ERB Executable

The erb executable gives command-line access to ERB template processing.

The executable is installed with ERB, which is part of the Ruby installation.

For a quick summary, type:

$ erb --help

The format of the command is erb [_options_] [_filepaths_], where:

• options are zero or more options.

• filepaths are zero or more paths to files, each containing an plain text that can include ERB tags.

Filepaths

With one or more filepaths given, erb reads all the given files as a single template; that is, erb processes multiple files into a single result:

$ cat t.erb
<%= RUBY_VERSION %>
<%= Time.now %>
$ cat u.erb
% Encoding.list.take(4).each do |encoding|
* <%= encoding %>
% end
$ erb t.erb u.erb
3.4.5
2025-09-24 00:23:00 +0100
* ASCII-8BIT
* UTF-8
* US-ASCII
* UTF-16BE

There is a special “filepath”, '-', that specifies the standard input:

$ echo "<%= RUBY_VERSION %>" | erb u.erb -
* ASCII-8BIT
* UTF-8
* US-ASCII
* UTF-16BE
3.4.5

Any filepath, including '-', may be repeated.

With no filepaths given, erb reads and processes the standard input:

$ echo "<%= RUBY_VERSION %>" | erb # Prints the ERB version string.

Options

-d, --debug: Set $DEBUG

Use option -d or --debug to turn on debugging output:

$ echo "<%= $DEBUG %>" | erb
"false"
$echo "<%= $DEBUG %>" | erb --debug
"true"

-E, --encoding: Set Encodings

Use option -E or --encoding to set the default external encoding to _ex_ and, if _in_ is given, to set the default internal encoding to _in_.

Each encoding, ex and in, must be the name of an Encoding:

erb  -E ASCII-8BIT:ASCII-8BIT t.erb

-h, --help: Print Help

Use option -h or --help to print erb help text:

$ erb --help

-n: Print Source with Line Numbers

Use option -n with option -x to print the output of ERB#src, with numbered lines:

$ cat t.erb
<%= RUBY_VERSION %>
<%= Time.now %>
$ erb -n -x t.erb
  1 #coding:UTF-8
  2 _erbout = +''; _erbout.<<(( RUBY_VERSION ).to_s); _erbout.<< "\n".freeze
  3 ; _erbout.<<(( Time.now ).to_s); _erbout.<< "\n".freeze
  4 ; _erbout

Using option -n without option -x has no effect:

$ erb -n t.erb
3.4.5
2025-09-23 02:44:57 +0100

-P: Disable Execution Tag Shorthand

By default, erb enables execution tag shorthand:

$ cat u.erb
% Encoding.list.take(4).each do |encoding|
* <%= encoding %>
% end
$ erb u.erb
* ASCII-8BIT
* UTF-8
* US-ASCII
* UTF-16BE

You can use option -P to disable the shorthand:

$ erb -P u.erb # Raises NameError: "undefined local variable or method 'encoding'"

-r: Load Library

You can use option -r to load a library; the option may be given multiple times, to load multiple libraries:

$ erb -r csv -r bigdecimal t.erb

-T: Set Trim Mode

You can use option -T to set the trim mode.

The values for the option are:

• '0', meaning '%'; enable execution tag shorthand; see execution tag shorthand.

• '1', meaning '%>': enable execution tag shorthand and omit newline for each line ending with '%>'; see suppressing unwanted newlines.

• '2', meaning '<>': to suppress the trailing newline for each line that both begins with '<%' and ends with '%>'; see suppressing unwanted newlines.

• '-', meaning '%-': enable execution tag shorthand and omit each blank line ending with '-%>'. see execution tag shorthand and suppressing unwanted blank lines.

Example:

$ erb -T 0 t.erb

-U: Set Default Encodings to UTF-8

You can use option -U to set both external and internal encodings to UTF-8:

$ erb -U t.erb

-v: Set $VERBOSE

Use option -v to turn on verbose output:

$ $ "<%= $VERBOSE %>" | erb
"false"
$ echo "<%= $VERBOSE %>" | erb -v
"true"

-v: Print ERB Version

Use option --version to print the ERB version string:

$ erb --version

-x: Print Source

Use option -x to print the output of ERB#src, which is the Ruby code that is to be run when ERB#result is called:

$ cat t.erb
<%= RUBY_VERSION %>
<%= Time.now %>
$ erb -x t.erb
#coding:UTF-8
_erbout = +''; _erbout.<<(( RUBY_VERSION ).to_s); _erbout.<< "\n".freeze
; _erbout.<<(( Time.now ).to_s); _erbout.<< "\n".freeze
; _erbout

--: End of Options

You can use option '--' to declare the end of options in the erb command; erb treats each word following as a filepath (even if it looks like an option):

erb -- --help # Raises Errno::ENOENT: "No such file or directory @ rb_sysopen - --help"

name=value: Set the Value of a Variable

You can use option name=value to set the value of the variable named name to the given value.

The option may be given multiple times to set multiple variables:

$ echo "<%= foo %> <%= bar %>" | erb foo=1 bar=2
"1 2"