Re: OT: bash parameter substitutions

[Tom H]

On Mon, Nov 5, 2018 at 7:51 AM Mike Wright  wrote:
> There is such a thing as too terse. Can somebody explain what this
> cryptic snippet means? I get the "substitute part; it's the
> "error,exit" part that has me confounded.
>
> +------------------+--------------------+---------------+--------------+
> |                  | parameter          | parameter     | parameter    |
> |                  | Set and Not Null   | Set But Null  | Unset        |
> +------------------+--------------------+---------------+--------------+
> |${parameter:?word}|substitute parameter|error,exit     |error,exit    |
> |${parameter?word} |substitute parameter|substitute null|error,exit    |
> +------------------+--------------------+---------------+--------------+

In the case of ":?word" and "unset", for example, it'll print "word",
and exit the script if it's running within a script.

th macbook12 ~ $ bash --version
GNU bash, version 4.4.23(1)-release (x86_64-apple-darwin18.0.0)
...

th macbook12 ~ $ echo ${PS1:?fedora}
\u \h \w $

th macbook12 ~ $ echo ${PS5:?fedora}
-bash: PS5: fedora

th macbook12 ~ $ vi mike.sh

th macbook12 ~ $ chmod +x mike.sh

th macbook12 ~ $ cat mike.sh
#!/bin/sh
one=ONE
three=THREE
echo "print one"
echo ${one:?fedora}
echo
echo "print two"
echo ${PS5:?fedora}
echo
echo "print three"
echo ${three}

th macbook12 ~ $ ./mike.sh
print one
ONE

print two
./mike.sh: line 8: PS5: fedora