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Project Notes

Indirect References

Indirect variable de-referencing in bash scripts.

Notes

Indirect references: using a variable that holds the name of another variable. Indirection like that makes it possible to do all kinds of programming tricks in Bash that one really should not: mimic 2D/3D arrays etc.

But it can be useful for simple cases.

Old Style

The old approach is actually quite tricky and doesn’t actually need any new features.

If our variable of interest is called “X_value”:

X_value="the actual value"

An we have another variable called “variable_name” that will holder the name of the variable we will lookup:

variable_name="X_value"

Then we can evaluate the result of echoing “variable_name” in a variable format:

eval \$$variable_name

i.e. \$$variable_name will produce the string “$X_value”, which is then evaluated as a command, returning “the actual value”

New Syntax

A dollar-bang syntax was introduced in Bash version 2.

Instead of:

eval \$$variable_name

Simply:

${!variable_name}

Running the Example

See examples.sh for a full example and test:

$ ./examples.sh
Three variables are defined:
  ${X_value}: "the X value"
  ${Y_value}: "the Y value"
  ${Z_value}: "the Z value"

We have a variable "${selected}" that selects for X, Y or Z
Currently:
  ${selected}: "X"

And ${selected} is used to construct the variable name we are interested in, stored in "$variable_name":
  variable_name="${selected}_value"
Currently:
  ${variable_name}: "X_value"

And finally we indirectly lookup the value of the variable referenced in ${variable_name}:
  selected_value=${!variable_name}
Currently:
  ${selected_value}: "the X value"
Success! ${selected_value} is returning the correct value

Changing "${selected}" to Y
Currently:
  ${selected}: "Y"
  ${variable_name}: "Y_value"
  ${selected_value}: "the Y value"
Success! ${selected_value} is returning the correct value

Older/alternative syntax for indirectly lookup is to evaluate with double-dollars: \$${variable_name}:
  eval selected_value=\$${variable_name}
Currently:
  ${selected}: "Z"
  ${variable_name}: "Z_value"
  ${selected_value}: "the Z value"
Success! ${selected_value} is returning the correct value

Credits and References

About LCK#177 Bash
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This page is a web-friendly rendering of my project notes shared in the LittleCodingKata GitHub repository.

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