What does the substitution ${!var_name+x} mean?
I found a script that has a function that checks if a variable is set but i don't understand it very well.
check_if_variable_is_set() {
var_name=$1
if [ -z "${!var_name+x}" ]; then
false
else
true
fi
}
What exactly happens with this substitution ?
bash shell-script variable-substitution
asked 11 hours ago
Karim ManaouilKarim Manaouil
1919
add a comment |
I found a script that has a function that checks if a variable is set but i don't understand it very well.
check_if_variable_is_set() {
var_name=$1
if [ -z "${!var_name+x}" ]; then
false
else
true
fi
}
What exactly happens with this substitution ?
bash shell-script variable-substitution
asked 11 hours ago
Karim ManaouilKarim Manaouil
1919
related ${!FOO} and zsh.
– αғsнιη
10 hours ago
add a comment |
I found a script that has a function that checks if a variable is set but i don't understand it very well.
check_if_variable_is_set() {
var_name=$1
if [ -z "${!var_name+x}" ]; then
false
else
true
fi
}
What exactly happens with this substitution ?
bash shell-script variable-substitution
asked 11 hours ago
Karim ManaouilKarim Manaouil
1919
I found a script that has a function that checks if a variable is set but i don't understand it very well.
check_if_variable_is_set() {
var_name=$1
if [ -z "${!var_name+x}" ]; then
false
else
true
fi
}
What exactly happens with this substitution ?
bash shell-script variable-substitution
bash shell-script variable-substitution
asked 11 hours ago
Karim ManaouilKarim Manaouil
1919
asked 11 hours ago
Karim ManaouilKarim Manaouil
1919
asked 11 hours ago
Karim ManaouilKarim Manaouil
1919
asked 11 hours ago
Karim ManaouilKarim Manaouil
1919
asked 11 hours ago
Karim ManaouilKarim Manaouil
1919
1919
related ${!FOO} and zsh.
– αғsнιη
10 hours ago
add a comment |
related ${!FOO} and zsh.
– αғsнιη
10 hours ago
related ${!FOO} and zsh.
– αғsнιη
10 hours ago
related ${!FOO} and zsh.
– αғsнιη
10 hours ago
add a comment |
1 Answer
1
active
oldest
votes
In the bash shell, ${!var} is a variable indirection. It expands to the value of the variable whose name is kept in $var.
The variable expansion ${var+value} is a POSIX expansion that expands to value if the variable var is set (no matter if its value is empty or not).
Combining these, ${!var+x} would expand to x if the variable whose name is kept in $var is set.
Example:
$ foo=hello
$ var=foo
$ echo "${!var+$var is set, its value is ${!var}}"
foo is set, its value is hello
$ unset foo
$ echo "${!var+$var is set, its value is ${!var}}"
(empty line as output)
The function in the question could be shortened to
check_if_variable_is_set () { [ -n "${!1+x}" ]; }
or even:
check_if_variable_is_set () { [ -v "$1" ]; }
or even:
check_if_variable_is_set()[[ -v $1 ]]
Where -v is a bash test on a variable name which will be true if the named variable is set, and false otherwise.
POSIXly, it could be written:
check_if_variable_is_set() { eval '[ -n "${'"$1"'+x}" ]'; }
Note that all those are potential command injection vulnerabilities if the argument to that function could end up being under control of an attacker. Try for instance with check_if_variable_is_set 'a[$(id>&2)]'.
To guard against that, you may want to verify that the argument is a valid variable name first. For variables:
check_if_variable_is_set() {
case $1 in
("" | *[![:alnum:]_]* | [![:alpha:]_]*) false;;
(*) eval '[ -n "${'"$1"'+x}" ]'
esac
}
(note that [[:alpha:]] will check for alphabetical characters in your locale while some shells only accept alphabetical characters from the portable character set in their variable)
edited 9 hours ago
Stéphane Chazelas
306k57577931
answered 11 hours ago
KusalanandaKusalananda
131k17249408
There is nothing on earth more complete than this. You deserve a cookie for that.
– Karim Manaouil
8 hours ago
@KarimManaouil A third of that cookie goes to Stéphane though :-)
– Kusalananda
8 hours ago
add a comment |
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
In the bash shell, ${!var} is a variable indirection. It expands to the value of the variable whose name is kept in $var.
