How do you shell expand your variables and why?
How do you shell expand your variables and why?
29 comments
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find “$(echo $HOME > variable_holder.txt && cat variable_holder.txt)/$(cat alphabet.txt | grep “d”) $(cat alphabet.txt | grep “o”)$(cat alphabet.txt | grep “c”)$(cat alphabet.txt | grep “s”)”
This is the easiest method
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no
eval? -
when you're paid by character written
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This really enterprises my bash.
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@ilegadh You don't need
catin cases whengrep "d" alphabet.txtcan read from file too. Edit: But obviously your comment was more of a joke to over complicate it. So never mind then. -
What should I search to better understand what is written here? Don't mind learning myself, just looking for the correct keywords. Thanks!
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Read the Bash manual. That one patter on the GP is called "Command Substitution", you can search for it.
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This comment is a joke and you wouldn't want to do it like that in reality, but here are some related keywords you could look up: "Unix cat", "Unix pipeline", "grep", "output redirection", "command substitution".
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ExplainShell should help
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First one, then the other, then I forget the quotes, then I put them in single quotes by accident, then I utilize that "default value" colon syntax in case I'm missing HOME , then I just stick to ~ for the rest of the file.
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Typically
find "$HOME/docs", but with a few caveats:- In Zsh or Fish, the quotes are unnecessary:
find $HOME/docs - If I'm using anything potentially destructive:
mv "${HOME:?}/bin" ... - Of course, if it's followed by a valid identifier character, I'll add braces:
"${basename}_$num.txt"
- In Zsh or Fish, the quotes are unnecessary:
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I do what the linter tells me to: https://github.com/koalaman/shellcheck
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29 comments
@zephyr
echo "${HOME}/docs"This is the best way. It's also the way the Shellcheck
wantssometimes recommends.@bloopernova As you mention it, here the links for anyone interested: Online tool https://www.shellcheck.net/ and you can install it locally too https://github.com/koalaman/shellcheck .
This isn't true. Shellcheck doesn't insist on braces unless it thinks you need them.
This is the way
I also do this so the variables are more easily spotted.
This is the right way
This has never stuck with me, and I hadn't thought about why until now. I have two reasons why I will always write
${x}_$y.zinstead of${x}_${y}.z:$x_being expanded as${x_}."$#array[3]"actually prints the length of the third item inarray, rather than (Bash:) the number of positional parameters, then the string'array[3]'.@gamma I just use them out of consistency and principle, so I don't need to think in which case it is required or not.
The difference between the two seems different to what's in the OP. Is there a typo here?