Environment variable substitution using Sed
Suppose we have a text file config.ini looks something like this:
[MSSQLSERVER] Driver=INSTHOME/foo/foo.so [SYBASE] Driver=INSTHOME/bar/bar.so ...
We want to replace all the appearance of INSTHOME with the value we hold in $HOME. Here is what I do initially:
sed -i -e "s/INSTHOME/$HOME/g" config.ini
- s is used to replace the found expression INSTHOME with $HOME
- g stands for "global", which means to do this find & replace for the whole line. If you leave off the g and INSTHOME appears twice on the same line, only the first INSTHOME is changed to $HOME
- -i is used to edit in place on filename
- -e is to indicate the expression/command to run
I use double quotes " to expand any variable appeard inside ". In this case, $HOME.
However, when I type this in and I got the following error:
sed: -e expression #1, char 13: unknown option to `s'
Why did this error happen? That confused me for a while. Then, I try to simulate what the program will do for the above expression:
sed -i -e "s/INSTHOME//home/iidev20/g" config.ini
Ah! This expansion result doesn't make sense at all because sed expression inside " needs to follow:
So, the first thought comes to me is to escape all / in the expression:
sed -i -e "s/INSTHOME/\/home\/iidev20/g" config.ini
This can work but it has two severe drawbacks:
- I'm hardcoding the value. If $HOME no longer holds /home/iidev20, then my command breaks again, and this hinders portability.
- The readability of this code is too bad. Probably okay for Perl programmer but still, not quite friendly.
To address these two issues, I find the following about GNU sed:
(The % may be replaced by any other single character.)This also matches the regular expression regexp, but allows one to use a different delimiter than /. This is particularly useful if the regexp itself contains a lot of slashes, since it avoids the tedious escaping of every /. If regexp itself includes any delimiter characters, each must be escaped by a backslash ().
Essentially, we don't have to use / as our delimiter for the expression, especially when the pattern itself contains a lot of slashes (i.e. file path in my case).
so, I decide to use | as the delimiter:
sed -i "s|INSTHOME|$HOME|g" config.ini
I can also use single quote ' but the command should be modified like the below by leaving out to-be-expanded variable name outside of single quotes.
sed -i 's|INSTHOME|'$HOME'|g' config.ini
Now, everything works nice and clean.