subst - substitute $variables in text

subst parses its arguments as KEY=VALUE pairs, substitutes $KEYs by VALUEs in stdin, and writes the result to stdout.

• Get it
• Use it
  • Examples
• Comments

Get it

Using go get:

go get -u github.com/sgreben/subst

Or download the binary from the releases page.

# Linux
curl -LO https://github.com/sgreben/subst/releases/download/1.1.1/subst_1.1.1_linux_x86_64.zip
unzip subst_1.1.1_linux_x86_64.zip

# OS X
curl -LO https://github.com/sgreben/subst/releases/download/1.1.1/subst_1.1.1_osx_x86_64.zip
unzip subst_1.1.1_osx_x86_64.zip

# Windows
curl -LO https://github.com/sgreben/subst/releases/download/1.1.1/subst_1.1.1_windows_x86_64.zip
unzip subst_1.1.1_windows_x86_64.zip

Use it

subst uses key=value mappings given as CLI arguments, reads from stdin, and writes to stdout. The stdin input is line-buffered.

subst [OPTIONS] [KEY=VAL [KEY=VAL...]]

Usage of subst:
  -q    suppress all logs
  -escape string
        set the escape string. if set, a '$' preceded by the escape string does not lead to expansion
  -unknown value
        handling of unknown keys, one of [ignore empty error] (default ignore)

Examples

$ cat template.yaml
name: $NAME
url: $URL
comment: $COMMENT

$ cat template.yaml | subst -q NAME=jp  URL=github.com/sgreben/jp COMMENT=
name: jp
url: github.com/sgreben/jp
comment:

$ cat template.yaml | subst -q NAME=jp
name: jp
url: $URL
comment: $COMMENT

$ cat template.yaml | subst -q -unknown=empty NAME=jp
name: jp
url:
comment:

Without the -q flag, subst also prints to stderr:

2018/04/16 15:18:37 undefined key: $URL, using value "$URL"
2018/04/16 15:18:37 undefined key: $COMMENT, using value "$COMMENT"

You can substitute environment variables (like envsubst) by letting the shell substitute their values:

$ subst USER="$USER" SHELL="$SHELL"
$USER
sgreben
$SHELL
/bin/zsh

To ignore occurrences of defined variables, you can define an escape string using the -escape flag:

$ echo 'unescaped: $X, ${X}. escaped: \$X, \${X}.' | subst -escape '\' X=value
unescaped: value, value. escaped: $X, ${X}.

Comments

Feel free to leave a comment or create an issue.