/bin/dash: Bad substitution

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