Environment variable substitution in sed

Question

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?

Solution 1

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"'@'

Solution 2

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.

Solution 3

Another easy alternative:

Since $PWD will usually contain a slash /, use | instead of / for the sed statement:

sed -e "s|xxx|$PWD|"

Solution 4

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

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

Solution 5

一. bad way: change delimiter

sed 's/xxx/'"$PWD"'/'
sed 's:xxx:'"$PWD"':'
sed 's@xxx@'"$PWD"'@'

maybe those not the final answer,

you can not known what character will occur in $PWD, / : OR @.
if delimiter char in $PWD, they will break the expression

the good way is replace(escape) the special character in $PWD.

二. good way: escape delimiter

for example: try to replace URL as $url (has : / in content)

x.com:80/aa/bb/aa.js

in string $tmp

<a href="URL">URL</a>

A. use / as delimiter

escape / as \/ in var (before use in sed expression)

## step 1: try escape
echo ${url//\//\\/}
x.com:80\/aa\/bb\/aa.js   #escape fine

echo ${url//\//\/}
x.com:80/aa/bb/aa.js      #escape not success

echo "${url//\//\/}"
x.com:80\/aa\/bb\/aa.js   #escape fine, notice `"`


## step 2: do sed
echo $tmp | sed "s/URL/${url//\//\\/}/"
<a href="x.com:80/aa/bb/aa.js">URL</a>

echo $tmp | sed "s/URL/${url//\//\/}/"
<a href="x.com:80/aa/bb/aa.js">URL</a>

OR

B. use : as delimiter (more readable than /)

escape : as \: in var (before use in sed expression)

## step 1: try escape
echo ${url//:/\:}
x.com:80/aa/bb/aa.js     #escape not success

echo "${url//:/\:}"
x.com\:80/aa/bb/aa.js    #escape fine, notice `"`


## step 2: do sed
echo $tmp | sed "s:URL:${url//:/\:}:g"
<a href="x.com:80/aa/bb/aa.js">x.com:80/aa/bb/aa.js</a>

View more solutions

Comments

[derobert]

$PWD contains a / which is ending the substitute command.

[RomanM]

@derobert: tnx. One of the solutions addresses this ...

[bobbogo]

Use set -x in the shell to get the shell to echo each command just before it executes them. This can clear up a lot of confusion. (Also, I often use set -u to make de-referencing unset variables a hard error. (See set -e too.))

[ThorSummoner]

I was hoping to find a way for sed to handle the environment variables as not to leak the values into the process table, seems like sed is the wrong tool for installing secrets according to all the answers in this thread

[RomanM]

but PWD is defined by the shell ... if i go echo $PWD i get the pwd

[RomanM]

but, then what character can i use i know for sure will not appear in a path name ?

[Norman Ramsey]

You can have several candidates like @#%! and check with a case expression to find if $PWD has them. E.g., case "$PWD" of @) ;; *) delim="@" ;; esac; repeat until $delim is not empty.

[einpoklum]

So, how do you escape the environment variables?

[Eddie]

The selected answer describes a workaround ... don't use slash for the delimiter

[Thales Ceolin]

There's another alternative instead of using double quotes. See my answer below.

[Vladislavs Dovgalecs]

This is cleaner than messing with double quotes, single quotes etc.

[blubberdiblub]

Which will become a problem as soon as your current working directory contains that other character.

[Dan]

You said "an alternative to using double quotes" and yet your example uses double quotes?

[Boyd Hemphill]

In my specific case, $2 was a file path, so sed was barfing due to interpreting the / in the contents of $2. This was exactly what I needed to get past it. Thanks fora great tip!

[izikandrw]

this is what I had to use to get the environment variable to expand correctly in this command: sed -i "s/127.0.0.1/127.0.0.1 localhost $HOSTNAME/" hosts

[mjp]

This is neat but it is not working when you want to do some more complex substitutions, such as "2,$s/^/$foo/" as $s gets interpreted as a variable too and it should not.

[Christian]

I guess the point is it doesn't use double-quotes directly around $PWD...?

[andov]

I was struggling to get this working in Azure DevOps YAML pipelines. This comment helped me to successfully use this trick to tokenize some configuration files.

[Guchelkaben]

You can use another delimiter for sed. You dont need to use / you can use , as well if your environment variable is an url.

[Max Waterman]

What if the string contains a \ followed by an n - how to stop sed from converting that into a single newline character?

[MoRe]

This alone didn't work as I still needed a different delimiter for paths like the other answers suggest.

[NameOfTheRose]

@mjp, what you say is correct but I think one may write it as "2,$ s/^/$foo/"

[bananaforscale]

Nice clean pattern but the -e (--expression) is not needed here, since this example is not stringing several expressions together.