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Old 05-18-2012, 01:48 AM   #1
omega341991
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how to use a variable for a terminal command


how can i use a variable( user input ) in command line for a command

eg: variable= eth0

ifconfig variable
 
Old 05-18-2012, 01:52 AM   #2
chrism01
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Basically, use a leading '$' when reading a var, no leading $ when setting it eg
Code:
# NB: there must be NO whitespace around '=' in the assignment statement
var=value
echo $var
Try this good tutorial http://rute.2038bug.com/index.html.gz
 
Old 05-18-2012, 01:54 AM   #3
omega341991
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bu can the "var" be used with ifconfig?
 
Old 05-18-2012, 02:47 AM   #4
Nylex
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Of course. The shell will get the variable's value and pass it to the command.
 
Old 05-18-2012, 03:55 AM   #5
colucix
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Quote:
Originally Posted by omega341991 View Post
bu can the "var" be used with ifconfig?
Yes. As Nylex said. This is the way the shell acts: it splits a command line into words and if it encounters special characters the shell makes the proper substitutions. After the command line has been somehow rebuilt, it is executed. For example, if you type:
Code:
ifconfig $variable
the shell splits this line into two words. The first word must be a built-in or an external command, which will be searched inside the directories listed in the PATH environment variable (unless you use the absolute path). The second word contains a $ followed by a valid token for variable names, the shell searches in memory the value of that variable and substitute it. The resulting command is:
Code:
ifconfig eth0
and this one is the actual statement that will be executed. More details about shell substitutions (also called expansions), here.
 
Old 05-18-2012, 03:57 AM   #6
omega341991
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Its not working in my python code

import os
import subprocess

os.system("var=wlan0")
subprocess.call("timeout 2 tcpdump -XX -nnvvv -p -e -S -U -f -i $var > logt.txt 2>&1", shell=True)


its not working and the result shown is :

tcpdump: option requires an argument -- 'i'
tcpdump version 4.2.1
libpcap version 1.1.1
Usage: tcpdump [-aAbdDefhHIKlLnNOpqRStuUvxX] [ -B size ] [ -c count ]
[ -C file_size ] [ -E algo:secret ] [ -F file ] [ -G seconds ]
[ -i interface ] [ -M secret ]
[ -r file ] [ -s snaplen ] [ -T type ] [ -w file ]
[ -W filecount ] [ -y datalinktype ] [ -z command ]
[ -Z user ] [ expression ]
 
Old 05-18-2012, 04:01 AM   #7
pan64
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Of course, they were talking about shells, not python, you have forgotten to mention it.
in python you need to construct the command before executing that call (see string operations)
 
Old 05-18-2012, 04:03 AM   #8
omega341991
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Sorry about that. So how do i make the code execute?
 
Old 05-18-2012, 04:42 AM   #9
pan64
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Code:
var = "wlan0"
command = " ".join ( ['timeout 2 tcpdump -XX -nnvvv -p -e -S -U -f -i', var, '> logt.txt 2>&1')
subprocess.call(command)

...
probably works, but I have not tested
 
Old 05-18-2012, 05:05 AM   #10
omega341991
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stil doesnt work
the error shown is :

command = " ".join ( 'timeout 2 tcpdump -XX -nnvvv -p -e -S -U -f -i', var, '> logt.txt 2>&1')
TypeError: join() takes exactly one argument (3 given)
 
Old 05-18-2012, 05:07 AM   #11
Mark1986
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Put a $ before var and remove the comma?

command = " ".join ( 'timeout 2 tcpdump -XX -nnvvv -p -e -S -U -f -i', $var'> logt.txt 2>&1')
 
Old 05-18-2012, 05:11 AM   #12
pan64
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no, do not put $ sign, I have made a typo:
command = " ".join ( ['timeout 2 tcpdump -XX -nnvvv -p -e -S -U -f -i', var, '> logt.txt 2>&1' ] )
 
Old 05-18-2012, 06:14 AM   #13
omega341991
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still doesnt work its a bigger error now


File "test.py", line 18, in <module>
subprocess.call(command)
File "/usr/lib/python2.7/subprocess.py", line 493, in call
return Popen(*popenargs, **kwargs).wait()
File "/usr/lib/python2.7/subprocess.py", line 679, in __init__
errread, errwrite)
File "/usr/lib/python2.7/subprocess.py", line 1249, in _execute_child
raise child_exception
OSError: [Errno 2] No such file or directory
 
Old 05-18-2012, 06:50 AM   #14
pan64
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maybe this is missing from the end of the line:
subprocess.call(command, shell=True)
also you can try to use timeout with full path
 
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Old 05-18-2012, 07:40 AM   #15
omega341991
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its works now
thanks a lot pan64
 
  


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