Question

I have a bash script that lists subprograms/processes that could if the user chooses to, insert startupflags to a specific program. I want to match strings in the below formats and depending on which pgm the user chooses I want to insert/replace the string with the new flag infront of {PGMPATH}/pgm. The existing programs are listed in a startupfile according to something like this:

start -existingFlag ${PGMPATH}/pgm
start -existingFlag -anotherExistingFlag ${PGMPATH}/anotherPgm
start -existingFlag -anotherFlag ${PGMPATH}/yetAnotherPgm otherStuff

But to start with I try to match toward a hardcoded string (in the future toward the lines in the startup file):

start -existingFlag ${PGMPATH}\/pgm*

and replace it with a new line looking like this:

*start -existingFlag -newFlag ${PGMPATH}\/pgm*

From script:

existingString="start -existingFlag ${PGMPATH}\/pgm"
newString="start -existingFlag -newFlag ${PGMPATH}\/pgm"
sed 's/$replaceString/$newString/g' $STARTUPCONFFILE

This works (the string is replaced) as long as there is no ‘$’ (just before {PGMPATH}) in the strings, but as soon as I add ‘$’ as in ${PGMPATH} SED doesn’t replace. I have tried a lot but I can’t get it to work.

Suggestions?

Answer 1

You need double quotes for the shell to expand variables:

$ set newString=1                                                         
$ set replaceString=one

# using single quotes: no expansion -> no replacement!              
$ echo one | sed 's/$replaceString/$newString/g'
one

# using double quotes: expansion -> replacement!
$ echo one | sed "s/$replaceString/$newString/g"
1