Question

everyone, i have a problem in bash programming, that is I don’t know what’s mean in bash
script:

${parameter:-word}
${parameter:=word}
${parameter:?word}
${parameter:+word}
${parameter:offset}
${parameter:offset:length}
${!prefix*}
${#parameter}
${parameter#word}
${parameter##word}
${parameter%word}
${parameter%%word}
${parameter/pattern/string}
${parameter//pattern/string}

this usage, thank you for answer me .

Answer 1

If you just open up a terminal window and enter man bash, you’ll be able to find all of these under Parameter Expansion.

In fact, since these are exactly the form in which the man page has them (using parameter and word) and in the same order you have them, I’m surprised you don’t know this already.

One example is ${parameter:-word} which states:

Use Default Values. If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.

So, for that case, ${xyzzy:-plugh} will give you the value of ${xyzzy} if it’s set, otherwise you’ll get the literal "plugh".

I won’t go through them all, the bash man page explains in full detail. “Teach a man to fish …” and all that stuff.