I want to substitute a value for a variable in a file with Bash.

Why don't you use envsubst?Expand the environment variables.

cat sample.json | ver=123envsubst 

Reference

• I want a low-functioning, portable template engine→That's where envsubst is -Qiita
• If you want to run a shell script without exporting environment variables in bash, you can replace them by writing them before the command - code progression
  • ver=123 describes what you are doing

• ver=123 describes what you are doing

The jq command, which is well known as a JSON parser, has the option to treat JSON in the form shown in the questionnaire as a template: Using jq as a template engine.

$cat sample.json
{
    "red": "aaaaa",
    "green"—$ver,
    "yellow": "cccccc",
    "blue": "dddddd"
}
$ cat test.sh
ver=123
jq-n --argjson ver"\"$ver\"-f sample.json>output.json
$ bash test.sh
$ cat output.json
{
  "red": "aaaaa",
  "green": "123",
  "yellow": "cccccc",
  "blue": "dddddd"
}

If the syntax of the template is simple, you can also use the sed command to handle it.

$cat sample.json
{
    "red": "aaaaa",
    "green": "$ver",
    "yellow": "cccccc",
    "blue": "dddddd"
}
$ cat test.sh
ver=123
sed"s/\$ver/$ver/g"sample.json>output.json
$ bash test.sh
$ cat output.json
{
    "red": "aaaaa",
    "green": "123",
    "yellow": "cccccc",
    "blue": "dddddd"
}

You can add the following one liner to the sh script and use extcat instead of cat to get the results you requested.

I also added the sh scripted version.This one is easier to use.

One Liner

function extcat() {cat$@|bash<<"cat<<"\"$(cat)"\"";};export-fextcat

Note
　In addition to expanding environment variables.Command replacement is also done, so be careful when using it.

If you don't mind, please use it.

Simple Template Engine extcat coneta #1

test.sh

#!/bin/bash
function extcat() {cat$@|bash<<"cat<<"cat<"\"$(cat)"\"";};export-fxtcat
ver="123" extcat docs/template.json>sample.json

Or

#!/bin/bash
function extcat() {cat$@|bash<<"cat<<"cat<"\"$(cat)"\"";};export-fxtcat
export ver="123"
extcat docs/template.json>sample.json

sample.json

{
    "red": "aaaaa",
    "green": "123",
    "yellow": "cccccc",
    "blue": "dddddd"
}

docs/template.json

You must include " at the beginning of the template, " at the end, and $ver around '.

"'{
    "red": "aaaaa",
    "green": "'$ver',
    "yellow": "cccccc",
    "blue": "dddddd"
}'"

Template variable replacement uses the bash feature.
Because the template is evaluated after being enclosed in double quotation marks ", if you use " in the template, you must change the quotation to single quotation marks ' and resolve the quotation before and after ${variable name}.

Another way is to escape " with \.

{
    \"red\":\"aaaa\",
    \"green\":\"$ver\",
    \"yellow\":\"cccccc\",
    \"blue\":\"dddddd\"
}

Directory Configuration

--test.sh
--docs/template.json
--sample.json

This is the sh scripted version of extcat.
There is no need to escape double quotation marks " in the template.

extcat.sh

#!/bin/sh
 cat<<^D|sh
 cat<^D2
`cat$@`
^D2
^D

^D is supposed to be a control character.Copy paste and control character ^D moves even if it becomes normal string^D

How-to

ver="123"./extcat.sh docs/template.json>sample.json

Results (sample.json)

{
    "red": "aaaaa",
    "green": "123",
    "yellow": "cccccc",
    "blue": "dddddd"
}

Templates (docs/template.json)

{
    "red": "aaaaa",
    "green": "$ver",
    "yellow": "cccccc",
    "blue": "dddddd"
}