How variables inside braces are evaluated
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
edited Feb 12 at 5:25
Rui F Ribeiro
40.5k1479137
asked May 29 '16 at 21:39
WLIONWLION
1553
add a comment |
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
edited Feb 12 at 5:25
Rui F Ribeiro
40.5k1479137
asked May 29 '16 at 21:39
WLIONWLION
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
add a comment |
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
edited Feb 12 at 5:25
Rui F Ribeiro
40.5k1479137
asked May 29 '16 at 21:39
WLIONWLION
1553
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
edited Feb 12 at 5:25
Rui F Ribeiro
40.5k1479137
asked May 29 '16 at 21:39
WLIONWLION
1553
edited Feb 12 at 5:25
Rui F Ribeiro
40.5k1479137
asked May 29 '16 at 21:39
WLIONWLION
1553
edited Feb 12 at 5:25
Rui F Ribeiro
40.5k1479137
edited Feb 12 at 5:25
Rui F Ribeiro
40.5k1479137
edited Feb 12 at 5:25
Rui F Ribeiro
40.5k1479137
40.5k1479137
asked May 29 '16 at 21:39
WLIONWLION
1553
asked May 29 '16 at 21:39
WLIONWLION
1553
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
add a comment |
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
add a comment |
1 Answer
1
active
oldest
votes
These are all various forms of parameter expansion with alternatives:
${var:-val}is replaced byvalifvaris unset or null,${var}otherwise (sovalis a "default value");
${var:=val}first assignsvaltovarifvaris unset or null, and then (in all cases) is replaced by${var};
${var:+val}is replaced with nothing ifvaris unset or null,valotherwise.
Omitting the : drops the "or null" part of all these definitions.
This is all described in the bash(1) manpage, and in POSIX.
Some examples might help:
unset a
echo "${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 :.
answered May 30 '16 at 9:26
Stephen KittStephen Kitt
173k24396471
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$ignorecaseunquoted on the cmd line...which is only safe if you know exactly what$ignorecasecan contain.
– cas
May 30 '16 at 12:47
Also, POSIX Parameter Expansions: pubs.opengroup.org/onlinepubs/9699919799.2018edition/utilities/…
– Kusalananda
Feb 12 at 7:03
add a comment |
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These are all various forms of parameter expansion with alternatives:
${var:-val}is replaced byvalifvaris unset or null,${var}otherwise (sovalis a "default value");
${var:=val}first assignsvaltovarifvaris unset or null, and then (in all cases) is replaced by${var};
${var:+val}is replaced with nothing ifvaris unset or null,valotherwise.
Omitting the : drops the "or null" part of all these definitions.
This is all described in the bash(1) manpage, and in POSIX.
Some examples might help:
unset a
echo "${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 :.
answered May 30 '16 at 9:26
Stephen KittStephen Kitt
173k24396471
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$ignorecaseunquoted on the cmd line...which is only safe if you know exactly what$ignorecasecan contain.
– cas
May 30 '16 at 12:47
Also, POSIX Parameter Expansions: pubs.opengroup.org/onlinepubs/9699919799.2018edition/utilities/…
– Kusalananda
Feb 12 at 7:03
add a comment |
These are all various forms of parameter expansion with alternatives:
${var:-val}is replaced byvalifvaris unset or null,${var}otherwise (sovalis a "default value");
${var:=val}first assignsvaltovarifvaris unset or null, and then (in all cases) is replaced by${var};
${var:+val}is replaced with nothing ifvaris unset or null,valotherwise.
Omitting the : drops the "or null" part of all these definitions.
This is all described in the bash(1) manpage, and in POSIX.
Some examples might help:
unset a
echo "${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 :.
answered May 30 '16 at 9:26
Stephen KittStephen Kitt
173k24396471
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$ignorecaseunquoted on the cmd line...which is only safe if you know exactly what$ignorecasecan contain.
– cas
May 30 '16 at 12:47
Also, POSIX Parameter Expansions: pubs.opengroup.org/onlinepubs/9699919799.2018edition/utilities/…
– Kusalananda
Feb 12 at 7:03
add a comment |
These are all various forms of parameter expansion with alternatives:
${var:-val}is replaced byvalifvaris unset or null,${var}otherwise (sovalis a "default value");
${var:=val}first assignsvaltovarifvaris unset or null, and then (in all cases) is replaced by${var};
${var:+val}is replaced with nothing ifvaris unset or null,valotherwise.
Omitting the : drops the "or null" part of all these definitions.
This is all described in the bash(1) manpage, and in POSIX.
Some examples might help:
unset a
echo "${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 :.
answered May 30 '16 at 9:26
Stephen KittStephen Kitt
173k24396471
These are all various forms of parameter expansion with alternatives:
${var:-val}is replaced byvalifvaris unset or null,${var}otherwise (sovalis a "default value");
${var:=val}first assignsvaltovarifvaris unset or null, and then (in all cases) is replaced by${var};
${var:+val}is replaced with nothing ifvaris unset or null,valotherwise.
Omitting the : drops the "or null" part of all these definitions.
This is all described in the bash(1) manpage, and in POSIX.
Some examples might help:
unset a
echo "${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 :.
answered May 30 '16 at 9:26
Stephen KittStephen Kitt
173k24396471
edited Feb 12 at 8:16
answered May 30 '16 at 9:26
Stephen KittStephen Kitt
173k24396471
answered May 30 '16 at 9:26
Stephen KittStephen Kitt
173k24396471
answered May 30 '16 at 9:26
Stephen KittStephen Kitt
173k24396471
173k24396471
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$ignorecaseunquoted on the cmd line...which is only safe if you know exactly what$ignorecasecan contain.
– cas
May 30 '16 at 12:47
Also, POSIX Parameter Expansions: pubs.opengroup.org/onlinepubs/9699919799.2018edition/utilities/…
– Kusalananda
Feb 12 at 7:03
add a comment |
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$ignorecaseunquoted on the cmd line...which is only safe if you know exactly what$ignorecasecan contain.
– cas
May 30 '16 at 12:47
Also, POSIX Parameter Expansions: pubs.opengroup.org/onlinepubs/9699919799.2018edition/utilities/…
– Kusalananda
Feb 12 at 7:03
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
Also, POSIX Parameter Expansions: pubs.opengroup.org/onlinepubs/9699919799.2018edition/utilities/…
– Kusalananda
Feb 12 at 7:03
Also, POSIX Parameter Expansions: pubs.opengroup.org/onlinepubs/9699919799.2018edition/utilities/…
– Kusalananda
Feb 12 at 7:03
add a comment |
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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