sed with variables containing /

Question

i have a text file containing some content like that,

some content  
replace content  
other content

and i have some variables, looking like this:

PATH="/path/to/scriptdir"
PARAMETERS="para1 para2"
FILETOCHANGE="/foo/bar"

Now i want to replace the content in the text file that begins with replace, with the content of the variables PATH and PARAMETERS. The text file should look like that, after the replace:

some content
/path/to/scriptdir/script para1 para2
other content

I just tried the a couple of sed commands, but always i get an error. For example

sed -i "s/^replace.*$/$PATH/script $PARAMETERS/g" $FILETOCHANGE
sed: -e expression #1, char 12: unknown option to `s'

or

sed -i "s|^replace.*$|$PATH/script $PARAMETERS|g" $FILETOCHANGE
sed: -e expression #1, char 58: unterminated `s' command

Can you tell me, what I'm doing wrong?

Answer 1

(1) Do not use the variable PATH for this. It has a special meaning for the shell. I choose SCRIPTDIR here:

SCRIPTDIR="/path/to/scriptdir"
PARAMETERS="para1 para2"
FILETOCHANGE="/foo/bar"

(2) You do not need a slash (/) per se in sed's substitution command: s#this#that# or s|this|that| works as well as s/this/that/g. Sed just uses the character directly after the s as the separator for the rest of the s (substitute) command. You can use this to avoid the problem with the slashes in your pattern and replacement string: when you use e.g. | as a seperator, the slashes do not have any special meaning for sed.

So, this will work:

sed "s%^replace.*$%$SCRIPTDIR/script $PARAMETERS%g" $FILETOCHANGE

Answer 2

The ultimate solution is to switch from sed to Perl which handles variables as first class citizens, not as macros to expand as the shell does.

path=/path/to/scriptdir parameters="para1 para2" perl -i~ -pe \
    's|^replace.*$|$ENV{path}/script $ENV{parameters}|g' -- "$filetochange"

• -p reads the input line by line and prints each line after processing it
• the %ENV hash contains the environment variables, assigning to them at the beginning of a command makes them exported for the command
• s/// (change the delimiters as you wish) works similarly to sed, see perlop