51

I'm trying to perform environment variable replacement through envsubst, but I want to only replace specific variables.

From the docs I should be able to tell envsubst to only replace certain variables but I'm failing to be able to do that.

For example, if I have a file containing:

VAR_1=${VAR_1}
VAR_2=${VAR_2}

how should I execute envsubst so that it only replaces the reference to ${VAR_1}?

3

Before calling envsubst you should use export using single quotes to get back VAR_1 modified. As in:

export VAR_1='somevalue'

For more details, please see:

How to substitute shell variables in complex text files

  • 14
    Correct answer is below – Craig Mar 19 '18 at 17:40
123

Per the man page:

envsubst [OPTION] [SHELL-FORMAT]

If a SHELL-FORMAT is given, only those environment variables that are referenced in SHELL-FORMAT are substituted; otherwise all environment variables references occurring in standard input are substituted.

Where SHELL-FORMAT strings are "strings with references to shell variables in the form $variable or ${variable}[...] The variable names must consist solely of alphanumeric or underscore ASCII characters, not start with a digit and be nonempty; otherwise such a variable reference is ignored.".
So, one has to pass the respective variables names to envsubst in a shell format string (obviously, they need to be escaped/quoted so as to be passed literally to envsubst). Example:

input file e.g. infile:

VAR1=${VAR1}
VAR2=${VAR2}
VAR3=${VAR3}

and some values like

export  VAR1="one" VAR2="two" VAR3="three"

then running

envsubst '${VAR1} ${VAR3}' <infile

or

envsubst '${VAR1},${VAR3}' <infile

or

envsubst '${VAR1}
${VAR3}' <infile

outputs

VAR1=one
VAR2=${VAR2}
VAR3=three

Or, if you prefer backslash:

envsubst \$VAR1,\$VAR2 <infile

produces

VAR1=one
VAR2=two
VAR3=${VAR3}
  • 12
    why is this not the accepted answer? – Qsiris Jan 11 '18 at 13:39
  • 3
    Yes this is the right answer! – Thomas Decaux Feb 26 '18 at 9:31
  • 3
    If you know a variable prefix to use to limit the variables that can be substituted: envsubst "$(printf '${%s} ' ${!PREFIX*})" < infile – Sam Liddicott Jan 22 at 15:13
  • Or if you have a list of them in SUBST_VARS then: envsubst "$(printf '${%s} ' $SUBST_VARS)" < infile – Sam Liddicott Jan 22 at 15:15

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