Using a variable in sed command

Question

I have the following command to insert a string variable to a file after line 1:

sed '1a\"$str_builder"' certs.json > certs-new.json

I have used double quotes around the variable as the answers in other similar posts have said to do (so don't mark as duplicate pls), but it still doesn't work and just inserts the string $str_builder

The file certs.json looked like:

[
    {
        "url" : "www.google.com",
    .
    .

And certs-new.json looks like:

[
$str_builder
    {
        "url" : "www.google.com",
    .
    .

@AbhijitPritam just a string block to insert as a new JSON entry — tnoel999888
– tnoel999888, Commented Apr 17, 2018 at 11:24

Alex Stiff · Accepted Answer · 2018-04-17 11:55:44Z

1

Use double quotes around the whole thing and remove the backslash.

sed "1a$str_builder" certs.json > certs-new.json

edited Apr 17, 2018 at 11:55

answered Apr 17, 2018 at 10:50

Alex Stiff

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4 Comments

tnoel999888 Over a year ago

thanks for the suggestion but this doesn't work unfortunately, just writes $str_builder

Alex Stiff Over a year ago

Ah - if you're using sed to insert lines then you don't need the backslash character. Try without it: sed "1a$str_builder" certs.json > certs-new.json

tnoel999888 Over a year ago

Thanks this worked!! If you edit your original answer or write a new one I can accept it

tnoel999888 Over a year ago

Just one thing, it's removing the first backslash and so the first tab \t is coming through as just a t, I've tried changing it to \\t but it's still coming through as just t

John Kugelman · Accepted Answer · 2018-04-17 11:31:49Z

0

Single quotes inhibit variable expansion. You need to use double quotes.

sed "1a$str_builder" certs.json > certs-new.json

If you want double quotes inside the outer quotes, the inner ones need to be escaped.

sed "1a\"$str_builder\"" certs.json > certs-new.json

You'd probably be better off using jq to modify your JSON data, though. It's a smarter tool designed expressly for JSON manipulation.

edited Apr 17, 2018 at 11:31

answered Apr 17, 2018 at 11:01

John Kugelman

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1 Comment

tnoel999888 Over a year ago

Thanks, but this also inserts a pair of " around the expanded variable. And thanks for the tip about jq!

Abhijit Pritam Dutta · Accepted Answer · 2018-04-17 11:39:30Z

0

I tried sed command in my system from the below two answer and got error sed: command garbled: hence quickly giving below solution without a sed command for the time being. Will check once get time how to fix it with sed.

 #first print the first line of file certs.json
 head -1 certs.json > certs-new.json
 #Now insert value of variable str_builder into certs-new.json as second line
 echo "$str_builder" >> certs-new.json

 #now insert all the lines except first line from certs.json to certs-new.json
 tail +2 certs.json >> certs-new.json

answered Apr 17, 2018 at 11:39

Abhijit Pritam Dutta

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