2

I want to write a script 'test.sh' that will take a user's input and replace all special characters with a '\' + the character. My script:

#!/bin/bash
echo 'input='"$1"
arg=`echo "$1" | sed 's:[]\[\^\$\.\*\/]:\\&:g'`
echo 'modified input='"$arg"

My command works on the string 'xy', which does not have any special characters. I run this in my terminal:

test.sh xy

And I obtain:

input=xy
modified input=xy

However, when I run this:

test.sh x.y

I get:

input=x.y
modified input=x&y

I don't understand why this script is not working. I expect to get:

input=x.y
modified input=x\.y

I think the problem lies in this sed command but I'm not sure where:

sed 's:[]\[\^\$\.\*\/]:\\&:g'
Improve this question

2 Answers 2

Reset to default
4

I don't think you are capturing the group right. Try this?

sed 's:\([]\[\^\$\.\*\/]\):\\\1:g'

Edit: nevermind, I wasn't aware of the relevance of &. The problem is you need to extra escape the slashes in replacement because of the parsing and passing to sub shell.

sed 's:[]\[\^\$\.\*\/]:\\\\&:g'

Also, if you used $(command) instead of backticks, you don't need the extra \'s

Improve this answer
2
  • Yes, if I add two more '\' as you did will work. But I don't see why they are needed. Is there a reference you can point me to understand this? Also, not sure what you mean regarding $(command) Commented Nov 7, 2012 at 22:15
  • 1
    enclosing commands in backticks uses simple substitution - so the the command is parsed by the current shell. $(command) opens a subshell, so anything inside the parens runs as if on its own command line. See for more info: wiki.bash-hackers.org/syntax/expansion/cmdsubst Commented Nov 8, 2012 at 14:58
1

So the task is to write a sed substitution that escapes all special characters (defined as the characters []^$.*/, and I added \ too) in a string.

The substitution might look like

s/[][\^$.*/]/\\&/g

Note that we don't really have to escape any of the characters in the [...] set (the \ is treated literally too, so we can't escape anything in there), and that the only thing we have to make sure of is that ] is in the very beginning of it.

So the script (rewritten as a shell function, and revamped in other ways) becomes

function escape
{
    printf '%s\n' "$1" | sed 's/[][\^$.*/]/\\&/g'
}

Testing:

$ escape 'hello world'
hello world

$  escape '*.*'
\*\.\* 

$ escape '\/'
\\\/

$ escape '/\'
\/\\

$ escape 's/[][\^$.*/]/\\&/g'
s\/\[\]\[\\\^\$\.\*\/\]\/\\\\&\/g
Improve this answer

You must log in to answer this question.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.