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I'm trying to jump start my Linux (Centos) knowledge and trying to understand what is "default" Shell Functions listed by declare -F. No idea what these functions is called, I just label them as "default". I googled it, but what I got is about scripting. Is there article which explain what are they for, where (defined in which file), purpose and usage. Or someone can give a short write-up? From the look of it, they look like utilities.

For example; declare -f _have

    PATH=$PATH:/usr/sbin:/sbin:/usr/local/sbin type $1 &>/dev/null

What is the type $1 do?

BTW, I created one from the shell prompt function App1 (), then { ls }, when I check using declare -f App1 found ls had became ls --color=auto.

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type is a builtin command to bash, so is described in the manual. Your _have function searches your path and 3 extra directories to see if you have a program (or alias or function), and returns a true or false value.

Your "default" functions are declared in files read by the files read at startup. You might find How to find the file where a bash function is defined? helpful. They exist because someone who put Centos together thought them to be useful.

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Thank icarus. I found type is a command, but as there is no ; as a separator, I think it is something else. Just tested VAR=Apple ls and VAR=Apple ; ls , the result is the same, now I know ; is not required. But now I wonder is there a difference.

Found where is start from; /etc/profile.d/bash_completion.sh, then /usr/share/bash-completion/bash_completion.
googled and found this https://github.com/scop/bash-completion Thanks!

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    Instead of thanking icarus by posting a new answer; upvote and/or accept their answer. Note that VAR=Apple ls only sets VAR for the duration of ls; it's gone afterwards. Using VAR=Apple; ls sets VAR in the current shell (but doesn't export it). – Jeff Schaller Dec 7 '18 at 18:30
  • Thanks Jeff, but I can't upvote. I will check can I accept as answer. – Yew Hang Dec 10 '18 at 3:22
  • Thanks Jeff, but I can't upvote (no reputation). BTW, I tried this VAR5=GG echo ${VAR5}test and the output is test, then I tried VAR5=GG ; echo ${VAR5}test and the output is as expected GGtest. Still trying to figure out, shouldn't the first one also returned GGtest?. As far as I know bash_completion.sh provide a set of utilities, but I still wonder is it important/commonly used? Will disabling it break things? It look dated in the distro, so it kind of make me curious. Anyway, I need to get my fundamental knowledge up first. – Yew Hang Dec 10 '18 at 3:39

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