208

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?

285

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/
  • 1
    How would one pass the "/g" option using this comma separated form? – blong Apr 1 '14 at 13:55
  • 5
    @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! – Brian Thomas Sep 9 '16 at 1:08
  • In case you are tempted to use \\0 instead of \0, it should not be done when \0 is enclosed in single quotes. Otherwise sed will substitute the pattern for a literal \0 instead of the whole match. – Lekensteyn Aug 23 '18 at 17:20
  • Did the trick for me. – jimh Dec 12 '18 at 17:20
85

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
  • 1
    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

protected by Community Nov 30 '18 at 16:21

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