1

I have one shell command that uses some argument that is being invoked from Python script :

cmd = 'sysbench --test=cpu --cpu-max-prime=20000 --num-threads=2 run > cpu_sysbench.txt'

Now I wanted to define -cpu-max-prime value as a varible and use it:

prime_numbers = "20000"
cmd = 'sysbench --test=cpu --cpu-max-prime=$prime_numbers --num-threads=2 run'

But when I run it I see few syntax errors:

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3 Answers 3

Reset to default
5

Try:

cmd = f'sysbench --test=cpu --cpu-max-prime={prime_numbers} --num-threads=2 run'

This is called an f-string, it's an easy way to format strings in Python >= 3.6. Of course you could also just use string concatenation or other methods, this is not at all specific to running shell commands.

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3 Comments

this is what I see:cmd = f'sysbench --test=cpu --cpu-max-prime=20000 --num-threads=2 run > cpu_sysbench.txt' ^ SyntaxError: invalid syntax
The difference is that here we're using fstrings.
@FilipMłynarski The OP appears to be using something older than 3.5, so f-strings aren't available.
2

Use str.format

Ex:

prime_numbers = "20000"
cmd = 'sysbench --test=cpu --cpu-max-prime={} --num-threads=2 run'.format(prime_numbers)
# --> sysbench --test=cpu --cpu-max-prime=20000 --num-threads=2 run
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5 Comments

Hello @Rakesh, actually the complete command is , "cmd = 'sysbench --test=cpu --cpu-max-prime=20000 --num-threads=2 run > cpu_sysbench.txt'"
Did you use cmd = 'sysbench --test=cpu --cpu-max-prime={} --num-threads=2 run > cpu_sysbench.txt'.format(prime_numbers) ?
Cool, this one worked for me, even for print it can be used ?
would like to use num-threads variable together with prime_numbers, what should be the exact syntax for same ?
cmd = 'sysbench --test=cpu --cpu-max-prime={} --num-threads={} run > cpu_sysbench.txt'.format(prime_numbers, num_threads) OR cmd = 'sysbench --test=cpu --cpu-max-prime={prime_numbers} --num-threads={num_threads} run > cpu_sysbench.txt'.format(prime_numbers=prime_numbers, num_threads=num_threads)
2

Don't use string formatting at all. Build a list consisting of the command name and its arguments as separate elements for use with subprocess.Popen (or one of its wrapper functions).

prime_numbers = "20000"
cmd = [
    'sysbench',
    '--test=cpu',
    '--cpu-max-prime',
    prime_numbers,
    '--num-threads=2',
    'run'
]

with open("cpu_sysbench.txt", "w") as f:
    subprocess.run(cmd, stdout=f)
    # or subprocess.call(cmd, stdout=f) in older Python 3.x

Update: to incorporate a grep command, use two subprocesses with their standard input and standard output joined.

with open("cpu_sysbench.txt", "w") as f:

    p1 = subprocess.Popen(cmd, stdout=subprocess.PIPE)
    # grep + cut can almost always be replaced by a single awk
    subprocess.call(['awk', '-F', ':', '/total time:/ { print $2 }'], stdin=p1.stdout, stdout=f)
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5 Comments

tried running abobe snippets but seeing this, AttributeError: 'module' object has no attribute 'run'
What version of Python are you using? subprocess.run was introduced in 3.5.
actually if I run it using python3 script_name it works but if just run it ./script_name it fails
Also, have to apply grep command on the cpu_sysbench.txt file, cmd = [ 'grep', 'total time:', "cpu_sysbench.txt" | cut -d ':' -f2 " ] but it seems to failing for syntax
Because | isn't an argument. If you want a good answer, you need to be clear about what you are actually asking.

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