regex - "sed" special characters handling

Question

we have an sed command in our script to replace the file content with values from variables

for example..

export value="dba01upcFusion_test"
sed -i "s%{"sara_ftp_username"}%$value%g" /home_ldap/user1/placeholder/Sara.xml

the sed command ignores the special characters like '' and replacing with string "dba01upcFusion_test" without '' It works If I do the export like export value='dba01upcFusion_test' (with '' surrounded with ‘’).. but unfortunately our client want to export the original text dba01upcFusion_test with single/double quotes and he don’t want to add any extra characters to the text. Can any one let me know how to make sed to place the text with special characters..

Before Replacement : Sara.xml

<?xml version="1.0" encoding="UTF-8"?>
<ser:service-account >
<ser:description/>
<ser:static-account>
<con:username>{sara_ftp_username}</con:username>
</ser:static-account>
</ser:service-account>

After Replacement : Sara.xml

<?xml version="1.0" encoding="UTF-8"?>
<ser:service-account>
<ser:description/>
<ser:static-account>
<con:username>dba01upcFusion_test</con:username>
</ser:static-account>
</ser:service-account>

Thanks in advance

Answer 1

You cannot robustly solve this problem with sed. Just use awk instead:

awk -v old="string1" -v new="string2" '
idx = index($0,old) {
    $0 = substr($0,1,idx-1) new substr($0,idx+length(old))
}
1' file

Ah, @mklement0 has a good point - to stop escapes from being interpreted you need to pass in the values in the arg list along with the file names and then assign the variables from that, rather than assigning values to the variables with -v (see the summary I wrote a LONG time ago for the comp.unix.shell FAQ at http://cfajohnson.com/shell/cus-faq-2.html#Q24 but apparently had forgotten!).

The following will robustly make the desired substitution (aa -> ef) on every search string found on every line:

$ cat tst.awk
BEGIN {
    old=ARGV[1]; delete ARGV[1]
    new=ARGV[2]; delete ARGV[2]
    lgthOld = length(old)
}
{
    head = ""; tail = $0
    while ( idx = index(tail,old) ) {
        head = head substr(tail,1,idx-1) new
        tail = substr(tail,idx+lgthOld)
    }
    print head tail
}

$ cat file
aa    a       a       aa

$ awk -f tst.awk 'aa' 'ef' file
ef    a       a       ef

The white space in file is tabs. You can shift ARGV[3] down and adjust ARGC if you like but it's not necessary in most cases.