The variable expansion ${var+value} is a POSIX expansion that expands to value if the variable var is set (no matter if its value is empty or not).
Combining these, ${!var+x} would expand to x if the variable whose name is kept in $var is set.
Example:
$ foo=hello
$ var=foo
$ echo "${!var+$var is set, its value is ${!var}}"
foo is set, its value is hello
$ unset foo
$ echo "${!var+$var is set, its value is ${!var}}"
(empty line as output)
The function in the question could be shortened to
check_if_variable_is_set () { [ -n "${!1+x}" ]; }
or even:
check_if_variable_is_set () { [ -v "$1" ]; }
or even:
check_if_variable_is_set()[[ -v $1 ]]
Where -v is a bash test on a variable name which will be true if the named variable is set, and false otherwise.
POSIXly, it could be written:
check_if_variable_is_set() { eval '[ -n "${'"$1"'+x}" ]'; }
Note that all those are potential command injection vulnerabilities if the argument to that function could end up being under control of an attacker. Try for instance with check_if_variable_is_set 'a[$(id>&2)]'.
To guard against that, you may want to verify that the argument is a valid variable name first. For variables:
check_if_variable_is_set() {
case $1 in
("" | *[![:alnum:]_]* | [![:alpha:]_]*) false;;
(*) eval '[ -n "${'"$1"'+x}" ]'
esac
}
(note that [[:alpha:]] will check for alphabetical characters in your locale while some shells only accept alphabetical characters from the portable character set in their variable)
edited 9 hours ago
Stéphane Chazelas
306k57577931
answered 11 hours ago
KusalanandaKusalananda
131k17249408
There is nothing on earth more complete than this. You deserve a cookie for that.
– Karim Manaouil
8 hours ago
@KarimManaouil A third of that cookie goes to Stéphane though :-)
– Kusalananda
8 hours ago
add a comment |
In the bash shell, ${!var} is a variable indirection. It expands to the value of the variable whose name is kept in $var.
The variable expansion ${var+value} is a POSIX expansion that expands to value if the variable var is set (no matter if its value is empty or not).
Combining these, ${!var+x} would expand to x if the variable whose name is kept in $var is set.
Example:
$ foo=hello
$ var=foo
$ echo "${!var+$var is set, its value is ${!var}}"
foo is set, its value is hello
$ unset foo
$ echo "${!var+$var is set, its value is ${!var}}"
(empty line as output)
The function in the question could be shortened to
check_if_variable_is_set () { [ -n "${!1+x}" ]; }
or even:
check_if_variable_is_set () { [ -v "$1" ]; }
or even:
check_if_variable_is_set()[[ -v $1 ]]
Where -v is a bash test on a variable name which will be true if the named variable is set, and false otherwise.
POSIXly, it could be written:
check_if_variable_is_set() { eval '[ -n "${'"$1"'+x}" ]'; }
Note that all those are potential command injection vulnerabilities if the argument to that function could end up being under control of an attacker. Try for instance with check_if_variable_is_set 'a[$(id>&2)]'.
To guard against that, you may want to verify that the argument is a valid variable name first. For variables:
check_if_variable_is_set() {
case $1 in
("" | *[![:alnum:]_]* | [![:alpha:]_]*) false;;
(*) eval '[ -n "${'"$1"'+x}" ]'
esac
}
(note that [[:alpha:]] will check for alphabetical characters in your locale while some shells only accept alphabetical characters from the portable character set in their variable)
edited 9 hours ago
Stéphane Chazelas
306k57577931
answered 11 hours ago
KusalanandaKusalananda
131k17249408
There is nothing on earth more complete than this. You deserve a cookie for that.
– Karim Manaouil
8 hours ago
@KarimManaouil A third of that cookie goes to Stéphane though :-)
– Kusalananda
8 hours ago
add a comment |
In the bash shell, ${!var} is a variable indirection. It expands to the value of the variable whose name is kept in $var.
The variable expansion ${var+value} is a POSIX expansion that expands to value if the variable var is set (no matter if its value is empty or not).
Combining these, ${!var+x} would expand to x if the variable whose name is kept in $var is set.
