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Safest way to store environment variable value in a file

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Safest way to store environment variable value in a file

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3

I am using bash and most interested in an answer for bash, but you can answer about how to do it in other shells if you want.

I have an environment variable, KEY, and I want to store the value of that environment variable in a a file. Currently I am doing

echo "${KEY}" > ./key.pem

In practice this seems to be OK, but in theory this breaks when KEY=-n.

$ export KEY=-n

$ echo BEGIN "${KEY}" END

BEGIN -n END

$ echo "${KEY}" END

END$

So, is there a better way to store the value of a single environment variable in a file? (Note that export and declare will include the name of the variable in their outputs, which is no good for me.)

bash scripting

share|improve this question

asked 9 hours ago

Old ProOld Pro

61222 gold badges66 silver badges1515 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  What will use the file? Will it be used to set the value again in a shell script, or by some other tool?
  
  – muru
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @drewbenn echo -- "$KEY" would not help. It would output -- -n.
  
  – Kusalananda♦
  8 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Kusalananda, I'm a bit disappointed that all my shells consistently output -- -n
  
  – ilkkachu
  8 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  This is one of the reasons why printf is always better than echo when you don't have complete control over what input you want to print.
  
  – terdon♦
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @ilkkachu Hmm... Usually, consistency is considered a good thing ;-)
  
  – Kusalananda♦
  7 hours ago
  
  
  

  
  
  
  

add a comment | 

3

I am using bash and most interested in an answer for bash, but you can answer about how to do it in other shells if you want.

I have an environment variable, KEY, and I want to store the value of that environment variable in a a file. Currently I am doing

echo "${KEY}" > ./key.pem

In practice this seems to be OK, but in theory this breaks when KEY=-n.

$ export KEY=-n

$ echo BEGIN "${KEY}" END

BEGIN -n END

$ echo "${KEY}" END

END$

So, is there a better way to store the value of a single environment variable in a file? (Note that export and declare will include the name of the variable in their outputs, which is no good for me.)

bash scripting

share|improve this question

asked 9 hours ago

Old ProOld Pro

61222 gold badges66 silver badges1515 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  What will use the file? Will it be used to set the value again in a shell script, or by some other tool?
  
  – muru
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @drewbenn echo -- "$KEY" would not help. It would output -- -n.
  
  – Kusalananda♦
  8 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Kusalananda, I'm a bit disappointed that all my shells consistently output -- -n
  
  – ilkkachu
  8 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  This is one of the reasons why printf is always better than echo when you don't have complete control over what input you want to print.
  
  – terdon♦
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @ilkkachu Hmm... Usually, consistency is considered a good thing ;-)
  
  – Kusalananda♦
  7 hours ago
  
  
  

  
  
  
  

add a comment | 

I am using bash and most interested in an answer for bash, but you can answer about how to do it in other shells if you want.

I have an environment variable, KEY, and I want to store the value of that environment variable in a a file. Currently I am doing

echo "${KEY}" > ./key.pem

In practice this seems to be OK, but in theory this breaks when KEY=-n.

$ export KEY=-n

$ echo BEGIN "${KEY}" END

BEGIN -n END

$ echo "${KEY}" END

END$

So, is there a better way to store the value of a single environment variable in a file? (Note that export and declare will include the name of the variable in their outputs, which is no good for me.)

bash scripting

share|improve this question

asked 9 hours ago

Old ProOld Pro

61222 gold badges66 silver badges1515 bronze badges

echo "${KEY}" > ./key.pem

$ export KEY=-n

$ echo BEGIN "${KEY}" END

BEGIN -n END

$ echo "${KEY}" END

END$

share|improve this question

asked 9 hours ago

Old ProOld Pro

61222 gold badges66 silver badges1515 bronze badges

share|improve this question

asked 9 hours ago

Old ProOld Pro

61222 gold badges66 silver badges1515 bronze badges

asked 9 hours ago

Old ProOld Pro

61222 gold badges66 silver badges1515 bronze badges

asked 9 hours ago

Old ProOld Pro

61222 gold badges66 silver badges1515 bronze badges

2 Answers
2

active

oldest

votes

3

If it's storing for the sake of reading it in later (in a bash script), just use declare -p KEY and then source the file to read it in again. If you just want to store the value, use printf '%sn' "$KEY" as you would do when you output any variable data.

