How variables inside braces are evaluated












5















I'm new to shell scripting and I came across these expressions



${var:-val} and ${var-val}



${var:+val} and ${var+val}



${var:=val} and ${var=val}



so how are they evaluated and what are the differences between them






bash shell scripting







share|improve this question




















  • 3





    IMO, not a duplicate. the linked answer only answers 1/3 of this question.

    – cas
    May 30 '16 at 3:06











  • What linked answer?

    – fpmurphy
    May 30 '16 at 9:25











  • @fpmurphy1 What does :- mean in a shell script.

    – Stephen Kitt
    May 30 '16 at 9:31
















5















I'm new to shell scripting and I came across these expressions



${var:-val} and ${var-val}



${var:+val} and ${var+val}



${var:=val} and ${var=val}



so how are they evaluated and what are the differences between them






bash shell scripting







share|improve this question




















  • 3





    IMO, not a duplicate. the linked answer only answers 1/3 of this question.

    – cas
    May 30 '16 at 3:06











  • What linked answer?

    – fpmurphy
    May 30 '16 at 9:25











  • @fpmurphy1 What does :- mean in a shell script.

    – Stephen Kitt
    May 30 '16 at 9:31














5












5








5


2






I'm new to shell scripting and I came across these expressions



${var:-val} and ${var-val}



${var:+val} and ${var+val}



${var:=val} and ${var=val}



so how are they evaluated and what are the differences between them






bash shell scripting







share|improve this question
















I'm new to shell scripting and I came across these expressions



${var:-val} and ${var-val}



${var:+val} and ${var+val}



${var:=val} and ${var=val}



so how are they evaluated and what are the differences between them






bash shell scripting




bash shell scripting






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited 44 mins ago









Rui F Ribeiro

40.1k1479136




40.1k1479136










asked May 29 '16 at 21:39









WLIONWLION

1553




1553








  • 3





    IMO, not a duplicate. the linked answer only answers 1/3 of this question.

    – cas
    May 30 '16 at 3:06











  • What linked answer?

    – fpmurphy
    May 30 '16 at 9:25











  • @fpmurphy1 What does :- mean in a shell script.

    – Stephen Kitt
    May 30 '16 at 9:31














  • 3





    IMO, not a duplicate. the linked answer only answers 1/3 of this question.

    – cas
    May 30 '16 at 3:06











  • What linked answer?

    – fpmurphy
    May 30 '16 at 9:25











  • @fpmurphy1 What does :- mean in a shell script.

    – Stephen Kitt
    May 30 '16 at 9:31








3




3





IMO, not a duplicate. the linked answer only answers 1/3 of this question.

– cas
May 30 '16 at 3:06





IMO, not a duplicate. the linked answer only answers 1/3 of this question.

– cas
May 30 '16 at 3:06













What linked answer?

– fpmurphy
May 30 '16 at 9:25





What linked answer?

– fpmurphy
May 30 '16 at 9:25













@fpmurphy1 What does :- mean in a shell script.

– Stephen Kitt
May 30 '16 at 9:31





@fpmurphy1 What does :- mean in a shell script.

– Stephen Kitt
May 30 '16 at 9:31










1 Answer
1






active

oldest

votes


















6














These are all various forms of parameter expansion with alternatives:





  • ${var:-val} is replaced by val if var is unset or null, ${var} otherwise (so val is a "default value");


  • ${var:=val} first assigns val to var if var is unset or null, and then (in all cases) is replaced by ${var};


  • ${var:+val} is replaced with nothing if var is unset or null, val otherwise.


Omitting the : drops the "or null" part of all these definitions.



This is all described in the bash(1) manpage.



Some examples might help:



unset a

echo "${a:-default}"


produces default, as does echo "${a-default}".



a=

echo "${a:-default}"


again produces default, but echo "${a-default}" outputs a blank line.



a=test

echo "${a:-default}"


produces test, as does echo "${a-default}".



unset a

echo "${a:=default}"


produces default, and a is now default (as confirmed by echo "${a}").



The + form might seem strange, but it is useful when constructing variables in several steps:



PATH="${PATH}${PATH:+:}/blah/bin"


will add : before /blah/bin only if PATH is non-empty, which avoids having a path starting with :.






share|improve this answer


























  • The :+ form is also useful for avoiding placing the empty string as an argument on a command line, while still properly double-quoting the variable. e.g. grep ${ignorecase:+"$ignorecase"} ... . The other, worse, alternative is to just use $ignorecase unquoted on the cmd line...which is only safe if you know exactly what $ignorecase can contain.

    – cas
    May 30 '16 at 12:47













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1 Answer
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1 Answer
1






active

oldest

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active

oldest

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active

oldest

votes









6














These are all various forms of parameter expansion with alternatives:





  • ${var:-val} is replaced by val if var is unset or null, ${var} otherwise (so val is a "default value");


  • ${var:=val} first assigns val to var if var is unset or null, and then (in all cases) is replaced by ${var};


  • ${var:+val} is replaced with nothing if var is unset or null, val otherwise.


Omitting the : drops the "or null" part of all these definitions.



This is all described in the bash(1) manpage.



Some examples might help:



unset a

echo "${a:-default}"


produces default, as does echo "${a-default}".



a=

echo "${a:-default}"


again produces default, but echo "${a-default}" outputs a blank line.



a=test

echo "${a:-default}"


produces test, as does echo "${a-default}".



unset a

echo "${a:=default}"


produces default, and a is now default (as confirmed by echo "${a}").



