When I read some tutorial today, I came over with the statements like ${file?my.file.txt} and ${file:?my.file.txt}, what do ${file?my.file.txt} and ${file:?my.file.txt} mean in Bash script?
asked Oct 5, 2014 at 15:20
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For your future questions, please do a little research before. I've seen you've asked a bit about Bash in particular, but your questions are hardly based on an actual problem and somewhat lack demonstration of research effort.– slhckOct 5, 2014 at 15:41
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@slhck Ok. In fact, I just wanna know the specific details. Sorry for that if it conflicts with the style here. Thanks for the patient answers, anyhow.– user3872279Oct 5, 2014 at 15:48
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1 Answer
From the manual – and it's always good to check that first:
${parameter:?word}If
parameteris null or unset, the expansion ofword(or a message to that effect ifwordis not present) is written to the standard error and the shell, if it is not interactive, exits. Otherwise, the value ofparameteris substituted.
So, it's a safeguard against unset variables, where you could show an error. Also,
Omitting the colon results in a test only for a parameter that is unset. Put another way, if the colon is included, the operator tests for both parameter’s existence and that its value is not null; if the colon is omitted, the operator tests only for existence.
But of course, the examples you gave are somewhat arbitrary and I wonder why the tutorial would even include them, let alone including them but not explaining what they do.
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After I make all these things clear, I will write a really high quality doc. Thx! Oct 5, 2014 at 16:00