Insert a variable in a text file [duplicate] - bash

Question

This question already has answers here:
Replace a string in shell script using a variable
(12 answers)
Closed 4 years ago.
I am trying to using sed -i command to insert a string variable in the 1st line of a text file.
This command work : sed -i '1st header' file.txt
But when i pass a variable this doesn't work.
example :
var=$(cat <<-END
This is line one.
This is line two.
This is line three.
END
)
sed -i '1i $var' file.txt # doesn't work
sed -i ’1i $var’ file.txt # doesn't work
Any help with this problem
Thank you

Answer 1

First, let's define your variable a simpler way:
$ var="This is line one.
This is line two.
This is line three."
Since sed is not good at working with variables, let's use awk. This will place your variable at the beginning of a file:
awk -v x="$var" 'NR==1{print x} 1' file.txt
How it works
-v x="$var"
This defines an awk variable x to have the value of shell variable $var.
NR==1{print x}
At the first line, this tells awk to insert the value of variable x.
1
This is awk's shorthand for print-the-line.
Example
Let's define your variable:
$ var="This is line one.
> This is line two.
> This is line three."
Let's work on this test file:
$ cat File
1
2
This is what the awk command produces:
$ awk -v x="$var" 'NR==1{print x} 1' File
This is line one.
This is line two.
This is line three.
1
2
Changing a file in-place
To change file.txt in place using a recent GNU awk:
awk -i inplace -v x="$var" 'NR==1{print x} 1' file.txt
On macOS, BSD or older GNU/Linux, use:
awk -v x="$var" 'NR==1{print x} 1' file.txt >tmp && mv tmp file.txt

Answer 2

Using printf...
$ var="This is line one.
This is line two.
This is line three.
"
Use cat - to read from stdin and then print into a new file. Move it to the original file if you want to modify it.
$ printf "$var" | cat - file > newfile && mv newfile file;

Answer 3

Not the best job for sed. What about a simple cat ?
cat - file.txt <<EOF > newfile.txt
This is line one.
This is line two.
This is line three.
EOF
# you can add mv, if you really want the original file gone
mv newfile.txt file.txt
And for the original problem - sed does not like newlines and spaces in it's 'program', you need to quote and escape the line breaks:
# this works
sed $'1i "abc\\\ncde"' file.txt
# this does not, executes the `c` command from the second line
sed $'1i "abc\ncde"' file.txt