So,

printf '%sn' "$KEY" >key.pem

or

printf 'BEGIN %s ENDn' "$KEY" >key.pem

or whatever you need to output.

Your issue occurs since -n is a valid option to echo in bash. The strings -e and -E (and combinations like -neEne) would also cause issues in bash, for the same reason. Depending on how bash is built or the environment or options, backslash characters in arguments may also be a problem.

These issues and more are outlined in the following Q/A:

• Why is printf better than echo?

share|improve this answer

edited 6 hours ago

answered 9 hours ago

Kusalananda♦Kusalananda

159k1818 gold badges315315 silver badges501501 bronze badges

add a comment | 

1

Many systems have a printenv command that outputs the contents of the given environment variable followed by one newline character to standard output:

printenv 'My Env Var' > file

Would store the content of the My Env Var variable followed by NL into file in all shells on those systems.

For environment variables that are mapped to shell variables (includes at least those whose name starts with an ASCII letter or underscore and are followed by 0 or more ASCII letters, digits or underscore, and are not otherwise special variables set by the shell), in Bourne-like shells (also works in fish), you can do the same with:

printf '%sn' "$ENVVAR" > file

(though if the variable is unset, that will still store an empty line into file)

In rc-like shells:

echo $ENVVAR > file

(same caveat as above)

In csh-like shells:

printf '%sn' $ENVVAR:q > file

(if the variable is not set, it will fail and not overwrite file)

Some implementations/versions of ksh don't have printf builtin. In those, you could also do:

print -r -- "$ENVVAR" > file

share|improve this answer

answered 6 hours ago

Stéphane ChazelasStéphane Chazelas

329k5757 gold badges640640 silver badges10081008 bronze badges

add a comment | 

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3

If it's storing for the sake of reading it in later (in a bash script), just use declare -p KEY and then source the file to read it in again. If you just want to store the value, use printf '%sn' "$KEY" as you would do when you output any variable data.

So,

printf '%sn' "$KEY" >key.pem

or

printf 'BEGIN %s ENDn' "$KEY" >key.pem

or whatever you need to output.

Your issue occurs since -n is a valid option to echo in bash. The strings -e and -E (and combinations like -neEne) would also cause issues in bash, for the same reason. Depending on how bash is built or the environment or options, backslash characters in arguments may also be a problem.

These issues and more are outlined in the following Q/A:

• Why is printf better than echo?

share|improve this answer

edited 6 hours ago

answered 9 hours ago

Kusalananda♦Kusalananda

159k1818 gold badges315315 silver badges501501 bronze badges

add a comment | 

3

If it's storing for the sake of reading it in later (in a bash script), just use declare -p KEY and then source the file to read it in again. If you just want to store the value, use printf '%sn' "$KEY" as you would do when you output any variable data.

So,

printf '%sn' "$KEY" >key.pem

or

printf 'BEGIN %s ENDn' "$KEY" >key.pem

or whatever you need to output.

Your issue occurs since -n is a valid option to echo in bash. The strings -e and -E (and combinations like -neEne) would also cause issues in bash, for the same reason. Depending on how bash is built or the environment or options, backslash characters in arguments may also be a problem.

These issues and more are outlined in the following Q/A:

• Why is printf better than echo?

share|improve this answer

edited 6 hours ago

answered 9 hours ago

Kusalananda♦Kusalananda

159k1818 gold badges315315 silver badges501501 bronze badges

add a comment | 

If it's storing for the sake of reading it in later (in a bash script), just use declare -p KEY and then source the file to read it in again. If you just want to store the value, use printf '%sn' "$KEY" as you would do when you output any variable data.

So,

printf '%sn' "$KEY" >key.pem

or

printf 'BEGIN %s ENDn' "$KEY" >key.pem

or whatever you need to output.

Your issue occurs since -n is a valid option to echo in bash. The strings -e and -E (and combinations like -neEne) would also cause issues in bash, for the same reason. Depending on how bash is built or the environment or options, backslash characters in arguments may also be a problem.

