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I am having difficulties figuring out some the nuances between single and double quotes within a variable context

I define:


then run these:

echo $'$foo'

Which echos $foo (meaning the first $ in my echo command is dropped)

echo $"$foo"

This echos pwd (which means the bash expands $foo, my variable, to its value)

echo $`$foo`

Finally this echos $~/scripts (I expected it to print ~/scripts and not $~/scripts)

can somebody help me figure out these differences?

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Ultimately, the last one prints $~/scripts because $`foo` is a combination of a literal $ followed by a command substitution; so the leading $ is interpreted as a literal $ and the trailing `$foo` as a command substitution.

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I removed the code tags and replaces with just ```````` characters etc. Feel free to rollback if you prefer the code tags – Tim Feb 18 at 12:53
@Tim Thanks, not sure what I did exactly but for some reason after your edited the post appeared just as disrupted as in my first revision, so I rolled it back... And now I see it looks ok in the revision. Not sure what happened, gonna test this in the formatting sandbox before rolling back to your version since I fear I might disrupt the post again. Regardless, thanks for the effort. – kos Feb 18 at 13:08
sure, no issues. Mine uses zero-width spaces, so be careful as you edit. – Tim Feb 18 at 13:09

First case:

echo $'$foo'

Which echos $foo (meaning the first $ in my echo command is dropped)

The first $ is run as a command. Echo doesn't require spaces between arguments, it just takes them.

So echo $'$foo' expands to run the command $ (which isn't a command so it ignores it) and then echo '$foo'. Anything in '' is taken at face value - it doesn't expand it or anything. You can put what you want in '' and nothing happens.

If you replace the $ with $$ it echos the process ID.


echo $"$foo"

This echos pwd (which means the bash expands $foo, my variable, to its value)

That's because "" doesn't prevent it expanding. "" is normally used to ensure there is a value to compare to.

If you have a variable that could be empty (e.g. it is from a grep result) then you want to compare to another value / variable, you have to put the first variable in "". This ensures that there is always a comparison to check, rather than just an empty section of code (as bash sees) which gives an error.

Finally, number 3. This is interesting.

echo $`$foo`

This echos $~/scripts (I expected it to print ~/scripts and not $~/scripts)

That's because (in this case) echo is printing the $ character first. Putting something in backticks causes the value of the variable to be run as a command (the better way to do it in more recent versions of bash is $(command))

So first it echos $ then it expands ​`$foo`​ to ​`pwd`​ which is run and returns ~/Scripts. But we had the $ print earlier - so the whole output is $~/Scripts.

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Nice work on some very difficult formatting! – Arronical Feb 18 at 12:42
@Arronical Thanks :) You can't see it but there are some zero-width spaces there for the last sentence. The things I do for imaginary internet points...! – Tim Feb 18 at 12:43
+1 Good answer. I would just add that quoting variables should be the default unless you have a specific reason not to - not just to handle possible null values. See… for more than you ever wanted to know about this. ;) – Joe Feb 25 at 8:47

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