/bin/dash: Bad substitution - string

Question

I need to do a string manipuilation in shell script (/bin/dash):
#!/bin/sh
PORT="-p7777"
echo $PORT
echo ${PORT/p/P}
the last echo fails with Bad substitution. When I change shell to bash, it works:
#!/bin/bash
PORT="-p7777"
echo $PORT
echo ${PORT/p/P}
How can I implement the string substitution in dash ?

Answer 1

The substitution you're using is not a basic POSIX feature (see here, in section 2.6.2 Parameter Expansion), and dash doesn't implement it.
But you can do it with any of a number of external helpers; here's an example using sed:
PORT="-p7777"
CAPITOLPORT=$(printf '%s\n' "$PORT" | sed 's/p/P/')
printf '%s\n' "$CAPITOLPORT"
BTW, note that I'm using printf '%s\n' instead of echo -- that's because some implementations of echo do unpredictable things when their first argument starts with "-". printf is a little more complicated to use (you need a format string, in this case %s\n) but much more reliable. I'm also double-quoting all variable references ("$PORT" instead of just $PORT), to prevent unexpected parsing.
I'd also recommend switching to lower- or mixed-case variables. There are a large number of all-caps variable that have special meanings, and if you accidentally use one of those it can cause problems.

Answer 2

Using parameter expansion:
$ cat foo.sh
#!/bin/sh
PORT="-p7777"
echo $PORT
echo ${PORT:+-P${PORT#-p}}
PORT=""
echo $PORT
echo ${PORT:+-P${PORT#-p}}
Run it:
$ /bin/sh foo.sh
-p7777
-P7777
Update:
$ man dash:
- -
${parameter#word} Remove Smallest Prefix Pattern.
$ echo ${PORT#-p}
7777
$ man dash
- -
${parameter:+word} Use Alternative Value.
$ echo ${PORT:+-P${PORT#-p}}
-P7777