I have a variable (extracted from a diff file result) with leading spaces that I want to insert into a file at a specific line. The variable:
add=" public function add($hardware_id = null)"
But when I insert the line with sed, it doesn't print the leading spaces:
$ sed "50i$add" file
...
public function add($hardware_id = null)
...
How to preserve the leading spaces?
To preserve the leading spaces when inserting a variable, you just have to escape once the spaces with a backslash \ after the insert command i, like this:
$ sed '50i\'"$add" file
public function add($hardware_id = null)
Is not specific to variables, it would be the same if the text was inserted literally.
Is not a POSIX specified behavior, which mandates a escaped literal newline before the text to be inserted.
The posixly way to accomplish is to use the i\ syntax. But since we are under the umbrella of double quotes we escape the backslash.
And the variable has to be made safe for this operation by escaping newline(s) and backslash (s)
add=' public function add($hardware_id \= null
)'
#------------------------------
# make variable safe to use
# by escaping backslash n newline
esc_add=$(printf '%s\n' "$add" |
sed -e 's:\\:&&:g;$!s:$:\\:g')
sed -e "
5i\\
$esc_add
" file | nl
1 ABC
2 2 3 4:blockA-1
3 7 9 4:blockA-2
4 1 2 5:blockA-3/
5 public function add($hardware_id \= null
6 )
7 ABC
8 13 11 17:blockB-1
9 2 1 1:blockB-2/
10 ABC
11 7 9 14:blockC-1
12 5 8 2:BlockC-2
13 9 9 9:BlockC-3
14 7 1 2:BlockC-4$
seds in the C locale or that will potentially leave some arbitrary command injection vulnerabilities. Try for instance with add=$'\u03b1\ne uname>&2' with GNU sed in a locale using BIG5 or BIG5-HKSCS charset (such as zh_HK on Ubuntu 20.04 for instance) where that U+03B1 (α) is encoded as 0xa3 0x5c (0x5c being also backslash). More generally, it's best to avoid inserting arbitrary data in the code argument of interpreters (like sed here), and when you still do, sanitise like you did but bear in mind backslash escaping is only reliable in the C locale.
Do yourself a favour and use perl or awk instead of sed here to do that:
add=$' anything including \\backslash\\
and newline and even non\x80-text or overlong lines '
ADD=$add perl -pe '$_ = "$ENV{ADD}\n$_" if $. == 5' < "$file"
Or (though this one could fail with overlong lines or NUL bytes in the contents and beware it will add a trailing newline if there was none originally):
ADD=$add LC_ALL=C awk 'NR == 5 {$0 = ENVIRON["ADD"] ORS $0}; 1' < "$file"
