Passing a variable to a subprocess call

Question

I'm trying to cut the first n lines of a file off a file, I calculate n and assign that value to myvariable

command = "tail -n +"myvariable" test.txt >> testmod.txt"
call(command)

I've imported call from subprocess. But I get a syntax error at myvariable.

You get a syntax error because you that's not valid Python code. — Martijn Pieters, Mar 19 '14 at 12:20
Your concatenation should be done this way: `cmd = "..." + myvar + "..."` — svvac, Mar 19 '14 at 12:23

score 5 · Accepted Answer · answered Mar 19 '14 at 12:23

There are two things wrong here:

Your string concatenation syntax is all wrong; that's not valid Python. You probably wanted to use something like:
```
command = "tail -n " + str(myvariable) + " test.txt >> testmod.txt"
```
where I assume that myvariable is an integer, not a string already.

Using string formatting would be more readable here:
```
command = "tail -n {} test.txt >> testmod.txt".format(myvariable)
```
where the str.format() method will replace {} with the string version of myvariable for you.
You need to tell subprocess.call() to run that command through the shell, because you are using >>, a shell-only feature:
```
call(command, shell=True)
```

@Gar: you could also use a list argument: `rc = call(["tail", "-n", str(myvariable), "test.txt"], stdout=open('testmod.txt', 'a'))` — jfs, Mar 23 '14 at 13:37

1 Answers1