454

I tried the following code to replace QQ with ZZ, but it doesn't do what I want:

var1=QQ
sed -i 's/$var1/ZZ/g' $file

However, this code does what I want:

sed -i 's/QQ/ZZ/g' $file

How do I use variables in sed?

2
601

The shell is responsible for expanding variables. When you use single quotes for strings, its contents will be treated literally, so sed now tries to replace every occurrence of the literal $var1 by ZZ.

Using double quotes

Use double quotes to make the shell expand variables while preserving whitespace:

sed -i "s/$var1/ZZ/g" "$file"

When you require the quote character in the replacement string, you have to precede it with a backslash which will be interpreted by the shell. In the following example, the string quote me will be replaced by "quote me" (the character & is interpreted by sed):

sed -i "s/quote me/\"&\"/" "$file"

Using single quotes

If you've a lot shell meta-characters, consider using single quotes for the pattern, and double quotes for the variable:

sed -i 's,'"$pattern"',Say hurrah to &: \0/,' "$file"

Notice how I use s,pattern,replacement, instead of s/pattern/replacement/, I did it to avoid interference with the / in \0/.

Example

The shell then runs the above command sed with the next arguments (assuming pattern=bert and file=text.txt):

-i
s,bert,Say hurrah to &: \0/,
text.txt

If file.txt contains bert, the output will be:

Say hurrah to bert: \0/
10
  • 2
    How would one pass the "/g" option using this comma separated form?
    – blong
    Apr 1 '14 at 13:55
  • 7
    @b.long It's a g option, so you would pass s,foo,bar,g instead.
    – Lekensteyn
    Apr 1 '14 at 14:32
  • 1
    your single quote example isparticularly useful, covers sed using regex pattern pretty nicely! Sep 9 '16 at 1:08
  • 6
    Or, for the uber-paranoid (like myself), sed 's/'"${foo}"'/replacement/'
    – leetbacoon
    Oct 4 '19 at 3:01
  • 1
    @leetbacoon Your paranoia saved me a hell lot of work. I couldn't use the above patterns for my task-specific reasons.
    – mnis
    Dec 1 '20 at 7:33
190

We can use variables in sed using double quotes:

sed -i "s/$var/r_str/g" file_name

If you have a slash / in the variable then use different separator, like below:

sed -i "s|$var|r_str|g" file_name
6
  • 29
    If you have a slash / in the variable => This saved me ! My variable is a url and it contains /. Switching to use | as separator fixed my issue
    – sonlexqt
    Sep 20 '18 at 4:39
  • 9
    Thumb up for "if you have a slash in the variable"
    – Tiina
    Apr 25 '19 at 2:07
  • 1
    @sonlexqt, related: stackoverflow.com/questions/9366816/… Mar 19 '20 at 6:33
  • 1
    Mannnn... I had a forward slash in my var and was pulling my hair on that.. this was a life saver... Thanks a ton.
    – Ajay Kumar
    Jan 6 at 23:57
  • Indeed the variables may contain paths and in that case, delimiters should be something other than slashes. Thanks for that great tip!
    – msoutopico
    May 17 at 11:12
1

To expand (pun intended) on @mani's answer,

  • this solution will work for regular expressions fed to other commands, such as perl
  • use double quotes around the regex (yes, even under Unixy systems) in order to expand the variable as expected
  • | may appear in your variable's value as well, so don't be scared to try other delimiters

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