Example:
$ foo=hello
$ var=foo
$ echo "${!var+$var is set, its value is ${!var}}"
foo is set, its value is hello
$ unset foo
$ echo "${!var+$var is set, its value is ${!var}}"
(empty line as output)
The function in the question could be shortened to
check_if_variable_is_set () { [ -n "${!1+x}" ]; }
or even:
check_if_variable_is_set () { [ -v "$1" ]; }
or even:
check_if_variable_is_set()[[ -v $1 ]]
Where -v is a bash test on a variable name which will be true if the named variable is set, and false otherwise.
POSIXly, it could be written:
check_if_variable_is_set() { eval '[ -n "${'"$1"'+x}" ]'; }
Note that all those are potential command injection vulnerabilities if the argument to that function could end up being under control of an attacker. Try for instance with check_if_variable_is_set 'a[$(id>&2)]'.
To guard against that, you may want to verify that the argument is a valid variable name first. For variables:
check_if_variable_is_set() {
case $1 in
("" | *[![:alnum:]_]* | [![:alpha:]_]*) false;;
(*) eval '[ -n "${'"$1"'+x}" ]'
esac
}
(note that [[:alpha:]] will check for alphabetical characters in your locale while some shells only accept alphabetical characters from the portable character set in their variable)
edited 9 hours ago
Stéphane Chazelas
306k57577931
answered 11 hours ago
KusalanandaKusalananda
131k17249408
In the bash shell, ${!var} is a variable indirection. It expands to the value of the variable whose name is kept in $var.
The variable expansion ${var+value} is a POSIX expansion that expands to value if the variable var is set (no matter if its value is empty or not).
Combining these, ${!var+x} would expand to x if the variable whose name is kept in $var is set.
Example:
$ foo=hello
$ var=foo
$ echo "${!var+$var is set, its value is ${!var}}"
foo is set, its value is hello
$ unset foo
$ echo "${!var+$var is set, its value is ${!var}}"
(empty line as output)
The function in the question could be shortened to
check_if_variable_is_set () { [ -n "${!1+x}" ]; }
or even:
check_if_variable_is_set () { [ -v "$1" ]; }
or even:
check_if_variable_is_set()[[ -v $1 ]]
Where -v is a bash test on a variable name which will be true if the named variable is set, and false otherwise.
POSIXly, it could be written:
check_if_variable_is_set() { eval '[ -n "${'"$1"'+x}" ]'; }
Note that all those are potential command injection vulnerabilities if the argument to that function could end up being under control of an attacker. Try for instance with check_if_variable_is_set 'a[$(id>&2)]'.
To guard against that, you may want to verify that the argument is a valid variable name first. For variables:
check_if_variable_is_set() {
case $1 in
("" | *[![:alnum:]_]* | [![:alpha:]_]*) false;;
(*) eval '[ -n "${'"$1"'+x}" ]'
esac
}
(note that [[:alpha:]] will check for alphabetical characters in your locale while some shells only accept alphabetical characters from the portable character set in their variable)
edited 9 hours ago
Stéphane Chazelas
306k57577931
answered 11 hours ago
KusalanandaKusalananda
131k17249408
edited 9 hours ago
Stéphane Chazelas
306k57577931
edited 9 hours ago
Stéphane Chazelas
306k57577931
edited 9 hours ago
Stéphane Chazelas
306k57577931
306k57577931
answered 11 hours ago
KusalanandaKusalananda
131k17249408
answered 11 hours ago
KusalanandaKusalananda
131k17249408
answered 11 hours ago
KusalanandaKusalananda
131k17249408
131k17249408
There is nothing on earth more complete than this. You deserve a cookie for that.
– Karim Manaouil
8 hours ago
@KarimManaouil A third of that cookie goes to Stéphane though :-)
– Kusalananda
8 hours ago
add a comment |
There is nothing on earth more complete than this. You deserve a cookie for that.
– Karim Manaouil
8 hours ago
@KarimManaouil A third of that cookie goes to Stéphane though :-)
– Kusalananda
8 hours ago
There is nothing on earth more complete than this. You deserve a cookie for that.
– Karim Manaouil
8 hours ago
There is nothing on earth more complete than this. You deserve a cookie for that.
– Karim Manaouil
8 hours ago
@KarimManaouil A third of that cookie goes to Stéphane though :-)
– Kusalananda
8 hours ago
@KarimManaouil A third of that cookie goes to Stéphane though :-)
– Kusalananda
8 hours ago
add a comment |
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related ${!FOO} and zsh.
– αғsнιη
10 hours ago