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Indirectly access environment variable

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2

Given I have in a bash script

ev=USER

How can I get the environment variable value for $USER using ev?

Tried naively doing:

echo ${"$"$ev}

which results in bad substitution.

I'd expect to get back whatever the value of $USER is.

thanks

bash variable

share|improve this question

edited 31 mins ago

Kusalananda

133k17253416

asked 39 mins ago

PaulBPaulB

1134

New contributor

PaulB is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

add a comment | 

2

Given I have in a bash script

ev=USER

How can I get the environment variable value for $USER using ev?

Tried naively doing:

echo ${"$"$ev}

which results in bad substitution.

I'd expect to get back whatever the value of $USER is.

thanks

bash variable

share|improve this question

edited 31 mins ago

Kusalananda

133k17253416

asked 39 mins ago

PaulBPaulB

1134

New contributor

PaulB is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

add a comment | 

Given I have in a bash script

ev=USER

How can I get the environment variable value for $USER using ev?

Tried naively doing:

echo ${"$"$ev}

which results in bad substitution.

I'd expect to get back whatever the value of $USER is.

thanks

bash variable

share|improve this question

edited 31 mins ago

Kusalananda

133k17253416

asked 39 mins ago

PaulBPaulB

1134

New contributor

PaulB is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

ev=USER

echo ${"$"$ev}

share|improve this question

edited 31 mins ago

Kusalananda

133k17253416

asked 39 mins ago

PaulBPaulB

1134

New contributor

PaulB is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share|improve this question

edited 31 mins ago

Kusalananda

133k17253416

asked 39 mins ago

PaulBPaulB

1134

New contributor

PaulB is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

edited 31 mins ago

Kusalananda

133k17253416

edited 31 mins ago

Kusalananda

133k17253416

asked 39 mins ago

PaulBPaulB

1134

New contributor

PaulB is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 39 mins ago

PaulBPaulB

1134

3 Answers
3

active

oldest

votes

4

By using an indirect expansion (also sometimes called "variable indirection"),

ev=USER

printf '%sn' "${!ev}"

This is described in the bash (5.0) manual, in the section titled "Parameter Expansion".

Or, by making ev a name reference (requires bash 4.3+),

declare -n ev=USER

printf '%sn' "$ev"

This is described in the bash (5.0) manual, just before the section called "Positional Parameters".

share|improve this answer

edited 24 mins ago

answered 35 mins ago

KusalanandaKusalananda

133k17253416

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Perfect thanks ... env_val="${!ev}"
  
  – PaulB
  25 mins ago
  
  
  

  
  
  
  

add a comment | 

1

If it's only about environment variables, as opposed to shell variables, then on most systems, you can use:

printenv -- "$ev"

For shell variables, with any Bourne-like shell, you can do:

eval 'printf "%sn" "${'"$ev"}'}"'

Or with zsh:

printf '%sn' "${(P)ev}"

Or with bash:

printf '%sn' "${!ev}"

All 3 are arbitrary command injection vulnerabilities if the content of $ev is not under your control.

share|improve this answer

answered 16 mins ago

Stéphane ChazelasStéphane Chazelas

308k57581939

add a comment | 

0

You can also evaluate the command after the vale for $ev has been substituted:

eval echo "$"$ev

The part "$"$ev resolves to $USER so eval executes echo $USER.

share|improve this answer

answered 15 mins ago

katoshkatosh

425

add a comment | 

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4

By using an indirect expansion (also sometimes called "variable indirection"),

ev=USER

printf '%sn' "${!ev}"

This is described in the bash (5.0) manual, in the section titled "Parameter Expansion".

Or, by making ev a name reference (requires bash 4.3+),

declare -n ev=USER

printf '%sn' "$ev"

This is described in the bash (5.0) manual, just before the section called "Positional Parameters".

share|improve this answer

edited 24 mins ago

answered 35 mins ago

KusalanandaKusalananda

133k17253416

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Perfect thanks ... env_val="${!ev}"
  
  – PaulB
  25 mins ago
  
  
  

  
  
  
  

add a comment | 

4

By using an indirect expansion (also sometimes called "variable indirection"),

ev=USER

printf '%sn' "${!ev}"

This is described in the bash (5.0) manual, in the section titled "Parameter Expansion".

