sed replace using string containing backslashes - sed

I need to replace text in a file with a Windows-style directory path containing backslash (REVERSE SOLIDUS) characters. I am already using an alternative expression delimiter. The backslashes appear to be treated as escape characters.
How can I keep the backslashes in the output?
$ echo DIR=foobar | sed -e "s#DIR=.*#$(cygpath -w $(pwd))#"
C:gwin64homelit
The desired output is:
C:\cygwin64\home\lit

You'll have to escape metacharacters in sed replacement pattern. Fortunately, there are only three of those: &, \, and a delimiter / (see this question and this). In your case, since you're using # for delimiter, you'll have to escape # instead of /.
You can create a helper shell function (like here):
escapeSubst() { sed 's/[&#\]/\\&/g'; }
and then pass your string through it before giving it to sed, like this:
$ echo DIR=foobar | sed -e "s#DIR=.*#$(cygpath -w $(pwd) | escapeSubst)#"
C:\cygwin64\home\lit

Related

sed replace with backslash, double quote, single quote

Can you give the sed command that will find \" and replace with \\' in a file.
For example line:
LOG_FN=\"file_name\"
will become
LOG_FN=\\'file_name\\'
By using this template:
sed -i 's/old-text/new-text/g' input.txt
I tried following sed commands:
sed -i 's/\\\"/\\\\\'/g' input.txt
sed -i "s/\\\"/\\\\'/g" input.txt
Unfortunately they fail because what I am looking for is a string substitution for \" while commands I tried change individual " characters.
You can't escape a single quote inside single quotes. Your second attempt needs more backslashes: Remember, inside double quotes, the shell processes one layer of backslashes, so you have to double each backslash which should make it through to sed.
sed "s/\\\\\"/\\\\\\\\'/g" input.txt
After the shell has processed the double-quoted string, the script which ends up being executed is
s/\\"/\\\\'/g
where the first pair of backslashes produce a literal backslash in the matching regex, and each pair of backslashes in the replacement produce one literal backslash in the output.
Demo: https://ideone.com/XqfwbV

search a string which contains "/" and replace using sed

How to search a pattern and remove the line using sed which contains special characters like "ranasnfs2:/SA_kits/prod"
I tried using a variable to hold the complete string and then recall the variable in sed command but it is not working.
echo $a
ranasnfs2:/SA_kits/prod
sed -i '/"$a"/d' test.txt
cat test.txt | grep -i SA
/SA_kits -rw,suid,soft,retry=4 ranasnfs2:/SA_kits/prod
You need to escape the slash character.
Use this for deleting lines which contain a /:
sed '/\//d' file

Escaping a variable with special characters within sed - comment and uncomment an arbitrary line of source code

I need to comment out a line in a crontab file through a script, so it contains directories, spaces and symbols. This specific line is stored in a variable and I am starting to get mixed up on how to escape the variable. Since the line changes on a regular basis I dont want any escaping in there. I don't want to simply add # in front of it, since I also need to switch it around and replace the line again with the original without the #.
So the goal is to replace $line with #$line (comment) with the possibility to do it the other way around (uncomment).
So I have a variable:
line="* * * hello/this/line & /still/this/line"
This is a line that occurs in a file, file.txt. Wich needs to get comment out.
First try:
sed -i "s/^${line}/#${line}/" file.txt
Second try:
sed -i 's|'${line}'|'"#${line}"'|g' file.txt
choroba's helpful answer shows an effective solution using perl.
sed solution
If you want to use sed, you must use a separate sed command just to escape the $line variable value, because sed has no built-in way to escape strings for use as literals in a regex context:
lineEscaped=$(sed 's/[^^]/[&]/g; s/\^/\\^/g' <<<"$line") # escape $line for use in regex
sed -i "s/^$lineEscaped\$/#&/" file.txt # Note the \$ to escape the end-of-line anchor $
With BSD/macOS sed, use -i '' instead of just -i for in-place updating without backup.
And the reverse (un-commenting):
sed -i "s/^#\($lineEscaped\)\$/\1/" file.txt
See this answer of mine for an explanation of the sed command used for escaping, which should work with any input string.
Also note how variable $lineEscaped is only referenced once, in the regex portion of the s command, whereas the substitution-string portion simply references what the regex matched (which avoids the need to escape the variable again, using different rules):
& in the substitution string represents the entire match, and \1 the first capture group (parenthesized subexpression, \(...\)).
For simplicity, the second sed command uses double quotes in order to embed the value of shell variable $lineEscaped in the sed script, but it is generally preferable to use single-quoted scripts so as to avoid confusion between what the shell interprets up front vs. what sed ends up seeing.
For instance, $ is special to both the shell and sed, and in the above script the end-of-line anchor $ in the sed regex must therefore be escaped as \$ to prevent the shell from interpreting it.
One way to avoid confusion is to selectively splice double-quoted shell-variable references into the otherwise single-quoted script:
sed -i 's/^'"$lineEscaped"'$/#&/' file.txt
awk solution
awk offers literal string matching, which obviates the need for escaping:
awk -v line="$line" '$0 == line { $0 = "#" $0 } 1' file.txt > $$.tmp && mv $$.tmp file.txt
If you have GNU Awk v4.1+, you can use -i inplace for in-place updating.
And the reverse (un-commenting):
awk -v line="#$line" '$0 == line { $0 = substr($0, 2) } 1' file.txt > $$.tmp &&
mv $$.tmp file.txt
Perl has ways to do the quoting/escaping for you:
line=$line perl -i~ -pe '$regex = quotemeta $ENV{line}; s/^$regex/#$ENV{line}/' -- input.txt

Escape backslash character in sed

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?
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/${path}/g" test.txt
D:\first\usr
This works.
Depending on which shell you are using, you may be able to use parameter expansion to double \ in your substitution string and prevent the \u interpretation:
path="first\usr"
sed -e "s/\\usr/${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).

Substitute with backslash in sed

I want to replace word with \word{sth} with sed.
I type in
sed -i s#word#\\word{sth}
but i am getting is word{sth} instead of \word{sth}
I tried with 1 slash also in the command
you should add four backslashes.
you need two to escape the backslash by the terminal, and two to escape it for sed. 2*2=4.
$ echo word|sed s#word#\\\\word{sth}#gi
\word{sth}
Consider enclosing sed expression with single-quotes '
sed -i 's#word#\\word{sth}#' file