Escape backslash character in sed

Question

I need to modify some Windows paths.

For instance,

D:\usr

to

D:\first\usr

So, I have created a variable.

$path = "first\usr"

then used the following command:

sed -i -e 's!\\usr!${path}/g;' test.txt

However, this ends up with the following:

D:\firstSr

How do I escape \u in sed?

possible duplicate of [Why does sed require 3 backslashes for a regular backslash?](http://stackoverflow.com/questions/2369314/why-does-sed-require-3-backslashes-for-a-regular-backslash) — Risadinha, Jul 15 '15 at 12:32
you need to double escape the content of new value also, not only the sed search pattern — NeronLeVelu, Jul 15 '15 at 12:35
What shell or scripting language are you using? `$path = "first\usr"` looks like Perl, but the rest doesn't... — Toby Speight, Feb 22 '16 at 11:51

score 2 · Accepted Answer · answered Feb 22 '16 at 03:00

Assuming your path variable was assigned properly (without spaces in the assignment: path='first\usr'), fixing step by step for an input file test.txt with one example path:

$ cat test.txt
D:\usr

Your original command

$ sed 's!\\usr!${path}/g;' test.txt
sed: -e expression #1, char 18: unterminated `s' command

doesn't do much, as you've mixed ! and / as the delimiter.

Fixing delimiters:
```
$ sed 's!\\usr!${path}!g;' test.txt
D:${path}
```
Now no interpolation happens at all because of the single quotes. I suspect these are just copy-paste mistakes, as you obviously got some output.
Double quotes:
```
$ sed "s!\\usr!${path}!g" test.txt
bash: !\\usr!${path}!g: event not found
```
Now this clashes with history expansion. We could escape the !, or use a different delimiter.
/ as delimiter:
```
$ sed "s/\\usr/${path}/g" test.txt
D:\firstSr
```
Now we're where the question actually started. ${path} expands to first\usr, but \u has a special meaning in GNU sed in the replacement string: it uppercases the following character, hence the S.

Even without the special meaning, \u would most likely just expand to u and the backslash would be gone.

Escaping the backslash:

$ path='first\\usr'
$ sed "s/\\usr/${path}/g" test.txt
D:\first\usr

This works.

score 0 · Answer 2 · edited Feb 22 '16 at 11:45

Depending on which shell you are using, you may be able to use parameter expansion to double \ in your substitution string and prevent the \u interpretation:

path="first\usr"
sed -e "s/\\usr/${path//\\/\\\\}/g" <<< "D:\usr"

The syntax for replacing a pattern with the shell parameter expansion is ${parameter/pattern/string} (one replacement) or ${parameter//pattern/string} (replace all matches).

This substitution is not specified by POSIX, but is available in Bash.

Where it is not available, you may need to filter $path through a process:

path=$(echo "$path" | sed 's/[][\\*.%$]/\\&/g')

(N.B. I have also quoted other sed metacharacters in this filter).

Escape backslash character in sed

2 Answers2