These issues and more are outlined in the following Q/A:

• Why is printf better than echo?

share|improve this answer

edited 6 hours ago

answered 9 hours ago

Kusalananda♦Kusalananda

159k1818 gold badges315315 silver badges501501 bronze badges

printf '%sn' "$KEY" >key.pem

printf 'BEGIN %s ENDn' "$KEY" >key.pem

share|improve this answer

edited 6 hours ago

answered 9 hours ago

Kusalananda♦Kusalananda

159k1818 gold badges315315 silver badges501501 bronze badges

answered 9 hours ago

Kusalananda♦Kusalananda

159k1818 gold badges315315 silver badges501501 bronze badges

answered 9 hours ago

Kusalananda♦Kusalananda

159k1818 gold badges315315 silver badges501501 bronze badges

1

Many systems have a printenv command that outputs the contents of the given environment variable followed by one newline character to standard output:

printenv 'My Env Var' > file

Would store the content of the My Env Var variable followed by NL into file in all shells on those systems.

For environment variables that are mapped to shell variables (includes at least those whose name starts with an ASCII letter or underscore and are followed by 0 or more ASCII letters, digits or underscore, and are not otherwise special variables set by the shell), in Bourne-like shells (also works in fish), you can do the same with:

printf '%sn' "$ENVVAR" > file

(though if the variable is unset, that will still store an empty line into file)

In rc-like shells:

echo $ENVVAR > file

(same caveat as above)

In csh-like shells:

printf '%sn' $ENVVAR:q > file

(if the variable is not set, it will fail and not overwrite file)

Some implementations/versions of ksh don't have printf builtin. In those, you could also do:

print -r -- "$ENVVAR" > file

share|improve this answer

answered 6 hours ago

Stéphane ChazelasStéphane Chazelas

329k5757 gold badges640640 silver badges10081008 bronze badges

add a comment | 

1

Many systems have a printenv command that outputs the contents of the given environment variable followed by one newline character to standard output:

printenv 'My Env Var' > file

Would store the content of the My Env Var variable followed by NL into file in all shells on those systems.

For environment variables that are mapped to shell variables (includes at least those whose name starts with an ASCII letter or underscore and are followed by 0 or more ASCII letters, digits or underscore, and are not otherwise special variables set by the shell), in Bourne-like shells (also works in fish), you can do the same with:

printf '%sn' "$ENVVAR" > file

(though if the variable is unset, that will still store an empty line into file)

In rc-like shells:

echo $ENVVAR > file

(same caveat as above)

In csh-like shells:

printf '%sn' $ENVVAR:q > file

(if the variable is not set, it will fail and not overwrite file)

Some implementations/versions of ksh don't have printf builtin. In those, you could also do:

print -r -- "$ENVVAR" > file

share|improve this answer

answered 6 hours ago

Stéphane ChazelasStéphane Chazelas

329k5757 gold badges640640 silver badges10081008 bronze badges

add a comment | 

Many systems have a printenv command that outputs the contents of the given environment variable followed by one newline character to standard output:

printenv 'My Env Var' > file

Would store the content of the My Env Var variable followed by NL into file in all shells on those systems.

For environment variables that are mapped to shell variables (includes at least those whose name starts with an ASCII letter or underscore and are followed by 0 or more ASCII letters, digits or underscore, and are not otherwise special variables set by the shell), in Bourne-like shells (also works in fish), you can do the same with:

printf '%sn' "$ENVVAR" > file

(though if the variable is unset, that will still store an empty line into file)

In rc-like shells:

echo $ENVVAR > file

(same caveat as above)

In csh-like shells:

printf '%sn' $ENVVAR:q > file

(if the variable is not set, it will fail and not overwrite file)

Some implementations/versions of ksh don't have printf builtin. In those, you could also do:

print -r -- "$ENVVAR" > file

share|improve this answer

answered 6 hours ago

Stéphane ChazelasStéphane Chazelas

329k5757 gold badges640640 silver badges10081008 bronze badges

printenv 'My Env Var' > file

printf '%sn' "$ENVVAR" > file

echo $ENVVAR > file

printf '%sn' $ENVVAR:q > file

print -r -- "$ENVVAR" > file

share|improve this answer

answered 6 hours ago

Stéphane ChazelasStéphane Chazelas

329k5757 gold badges640640 silver badges10081008 bronze badges

answered 6 hours ago

Stéphane ChazelasStéphane Chazelas

329k5757 gold badges640640 silver badges10081008 bronze badges

answered 6 hours ago

Stéphane ChazelasStéphane Chazelas

329k5757 gold badges640640 silver badges10081008 bronze badges