest way it would be  import os print os popen  command   readline    x     print x

User · Answer

using commands module import commands      Get high load process details      result   commands getoutput  ps aux   sort -nrk 3 3   head -n 1   print result  -- python 2x print  result  -- python 3x

User · Answer

If you re only interested in the output from the process  it s easiest to use subprocess  check output function    output   subprocess check output   command    arg1    arg2       Then output holds the program output to stdout  Check the link above for more info

[python] Linux command-line call not returning what it should from os.system?

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