Copying lines from one file to another using sed and shell variables

Question

I've been trying to figure out how to copy lines from one file to the end of another using sed with some shell variables.

sed -n "$var1,$var2'p'" file1.txt > file2.txt

I've tried that and many close variations but no cigar. Could someone please enlighten me about the correct syntax? Thanks.

EDIT

So I tried to suggestions below but now I get this error

sed: -e expression #1, char 2: invalid usage of line address 0
sed: -e expression #1, char 2: unknown command: `*'"

EDIT

Ok these suggestions helped and I found another issue and now my script is running flawlessly. Thank you so much!

Answer 1

Freddy's answer works, but lacks an explanation. The sed format that you are probably aiming for is something like

sed -n 4,6p file1.txt

Where I've used 4 and 6 as dummy numbers.

Let's examine your expression, using these dummy numbers again, and the echo command.

$ var1=4
$ var2=6
$ echo "$var1,$var2'p'"
4,6'p'

Because you enclosed the 's in the "s, they are interpreted literally, and you are literal 's to the sed command. Hence, when you try to run your command:

$ sed -n "$var1,$var2'p'" file1.txt
sed: -e expression #1, char 4: unknown command: `''

Solution

You can remove the 's from your expression, but this would change the variable name to $var2p instead. Hence, as Freddy suggests, you can use {…} to explicitly enclose the variable name(s). i.e.

sed -n "${var1},${var2}p" file1.txt

You can test if this works by putting an echo before the command. An alternative format is

sed -n "$var1,$var2"p file1.txt

but IMO it's less readable.

Finally, as steeldriver points out, if you are aiming to append to the file, use >> file2.txt instead of > file2.txt.

Answer 2

Use curly braces around your variable names:

sed -n "${var1},${var2}p" file1.txt > file2.txt

The braces around the first variable are not needed, but better make it consistent.