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If I pass variable as a positional argument to a function, is it possible to determine the name of the variable used from inside the function, or I can only access its value?

I have a function envir-export that I can use to define an environment variable and store it in a special environment file that would then get sourced.

envir-export ()
 {
  nm="$1"
  vl="$2"
     
  envirs_trkrc="${HOME}/Opstk/envirs"
  echo "export ${nm}=${vl}" >> ${envirs_trkrc}/envirs.rc

  source ${envirs_trkrc}/envirs.rc
  echo "$nm exported with value $vl"
 }

Currently I am using by the following call

envir-export VERBOS 8

Using the name VERBOS with value 8.

I would like to improve on this function, both in convenience, and also functionally.

For instance, if no value is supplied then I only export the name with a value being assigned.

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2 Answers 2

Reset to default
0

Common practice for achieving this is do something like

function envir-export () {
  name="${1%%=*}"
  value="${1##*=}"
  echo 'the name is '"$name"
  echo 'the value is '"$value"
}

envir-export VERBOS=8



$ function f () { name="${1%%=*}" ; value="${1##*=}" ; echo name "$name" ; echo value "$value" ; } ; f VERBOS=8
name VERBOS
value 8
$

Check the bash manpage and search for 'Parameter Expansion' for a howto on splitting the input into name and value.

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To output an export statement with or without a value depending on whether a value was given to the function:

write_export () {
    printf 'export %s\n' "$1${2+=${2@Q}}"
}

This writes the string export followed by the given variable name (the 1st argument). If the second argument, the value, is defined, an equal character (=) is added, followed by the quoted value.

The expansion ${parameter+word} is standard and will expand to word if the shell variable parameter is defined. The expansion ${parameter@Q} is an extension in the bash shell, expanding to the quoted value of the shell variable parameter.

Example runs of this function:

$ write_export empty
export empty

$ write_export foo bar
export foo='bar'

$ write_export lime 'the "value"'
export lime='the "value"'

$ write_export lemon "the 'value'"
export lemon='the '\''value'\'''

$ write_export multiline 'line 1
> line 2
> line 3'
export multiline=$'line 1\nline 2\nline 3'
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  • Note that while using ${2@Q} is certainly much better than doing no quoting at all, it produces non-portable and locale dependant output. For instance, try bash -c 'printf "export %s\n" "$1${2+=${2@Q}}"' bash var $'\n‣\'; echo you have been pawned #' | LC_ALL=zh_HK bash on a system where the zh_HK locale uses the BIG5HKSCS charset. See Escape a variable for use as content of another script for details. Commented May 14, 2024 at 10:22
  • Note that ${var@Q} is initially from mksh, copied later by bash (though there's of course prior art with $var:q or ${(q)var} and variants (${(qq)var} being the one producing portable output safe for reinput in Bourne-like shells (not to send to a terminal)) in zsh) Commented May 14, 2024 at 10:24

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