How lower case the following bash variables assignments in a single line of code?

Question

#!/usr/bin/env bash

readonly FQHN=${1:-`hostname -f`}
readonly HOSTNAME=${FQHN%%.*}

I am struggling with the syntax to assign FQHN to the lower case of either the first command line argument or the output of hostname -f in a single assignment.

I have tried to use the various ways to lower case a variable reference contents as given in this answer but have not been able to integrate them into my code without having to use something like "${HOSTNAME,,}" or "${FQHN,,}" for every reference since you can not nest string substitutions.

I am really looking or a solution that does not rely on mutability.

@Cyrus - I like this as well, can you add it as an answer so it can get upvoted? All the years I have used bash and I have never come across a mention of the declare builtin. — user177800
– user177800, Commented Mar 12, 2018 at 14:17

that other guy · Accepted Answer · 2018-03-12 04:49:03Z

1

without having to use something like "${HOSTNAME,,}" or "${FQHN,,}" for every reference

Just do it once and assign the result back to the variable:

FQHN=${1:-`hostname -f`}
readonly FQHN=${FQHN,,}
HOSTNAME=${FQHN%%.*}

I don't know why you want to do it in a single line, but you can join them with ; or &&:

FQHN=${1:-`hostname -f`}; readonly FQHN=${FQHN,,}
HOSTNAME=${FQHN%%.*}

Or if you want to do it in a single assignment, you can do it the slow and verbose way:

readonly FQHN=$(tr '[:upper:]' '[:lower:]' <<< "${1:-`hostname -f`}")
HOSTNAME=${FQHN%%.*}

edited Mar 12, 2018 at 4:49

answered Mar 12, 2018 at 4:31

that other guy

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2 Comments

tripleee Over a year ago

If you want the end result to be readonly, simply do that when you have the final value. Use a temporary variable (you can make that readonly, too!) if you want to be strict about only assigning a value once; but there really is no way to make it more compact than this.

user177800 Over a year ago

slow and verbose is not a problem if it is also clear and obvious, not that anything in bash is clear or obvious unless you already know what you are doing ...

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