How to add an attribute in a tag in XML file by command-line?

in sed "a" appends a pattern IN A NEW LINE.

what you want to do is replace (substitute). Let's use a colon as separator for clarity:

sed 's:\(<book.*\)\(/>\):\1 Book_Width="A"\2:'

anything in \( .. \) is a pattern memorized by the order of appearance and recalled by \indexnumber , e.g. \1 will reproduce the first pattern saved.

So we are memorizing <book name="Sed tutorial" price="250" as pattern 1 and /> as pattern 2 and just insert Book_Width="A" in the middle.

echo '<book name="Sed tutorial" price="250"/>' | sed 's:\(<book.*\)\(/>\):\1 Book_Width="A"\2:'
<book name="Sed tutorial" price="250" Book_Width="A"/>

Assuming that your XML file is literally just a single line with a book node, as shown in the question, you may add a Book_Width attribute to that book node using xq (part of yq from https://kislyuk.github.io/yq/) like so:

$ xq -x '.book."@Book_Width" |= "A"' file.xml
<book name="Sed tutorial" price="250" Book_Width="A"></book>

If you have data that you'd like to insert as the attribute's value:

$ myvariable='20 cm'
$ xq -x --arg val "$myvariable" '.book."@Book_Width" |= $val' file.xml
<book name="Sed tutorial" price="250" Book_Width="20 cm"></book>

With awk you can do something like this:

Supposing the line is in file file1.xml then:

awk -F '/' '{print $1,"Book_Width=\"A\"",FS,$2}' file1.xml

The solution would be to use sed and not use any external packages or commands.

echo '<book name="Sed tutorial" price="250"/>' |
sed 's/<book /& Book_Width="A"/'

Output:

<book Book_Width="A" name="Sed tutorial" price="250"/>

If you have a file.

sed 's/<book /& Book_Width="A"/' ${FILE_NAME}

or if you want to replace inplace

sed -i 's/<book /& Book_Width="A"/' ${FILE_NAME}

&: This symbol is a special character in sed that represents the entire matched string, i.e., <book(space). We use it here in the replacement pattern to include the original <book tag in the modified text.