How do I append text to the beginning and end of multiple text files in Bash?

To prepend text to a file you can use (with the GNU implementation of sed):

sed -i '1i some string' file

Appending text is as simple as

echo 'Some other string' >> file

The last thing to do is to put that into a loop which iterates over all the files you intend to edit:

for file in *.txt; do
  sed -i '1i Some string' "$file" &&
  echo 'Some other string' >> "$file"
done

You can use GNU sed

Like already illustrated, you can insert lines of text right before and after matching lines of a file with sed, using the i and a command respectively. What hasn't been shown is that you can do it with a one-liner and for multiple files at once.

The following will insert a line before the first 1i and after the last line $a. The insertions will be executed for all files matching the glob *.txt.

sed -i -e '1ivar language = {' -e '$a};' -- *.txt

Both i and a do not only work with line numbers, but also on every line that matches a given pattern. This would insert a comment whenever a line contains var y = 2;:

sed -i -- '/var y = 2;/i//initialize variable y' *.js

Fully POSIX compliant command, using ex:

for f in *.txt; do printf '%s\n' 0a 'var language = {' . '$a' '};' . x | ex "$f"; done

---

If you run the printf portion of the command by itself, you will see the exact editing commands that it is passing to ex:

0a
var language = {
.
$a
};
.
x

0a means "Append text after line 0" (in other words, before the first line). The next line is the literal text to "append" after line 0. The period (.) on a line by itself ends the text to be appended.

$a means to append text after the last line of the file.

x means to save the changes and exit.