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In my Bash environment I use variables containing spaces, and I use these variables within command substitution. Unfortunately I cannot find the answer on SE.

What is the correct way to quote my variables? And how should I do it if these are nested?

DIRNAME=$(dirname "$FILE")

or do I quote outside the substitution?

DIRNAME="$(dirname $FILE)"

or both?

DIRNAME="$(dirname "$FILE")"

or do I use back-ticks?

DIRNAME=`dirname "$FILE"`

What is the right way to do this? And how can I easily check if the quotes are set right?

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See also - unix.stackexchange.com/questions/97560/… –  Graeme Mar 6 '14 at 14:48
    
    
This is a good question, but given all the issues with embedded blanks, why would you make life hard on yourself by using them on purpose? –  Joe Mar 8 '14 at 4:03
1  
@Joe, with embedded blanks you mean space in the filenames? Personally I do not use them that often, but I am working with other peoples directories and files of which I am not certain if they contain spaces. Furthermore, I think it is better to get it right at once so I do not have to worry in the future. –  CousinCocaine Mar 8 '14 at 11:18
2  
Yes. What are we going to do with those "other people"? <G> –  Joe Mar 9 '14 at 16:24

3 Answers 3

up vote 44 down vote accepted

In order from worst to best:

  • DIRNAME="$(dirname $FILE)" will not do what you want if $FILE contains whitespace or globbing characters \[?*.
  • DIRNAME=`dirname "$FILE"` is technically correct, but backticks are not recommended for command expansion because of the extra complexity when nesting them.
  • DIRNAME=$(dirname "$FILE") is correct, but only because this is an assignment. If you use the command substitution in any other context, such as export DIRNAME=$(dirname "$FILE") or du $(dirname "$FILE"), the lack of quotes will cause trouble if the result of the expansion contain whitespace or globbing characters.
  • DIRNAME="$(dirname "$FILE")" is the recommended way. You can replace DIRNAME= with a command and a space without changing anything else, and dirname receives the correct string.

To improve even further:

  • DIRNAME="$(dirname -- "$FILE")" works if $FILE starts with a dash.
  • DIRNAME="$(dirname -- "$FILE"; printf x)" && DIRNAME="${DIRNAME%?x}" works even if $FILE ends with a newline, since $() chops off newlines at the end of output and dirname outputs a newline after the result. Sheesh dirname, why you gotta be different?

You can nest command expansions as much as you like. With $() you always create a new quoting context, so you can do things like this:

foo "$(bar "$(baz "$(ban "bla")")")"

You do not want to try that with backticks.

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2  
Clear answer to my question. When I nest these variables, can I just keep on quoting like I do now? –  CousinCocaine Mar 6 '14 at 15:07
    
Is there a reference/resource detailing the behavior of quotes within a command substition within quotes? –  AmadeusDrZaius Mar 4 at 18:43
1  
@AmadeusDrZaius "With $() you always create a new quoting context", so it's just like outside the outer quotes. There's nothing more to it, as far as I know. –  l0b0 Mar 4 at 23:05
    
@l0b0 Thanks, yeah, I found your explanation very clear. I was just wondering whether it was in a manual somewhere as well. I did find it (albeit unofficially) at wooledge. I guess if you read about order of substitution carefully, you could derive this fact as a result. –  AmadeusDrZaius Mar 4 at 23:13

You can always show the effects of variable quoting with printf.

Word splitting done on var1:

$ var1="hello     world"
$ printf '[%s]\n' $var1
[hello]
[world]

var1 quoted, so no word splitting:

$ printf '[%s]\n' "$var1"
[hello     world]

Word splitting on var1 inside $(), equivalent to echo "hello" "world":

$ var2=$(echo $var1)
$ printf '[%s]\n' "$var2"
[hello world]

No word splitting on var1, no problem with not quoting the $():

$ var2=$(echo "$var1")
$ printf '[%s]\n' "$var2"
[hello     world]

Word splitting on var1 again:

$ var2="$(echo $var1)"
$ printf '[%s]\n' "$var2"
[hello world]

Quoting both, easiest way to be sure.

$ var2="$(echo "$var1")"
$ printf '[%s]\n' "$var2"
[hello     world]

Globbing problem

Not quoting a variable can also lead to glob expansion of its contents:

$ mkdir test; cd test; touch file1 file2
$ var="*"
$ printf '[%s]\n' $var
[file1]
[file2]
$ printf '[%s]\n' "$var"
[*]

Note this happens after the variable is expanded only. It is not necessary to quote a glob during assignment:

$ var=*
$ printf '[%s]\n' $var
[file1]
[file2]
$ printf '[%s]\n' "$var"
[*]

Use set -f to disable this behaviour:

$ set -f
$ var=*
$ printf '[%s]\n' $var
[*]

And set +f to re-enable it:

$ set +f
$ printf '[%s]\n' $var
[file1]
[file2]
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2  
People tend to forget that word splitting is not the only problem, you may want to change your example to have var1='hello * world' to illustrate the globbing problem as well. –  Stéphane Chazelas Mar 6 '14 at 15:15

...aaand apparently rep doesn't transfer from one StackExchange site to another. Oh well, here's the comment I would have liked to have added to the accepted answer:

While I generally agree with @l0b0's answer here, I suspect the placement of bare backticks in the "worst to best" list is at least partly a result of the assumption that $(...) is available everywhere. I realize that the question specifies Bash, but there are plenty of times when Bash turns out to mean /bin/sh, which may not always actually be the full Bourne Again shell.

In particular, the plain Bourne shell won't know what to do with $(...), so scripts which claim to be compatible with it (e.g., via a #!/bin/sh shebang line) will likely misbehave if they are actually run by the "real" /bin/sh – this is of special interest when, say, producing init scripts, or packaging pre- and post-scripts, and can land one in a surprising place during installation of a base system.

If any of that sounds like something you're planning to do with this variable, nesting is probably less of a concern than having the script actually, predictably run. When it's a simple enough case and portability is a concern, even if I expect the script to usually run on systems where /bin/sh is Bash, I often tend to use backticks for this reason, with multiple assignments instead of nesting.

Having said all that, the ubiquity of shells which implement $(...) (Bash, Dash, et al.), leaves us in a good spot to stick with the prettier, easier-to-nest, and more recently preferred POSIX syntax in most cases, for all the reasons @l0b0 mentions.

Aside: this has shown up occasionally on StackOverflow, too –

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