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how does the expression b=$b:-“/data” work [duplicate]

What does `:-` mean in a shell scriptHow/why does this globbing expression work?What does : $param:=value mean?How does bash know how it is being invoked?Extract data from line and prepend to the line?What does this regex mean?Why does n expand to n in bash?How does storing the regular expression in a shell variable avoid problems with quoting characters that are special to the shell?Is it secure to use the following kinds of pathnames in `$PATH`?$@ used in a loop for mathMixing if ..; with command-grouping and [[ test ]] and logical operators

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0

This question already has an answer here:

• 
  What does `:-` mean in a shell script [duplicate]
  
  2 answers
  
  

I've run a cross a shell script with BUILDDIR=$BUILDDIR:-"/data" which , upon experimentation, takes the original BUILDDIR if it exists and isnt an empty string, and otherwise sets it to /data.
What I don't understand is how the expression works - why the : and - operators and how they work.

bash environment-variables

share|improve this question

edited Apr 2 at 15:48

Rui F Ribeiro

42k1483142

asked Apr 2 at 15:34

jeremy_rutmanjeremy_rutman

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marked as duplicate by Kusalananda♦ bash
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This question already has an answer here:

• 
  What does `:-` mean in a shell script [duplicate]
  
  2 answers
  
  

I've run a cross a shell script with BUILDDIR=$BUILDDIR:-"/data" which , upon experimentation, takes the original BUILDDIR if it exists and isnt an empty string, and otherwise sets it to /data.
What I don't understand is how the expression works - why the : and - operators and how they work.

bash environment-variables

share|improve this question

edited Apr 2 at 15:48

Rui F Ribeiro

42k1483142

asked Apr 2 at 15:34

jeremy_rutmanjeremy_rutman

159312

marked as duplicate by Kusalananda♦ bash
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Apr 2 at 16:20

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

add a comment | 

This question already has an answer here:

• 
  What does `:-` mean in a shell script [duplicate]
  
  2 answers
  
  

I've run a cross a shell script with BUILDDIR=$BUILDDIR:-"/data" which , upon experimentation, takes the original BUILDDIR if it exists and isnt an empty string, and otherwise sets it to /data.
What I don't understand is how the expression works - why the : and - operators and how they work.

bash environment-variables

share|improve this question

edited Apr 2 at 15:48

Rui F Ribeiro

42k1483142

asked Apr 2 at 15:34

jeremy_rutmanjeremy_rutman

159312

share|improve this question

edited Apr 2 at 15:48

Rui F Ribeiro

42k1483142

asked Apr 2 at 15:34

jeremy_rutmanjeremy_rutman

159312

share|improve this question

edited Apr 2 at 15:48

Rui F Ribeiro

42k1483142

asked Apr 2 at 15:34

jeremy_rutmanjeremy_rutman

159312

edited Apr 2 at 15:48

Rui F Ribeiro

42k1483142

edited Apr 2 at 15:48

Rui F Ribeiro

42k1483142

asked Apr 2 at 15:34

jeremy_rutmanjeremy_rutman

159312

asked Apr 2 at 15:34

jeremy_rutmanjeremy_rutman

159312

1 Answer
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1

It's one of (fortunately only) a handful of shortcuts done as part of parameter expansion. In short, there's the following, loosely defined:

• 
  $VAR:-value Use $VAR if possible, else value
  


• 
  $VAR:=value Use $VAR if possible, else set $VAR to value and use value
  


• 
  $VAR:?value (exit if $VAR is undefined) and $VAR:+value (opposite of :- ) exist, but I've never seen them in the wild.


• 
  $VAR:offset and $VAR:offset:length take substrings of $VAR.

(There's also a bunch of others that remove prefixes or suffixes or do general substitution; please see the bash info page linked above for those.)

share|improve this answer

answered Apr 2 at 15:56

Ulrich SchwarzUlrich Schwarz

10.1k13148

add a comment | 

1

It's one of (fortunately only) a handful of shortcuts done as part of parameter expansion. In short, there's the following, loosely defined:

• 
  $VAR:-value Use $VAR if possible, else value
  


• 
  $VAR:=value Use $VAR if possible, else set $VAR to value and use value
  


• 
  $VAR:?value (exit if $VAR is undefined) and $VAR:+value (opposite of :- ) exist, but I've never seen them in the wild.


• 
  $VAR:offset and $VAR:offset:length take substrings of $VAR.

(There's also a bunch of others that remove prefixes or suffixes or do general substitution; please see the bash info page linked above for those.)

share|improve this answer

answered Apr 2 at 15:56

Ulrich SchwarzUlrich Schwarz

10.1k13148

add a comment | 

1

It's one of (fortunately only) a handful of shortcuts done as part of parameter expansion. In short, there's the following, loosely defined:

• 
  $VAR:-value Use $VAR if possible, else value
  


• 
  $VAR:=value Use $VAR if possible, else set $VAR to value and use value
  


• 
  $VAR:?value (exit if $VAR is undefined) and $VAR:+value (opposite of :- ) exist, but I've never seen them in the wild.


• 
  $VAR:offset and $VAR:offset:length take substrings of $VAR.

(There's also a bunch of others that remove prefixes or suffixes or do general substitution; please see the bash info page linked above for those.)

share|improve this answer

answered Apr 2 at 15:56

Ulrich SchwarzUlrich Schwarz

10.1k13148

add a comment | 

It's one of (fortunately only) a handful of shortcuts done as part of parameter expansion. In short, there's the following, loosely defined:

• 
  $VAR:-value Use $VAR if possible, else value
  


• 
  $VAR:=value Use $VAR if possible, else set $VAR to value and use value
  


• 
  $VAR:?value (exit if $VAR is undefined) and $VAR:+value (opposite of :- ) exist, but I've never seen them in the wild.


• 
  $VAR:offset and $VAR:offset:length take substrings of $VAR.

(There's also a bunch of others that remove prefixes or suffixes or do general substitution; please see the bash info page linked above for those.)

share|improve this answer

answered Apr 2 at 15:56

Ulrich SchwarzUlrich Schwarz

10.1k13148

share|improve this answer

answered Apr 2 at 15:56

Ulrich SchwarzUlrich Schwarz

10.1k13148

answered Apr 2 at 15:56

Ulrich SchwarzUlrich Schwarz

10.1k13148

answered Apr 2 at 15:56

Ulrich SchwarzUlrich Schwarz

10.1k13148