Using SED -n with variables in a script

问题

I am trying to use sed in a script but it keeps failing. The commands on their own seem to work so I am not sure what I am doing wrong:

This is in Terminal in OS X (bash)

NOTE: file1.txt contains multiple lines of text

N=1
sed -n $Np < file1.txt > file2.txt
CONTENT=$(cat file2.txt)
echo $CONTENT

If I change $N to 1, it works perfectly

sed -n 1p <file1.txt >file2.txt
CONTENT=$(cat file2.txt)
echo $CONTENT

gives me

content of file2.txt

So basically, I am trying to copy the text from line 1 of a file to the start of line 2 of a file ... IF line 2 does not already start with the content of line 1.

回答1:

The shell doesn't know that you want $N and not $Np.

Do this: ${N}p

回答2:

Change:

sed -n $Np < file1.txt > file2.txt

to

sed -n ${N}p < file1.txt > file2.txt

You code has no clue what variable Np is...

回答3:

Since N was an integer, I ended up using the following.

sed -n `expr $N`p < file1 > file2

This also allows me to get the next line in a file using

sed -n `expr $N+1`p < file1 > file2

Thanks for your help!!!

回答4:

You should use >> for append redirection. > overwrites the original file.

How to append the output to a file?

来源：https://stackoverflow.com/questions/31374269/using-sed-n-with-variables-in-a-script