What is the $? (dollar question mark) variable in shell scripting?
I'm trying to learn shell scripting, and I need to understand someone else's code. What is the $? variable hold? I can't Google search the answer because they block punctuation characters.

What is the meaning of $? in a shell script? - Unix & Linux Stack Exchange
When going through one shell script, I saw the term "$?". What is the significance of this term?

Meaning of $? (dollar question mark) in shell scripts
What does echo $? mean in shell programming? true echo $? # echoes 0 false echo $? # echoes 1 From the manual: (acessible by calling man bash in your shell) ? Expands to the exit status of the most recently executed foreground pipeline. By convention an exit status of 0 means success, and non-zero return status means failure. Learn more about exit statuses on wikipedia. There are other special ...

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What does $# mean in shell? - Unix & Linux Stack Exchange
What does $# mean in shell? I have code such as if [ $# -eq 0 ] then I want to understand what $# means, but Google search is very bad for searching these kinds of things.

Difference between ${} and $() in a shell script - Super User
$(command) is “command substitution”. As you seem to understand, it runs the command, captures its output, and inserts that into the command line that contains the $(…); e.g., $ ls -ld $(date +%B).txt -rwxr-xr-x 1 Noob Noob 867 Jul 2 11:09 July.txt ${parameter} is “parameter substitution”. A lot of information can be found in the shell’s man page, bash (1), under the “ Parameter ...

What is the difference between $@ and $* in shell scripts?
In shell scripts, what is the difference between $@ and $*? Which one is the preferred way to get the script arguments? Are there differences between the different shell interpreters about this?

bash - What does <<< mean? - Unix & Linux Stack Exchange
it seems < is for passing file (or directory), << @ for passing multiple lines (similar to the banner command in cisco switches; as terminated by a custom string @ in this case), and <<< to pass a string (instead of file). test them yourself with cat and you'll grasp it very quickly.

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