How do I use variables in a sed command?

Question

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?

related: https://stackoverflow.com/questions/407523/escape-a-string-for-a-sed-replace-pattern — Ciro Santilli OurBigBook.com, Mar 28 '18 at 12:43

Lekensteyn · Accepted Answer · 2018-08-23T17:19:11.213

746

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/

edited Aug 23 '18 at 17:19

answered Nov 07 '11 at 17:20

Lekensteyn

174,277

2

How would one pass the "/g" option using this comma separated form? – blong Apr 01 '14 at 13:55
7

@b.long It's a g option, so you would pass s,foo,bar,g instead. – Lekensteyn Apr 01 '14 at 14:32
1

your single quote example isparticularly useful, covers sed using regex pattern pretty nicely! – Brian Thomas Sep 09 '16 at 01:08
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
using # as a delimiter is possible too: sed -i -e 's#$HOME/$SOME_PATH#$HOME/$ANOTHER_PATH#g' – vladkras Apr 15 '19 at 10:38
15

Or, for the uber-paranoid (like myself), sed 's/'"${foo}"'/replacement/' – leetbacoon Oct 04 '19 at 03:01
${foo} is the one working for me – cwhisperer Feb 20 '20 at 09:14
3

@leetbacoon Your paranoia saved me a hell lot of work. I couldn't use the above patterns for my task-specific reasons. – betelgeuse Dec 01 '20 at 07:33
@mnis Glad to hear that helped! – leetbacoon Dec 04 '20 at 20:09
@leetbacoon your approach is the only one that helped me with the following replacement: sed ':a;N;$!ba;s/UUID=[A-Fa-f0-9-]*/UUID='"${new_uuid}"'/2' </etc/fstab | sudo tee /etc/fstab2 > /dev/null – Anton Samokat Jan 14 '24 at 15:50

score 235 · Answer 2 · edited Mar 03 '17 at 20:37

235

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

edited Mar 03 '17 at 20:37

Zanna

70,465

answered Aug 07 '14 at 15:29

mani

2,351

41

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 04:39
12

Thumb up for "if you have a slash in the variable" – Tiina Apr 25 '19 at 02:07
1

@sonlexqt, related: https://stackoverflow.com/questions/9366816/sed-fails-with-unknown-option-to-s-error/9366940#9366940 – Gabriel Staples Mar 19 '20 at 06:33
2

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 06 '21 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 '21 at 11:12
1

The "|" was gold. Thanks! – mrsauravsahu Sep 13 '21 at 14:56
Looks like everyone here is replacing some URLs. Thank you guys – Yavor from Team-GPT Sep 29 '22 at 21:20

score 2 · Answer 3 · answered Dec 20 '20 at 07:02

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

How do I use variables in a sed command?

3 Answers3

Using double quotes

Using single quotes

Example

Linked

Related