5

Assuming I have a variable v="c d e f g" and a file with the content:

line1
$v
line3

How can I make a bash script print this file with the content of the variable in line 2 as if I was using a here-document like this:

#!/bin/bash

v="c d e f g"

cat << EOF
line1
$v
line3
EOF

Output:

line1
c d e f g
line3
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1 Answer 1

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10

Using envsubst to parse the file and replace the variables that corresponds to environment variables (in envsubst's environment):

$ v='c d e f g'
$ v=$v envsubst <file
line1
c d e f g
line3

Or, by exporting v first:

$ export v
$ envsubst <file
line1
c d e f g
line3

On my system, envsubst is distributed as part of the gettext package.

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3
  • Or do an explicit sed replacement... Commented Mar 30, 2024 at 4:16
  • 1
    @keshlam Assuming you don't know the value of the variable (it may be user input, for example), that may be impossible. If you use $v in the replacement text of a substitution comman in sed, it restricts the set of characters the variable can contain. It may , for example, not contain "&", or "\" followed by a digit, or the character that is used as delimiter in the substitution command. Depending on the sed implementation, you may even introduce a serious code injection vulnerability. Commented Mar 30, 2024 at 6:07
  • Granted, but (1) that isn't always a concern, (2) that ca Commented Mar 30, 2024 at 13:21

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