Environment variable substitution in sed

Roman_M_14587 0
Viewed 345882

If I run these commands from a script:

#my.sh
PWD=bla
sed 's/xxx/'$PWD'/'
...
$ ./my.sh
xxx
bla

it is fine.

But, if I run:

#my.sh
sed 's/xxx/'$PWD'/'
...
$ ./my.sh
$ sed: -e expression #1, char 8: Unknown option to `s'

I read in tutorials that to substitute environment variables from shell you need to stop, and 'out quote' the $varname part so that it is not substituted directly, which is what I did, and which works only if the variable is defined immediately before.

How can I get sed to recognize a $var as an environment variable as it is defined in the shell?

sed shell unix linux
12 Answers

Your two examples look identical, which makes problems hard to diagnose. Potential problems:

  1. You may need double quotes, as in sed 's/xxx/'"$PWD"'/'

  2. $PWD may contain a slash, in which case you need to find a character not contained in $PWD to use as a delimiter.

To nail both issues at once, perhaps

sed 's@xxx@'"$PWD"'@'
Norman_Ramsey_416610

In addition to Norman Ramsey's answer, I'd like to add that you can double-quote the entire string (which may make the statement more readable and less error prone).

So if you want to search for 'foo' and replace it with the content of $BAR, you can enclose the sed command in double-quotes.

sed 's/foo/$BAR/g'
sed "s/foo/$BAR/g"

In the first, $BAR will not expand correctly while in the second $BAR will expand correctly.

user_882520

You can use other characters besides "/" in substitution:

sed "s#$1#$2#g" -i FILE
user_10068630

With your question edit, I see your problem. Let's say the current directory is /home/yourname ... in this case, your command below:

sed 's/xxx/'$PWD'/'

will be expanded to

sed `s/xxx//home/yourname//

which is not valid. You need to put a \ character in front of each / in your $PWD if you want to do this.

user_577520

If your replacement string may contain other sed control characters, then a two-step substitution (first escaping the replacement string) may be what you want:

PWD='/a\1&b$_' # these are problematic for sed
PWD_ESC=$(printf '%s\n' "$PWD" | sed -e 's/[\/&]/\\&/g')
echo 'xxx' | sed "s/xxx/$PWD_ESC/" # now this works as expected
Joseph_Sturtevant_3170

for me to replace some text against the value of an environment variable in a file with sed works only with quota as the following:

sed -i 's/original_value/'"$MY_ENVIRNONMENT_VARIABLE"'/g' myfile.txt

BUT when the value of MY_ENVIRONMENT_VARIABLE contains a URL (ie https://andreas.gr) then the above was not working. THEN use different delimiter:

sed -i "s|original_value|$MY_ENVIRNONMENT_VARIABLE|g" myfile.txt
user_8233680
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