Passing multiple arguments from input file, to a command multiple times (Bash)

Question

I have found myself in several situations, where it would be handy to be able to feed a command arguments from an input file, on a per line basis. In the handful of times I've wanted to be able to do this, I've ended up finding a workaround or running the command multiple times manually.

I have a file input.txt which contains multiple lines, with an arbitrary number of arguments on each line. I am going to use my most recent need for this functionality as an example. I am trying to simply copy my iptables rules to ip6tables. I have run the command iptables -S > input.txt to generate the following file:

-P INPUT DROP
-P FORWARD DROP
-P OUTPUT DROP
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -p tcp -m tcp --dport 22 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 80 -m state --state ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m tcp --dport 443 -m state --state ESTABLISHED -j ACCEPT
-A OUTPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A OUTPUT -o lo -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 53 -m state --state NEW -j ACCEPT
-A OUTPUT -p udp -m udp --dport 53 -m state --state NEW -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 80 -m state --state NEW -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 443 -m state --state NEW -j ACCEPT

For each line, I would like to run something like sudo ip6tables $line, so that ip6tables runs with all the arguments from a given line, and N commands are run, where N is the number of lines in input.txt.

The answer to this question should be applicable to this scenario in general. I am not looking for a solution that only works for copying iptables rules to the IPv6 counterpart (through some functionality in iptables for example). I would also prefer a one liner that I can execute in the terminal if possible, instead of writing a bash script. Though, if a bash script turns out to be much less cumbersome than a one liner, then I would accept that as an answer as well.

acran · Accepted Answer · 2019-08-18 19:04:36Z

3

This sounds like the perfect job for xargs. For your iptables example it works like this:

xargs -L 1 sudo ip6tables < input.txt

xargs reads command arguments from stdin and executes the provided command with the arguments added to the command line. Here the arguments are piped in to stdin from the input.txt file. -L 1 limits the arguments to one line per execution, i.e. one line is added to the command line, the resulting command gets executed, continue with the next line etc.

answered Aug 18, 2019 at 19:04

acran

9,2582 gold badges25 silver badges40 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Léa Gris · Accepted Answer · 2019-08-18 19:14:15Z

1

If it is just for iptables your system may already have an appropriate iptables-restore command you just have to call with:

sudo iptables-restore input.txt

Now if input.txt is to be used to pass arguments to other commands, there i s an easy solution with Bash:

arguments is an array that is read (filled) from each line of input.txt

Iterate over each line of the input.txt file.

Pass the arguments array to your command like this:

#!/usr/bin/env bash

arguments=()

while read -r -a arguments; do
  your_command "${arguments[@]}"
done <input.txt

for a one-liner of the above:

while read -ra a;do your_command "${a[@]}";done<input.txt

Or for a POSIX compatible version (no array):

while read -r a;do set -- $a; your_command "$@";done <input.txt

edited Aug 18, 2019 at 19:14

answered Aug 18, 2019 at 18:48

Léa Gris

20.2k4 gold badges39 silver badges52 bronze badges

Collectives™ on Stack Overflow

Passing multiple arguments from input file, to a command multiple times (Bash)

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related