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Indirectly access environment variable

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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 1 hour ago

Kusalananda

133k17253416

asked 1 hour 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 1 hour ago

Kusalananda

133k17253416

asked 1 hour 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 1 hour ago

Kusalananda

133k17253416

asked 1 hour 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 1 hour ago

Kusalananda

133k17253416

asked 1 hour 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 1 hour ago

Kusalananda

133k17253416

asked 1 hour 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 1 hour ago

Kusalananda

133k17253416

edited 1 hour ago

Kusalananda

133k17253416

asked 1 hour 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 1 hour ago

PaulBPaulB

1134

3 Answers
3

active

oldest

votes

5

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 1 hour ago

answered 1 hour ago

KusalanandaKusalananda

133k17253416

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Perfect thanks ... env_val="${!ev}"
  
  – PaulB
  1 hour ago
  
  
  

  
  
  
  

add a comment | 

2

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 57 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 56 mins ago

katoshkatosh

425

add a comment | 

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5

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 1 hour ago

answered 1 hour ago

KusalanandaKusalananda

133k17253416

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Perfect thanks ... env_val="${!ev}"
  
  – PaulB
  1 hour ago
  
  
  

  
  
  
  

add a comment | 

5

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 1 hour ago

answered 1 hour ago

KusalanandaKusalananda

133k17253416

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Perfect thanks ... env_val="${!ev}"
  
  – PaulB
  1 hour 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 1 hour ago

answered 1 hour ago

KusalanandaKusalananda

133k17253416

ev=USER

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

declare -n ev=USER

printf '%sn' "$ev"

share|improve this answer

edited 1 hour ago

answered 1 hour ago

KusalanandaKusalananda

133k17253416

answered 1 hour ago

KusalanandaKusalananda

133k17253416

answered 1 hour ago

KusalanandaKusalananda

133k17253416

2

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 57 mins ago

Stéphane ChazelasStéphane Chazelas

308k57581939

add a comment | 

2

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 57 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 57 mins ago

Stéphane ChazelasStéphane Chazelas

308k57581939

printenv -- "$ev"

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

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

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

share|improve this answer

answered 57 mins ago

Stéphane ChazelasStéphane Chazelas

308k57581939

answered 57 mins ago

Stéphane ChazelasStéphane Chazelas

308k57581939

answered 57 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 56 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 56 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 56 mins ago

katoshkatosh

425

eval echo "$"$ev

share|improve this answer

answered 56 mins ago

katoshkatosh

425

answered 56 mins ago

katoshkatosh

425

answered 56 mins ago

katoshkatosh

425