The + form might seem strange, but it is useful when constructing variables in several steps:



PATH="${PATH}${PATH:+:}/blah/bin"


will add : before /blah/bin only if PATH is non-empty, which avoids having a path starting with :.






share|improve this answer


























  • The :+ form is also useful for avoiding placing the empty string as an argument on a command line, while still properly double-quoting the variable. e.g. grep ${ignorecase:+"$ignorecase"} ... . The other, worse, alternative is to just use $ignorecase unquoted on the cmd line...which is only safe if you know exactly what $ignorecase can contain.

    – cas
    May 30 '16 at 12:47


















6














These are all various forms of parameter expansion with alternatives:





  • ${var:-val} is replaced by val if var is unset or null, ${var} otherwise (so val is a "default value");


  • ${var:=val} first assigns val to var if var is unset or null, and then (in all cases) is replaced by ${var};


  • ${var:+val} is replaced with nothing if var is unset or null, val otherwise.


Omitting the : drops the "or null" part of all these definitions.



This is all described in the bash(1) manpage.



Some examples might help:



unset a

echo "${a:-default}"


produces default, as does echo "${a-default}".



a=

echo "${a:-default}"


again produces default, but echo "${a-default}" outputs a blank line.



a=test

echo "${a:-default}"


produces test, as does echo "${a-default}".



unset a

echo "${a:=default}"


produces default, and a is now default (as confirmed by echo "${a}").



The + form might seem strange, but it is useful when constructing variables in several steps:



PATH="${PATH}${PATH:+:}/blah/bin"


will add : before /blah/bin only if PATH is non-empty, which avoids having a path starting with :.






share|improve this answer


























  • The :+ form is also useful for avoiding placing the empty string as an argument on a command line, while still properly double-quoting the variable. e.g. grep ${ignorecase:+"$ignorecase"} ... . The other, worse, alternative is to just use $ignorecase unquoted on the cmd line...which is only safe if you know exactly what $ignorecase can contain.

    – cas
    May 30 '16 at 12:47
















6












6








6







These are all various forms of parameter expansion with alternatives:





  • ${var:-val} is replaced by val if var is unset or null, ${var} otherwise (so val is a "default value");


  • ${var:=val} first assigns val to var if var is unset or null, and then (in all cases) is replaced by ${var};


  • ${var:+val} is replaced with nothing if var is unset or null, val otherwise.


Omitting the : drops the "or null" part of all these definitions.



This is all described in the bash(1) manpage.



Some examples might help:



unset a

echo "${a:-default}"


produces default, as does echo "${a-default}".



a=

echo "${a:-default}"


again produces default, but echo "${a-default}" outputs a blank line.



a=test

echo "${a:-default}"


produces test, as does echo "${a-default}".



unset a

echo "${a:=default}"


produces default, and a is now default (as confirmed by echo "${a}").



The + form might seem strange, but it is useful when constructing variables in several steps:



PATH="${PATH}${PATH:+:}/blah/bin"


will add : before /blah/bin only if PATH is non-empty, which avoids having a path starting with :.






share|improve this answer















These are all various forms of parameter expansion with alternatives:





  • ${var:-val} is replaced by val if var is unset or null, ${var} otherwise (so val is a "default value");


  • ${var:=val} first assigns val to var if var is unset or null, and then (in all cases) is replaced by ${var};


  • ${var:+val} is replaced with nothing if var is unset or null, val otherwise.


Omitting the : drops the "or null" part of all these definitions.



This is all described in the bash(1) manpage.



Some examples might help:



unset a

echo "${a:-default}"


produces default, as does echo "${a-default}".



a=

echo "${a:-default}"


again produces default, but echo "${a-default}" outputs a blank line.



a=test

echo "${a:-default}"


produces test, as does echo "${a-default}".



unset a

echo "${a:=default}"


produces default, and a is now default (as confirmed by echo "${a}").



The + form might seem strange, but it is useful when constructing variables in several steps:



PATH="${PATH}${PATH:+:}/blah/bin"


will add : before /blah/bin only if PATH is non-empty, which avoids having a path starting with :.







share|improve this answer














share|improve this answer



share|improve this answer








edited May 30 '16 at 9:33

























answered May 30 '16 at 9:26









Stephen KittStephen Kitt

171k24386462




171k24386462













  • The :+ form is also useful for avoiding placing the empty string as an argument on a command line, while still properly double-quoting the variable. e.g. grep ${ignorecase:+"$ignorecase"} ... . The other, worse, alternative is to just use $ignorecase unquoted on the cmd line...which is only safe if you know exactly what $ignorecase can contain.

    – cas
    May 30 '16 at 12:47





















  • The :+ form is also useful for avoiding placing the empty string as an argument on a command line, while still properly double-quoting the variable. e.g. grep ${ignorecase:+"$ignorecase"} ... . The other, worse, alternative is to just use $ignorecase unquoted on the cmd line...which is only safe if you know exactly what $ignorecase can contain.

    – cas
    May 30 '16 at 12:47



















The :+ form is also useful for avoiding placing the empty string as an argument on a command line, while still properly double-quoting the variable. e.g. grep ${ignorecase:+"$ignorecase"} ... . The other, worse, alternative is to just use $ignorecase unquoted on the cmd line...which is only safe if you know exactly what $ignorecase can contain.

– cas
May 30 '16 at 12:47







The :+ form is also useful for avoiding placing the empty string as an argument on a command line, while still properly double-quoting the variable. e.g. grep ${ignorecase:+"$ignorecase"} ... . The other, worse, alternative is to just use $ignorecase unquoted on the cmd line...which is only safe if you know exactly what $ignorecase can contain.

– cas
May 30 '16 at 12:47




















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