add header, a command and file content to bash script

Based on this post, I want to automate this process, so i need to add a command and header to a bash script + the contents of a file to run ubuntu 18.04 and create a static arp table. Example my .sh:

lan=eth0
awk -F";" '{print $3 " " $2}' infile | xargs -I {} echo {} > outfile

note: infile content ip mac

output:

 192.168.1.1 00:e0:1e:b4:12:42 192.168.1.2 00:e0:1e:b4:13:43

First problem: empty header. Need "#!/bin/sh" or "#!/bin/bash", solved by:

sed -i -e '1i#!/bin/bash\'

Second problem: missing add arp command "arp -i $lan -s". Solved by:

sed -e 's/^/arp -i $lan -s /'

now i need to bind these commands to get this output:

 #!/bin/bash arp -i eth0 -s 192.168.1.1 00:e0:1e:b4:12:42 arp -i eth0 -s 192.168.1.2 00:e0:1e:b4:13:43

What should I fix at my command to add this content? thk

Update (solved, but...)

lan=eth0
awk -F";" '{print $3 " " $2}' infile | xargs -I {} echo {} | sed -e 's/^/arp -i $lan -s /' | sed -e '1i#!/bin/bash\' > outfile

Now the problem is variable $lan

 #!/bin/bash arp -i $lan -s 192.168.1.1 00:e0:1e:b4:12:42 arp -i $lan -s 192.168.1.2 00:e0:1e:b4:13:43

instead of

 #!/bin/bash arp -i eth0 -s 192.168.1.1 00:e0:1e:b4:12:42 arp -i eth0 -s 192.168.1.2 00:e0:1e:b4:13:43

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