Or, by making ev a name reference (requires bash 4.3+),

declare -n ev=USER

printf '%sn' "$ev"

This is described in the bash (5.0) manual, just before the section called "Positional Parameters".

share|improve this answer

edited 24 mins ago

answered 35 mins ago

KusalanandaKusalananda

133k17253416

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Perfect thanks ... env_val="${!ev}"
  
  – PaulB
  25 mins ago
  
  
  

  
  
  
  

add a comment | 

By using an indirect expansion (also sometimes called "variable indirection"),

ev=USER

printf '%sn' "${!ev}"

This is described in the bash (5.0) manual, in the section titled "Parameter Expansion".

Or, by making ev a name reference (requires bash 4.3+),

declare -n ev=USER

printf '%sn' "$ev"

This is described in the bash (5.0) manual, just before the section called "Positional Parameters".

share|improve this answer

edited 24 mins ago

answered 35 mins ago

KusalanandaKusalananda

133k17253416

ev=USER

printf '%sn' "${!ev}"

declare -n ev=USER

printf '%sn' "$ev"

share|improve this answer

edited 24 mins ago

answered 35 mins ago

KusalanandaKusalananda

133k17253416

answered 35 mins ago

KusalanandaKusalananda

133k17253416

answered 35 mins ago

KusalanandaKusalananda

133k17253416

1

If it's only about environment variables, as opposed to shell variables, then on most systems, you can use:

printenv -- "$ev"

For shell variables, with any Bourne-like shell, you can do:

eval 'printf "%sn" "${'"$ev"}'}"'

Or with zsh:

printf '%sn' "${(P)ev}"

Or with bash:

printf '%sn' "${!ev}"

All 3 are arbitrary command injection vulnerabilities if the content of $ev is not under your control.

share|improve this answer

answered 16 mins ago

Stéphane ChazelasStéphane Chazelas

308k57581939

add a comment | 

1

If it's only about environment variables, as opposed to shell variables, then on most systems, you can use:

printenv -- "$ev"

For shell variables, with any Bourne-like shell, you can do:

eval 'printf "%sn" "${'"$ev"}'}"'

Or with zsh:

printf '%sn' "${(P)ev}"

Or with bash:

printf '%sn' "${!ev}"

All 3 are arbitrary command injection vulnerabilities if the content of $ev is not under your control.

share|improve this answer

answered 16 mins ago

Stéphane ChazelasStéphane Chazelas

308k57581939

add a comment | 

If it's only about environment variables, as opposed to shell variables, then on most systems, you can use:

printenv -- "$ev"

For shell variables, with any Bourne-like shell, you can do:

eval 'printf "%sn" "${'"$ev"}'}"'

Or with zsh:

printf '%sn' "${(P)ev}"

Or with bash:

printf '%sn' "${!ev}"

All 3 are arbitrary command injection vulnerabilities if the content of $ev is not under your control.

share|improve this answer

answered 16 mins ago

Stéphane ChazelasStéphane Chazelas

308k57581939

printenv -- "$ev"

eval 'printf "%sn" "${'"$ev"}'}"'

printf '%sn' "${(P)ev}"

printf '%sn' "${!ev}"

share|improve this answer

answered 16 mins ago

Stéphane ChazelasStéphane Chazelas

308k57581939

answered 16 mins ago

Stéphane ChazelasStéphane Chazelas

308k57581939

answered 16 mins ago

Stéphane ChazelasStéphane Chazelas

308k57581939

0

You can also evaluate the command after the vale for $ev has been substituted:

eval echo "$"$ev

The part "$"$ev resolves to $USER so eval executes echo $USER.

share|improve this answer

answered 15 mins ago

katoshkatosh

425

add a comment | 

0

You can also evaluate the command after the vale for $ev has been substituted:

eval echo "$"$ev

The part "$"$ev resolves to $USER so eval executes echo $USER.

share|improve this answer

answered 15 mins ago

katoshkatosh

425

add a comment | 

You can also evaluate the command after the vale for $ev has been substituted:

eval echo "$"$ev

The part "$"$ev resolves to $USER so eval executes echo $USER.

share|improve this answer

answered 15 mins ago

katoshkatosh

425

eval echo "$"$ev

share|improve this answer

answered 15 mins ago

katoshkatosh

425

answered 15 mins ago

katoshkatosh

425

answered 15 mins ago

katoshkatosh

425