How can I parse a YAML file from a Linux shell script?

Here is a bash-only parser that leverages sed and awk to parse simple yaml files:

function parse_yaml {
   local prefix=$2
   local s="[[:space:]]*" w='[a-zA-Z0-9_]*' fs=$(echo @|tr @ '\034')
   sed -ne "s|^\($s\):|\1|" \
        -e "s|^\($s\)\($w\)$s:$s[\"']\(.*\)[\"']$s\$|\1$fs\2$fs\3|p" \
        -e "s|^\($s\)\($w\)$s:$s\(.*\)$s\$|\1$fs\2$fs\3|p"  $1 |
   awk -F$fs '{
      indent = length($1)/2;
      vname[indent] = $2;
      for (i in vname) {if (i > indent) {delete vname[i]}}
      if (length($3) > 0) {
         vn=""; for (i=0; i<indent; i++) {vn=(vn)(vname[i])("_")}
         printf("%s%s%s=\"%s\"\n", "'$prefix'",vn, $2, $3);
      }
   }'
}

It understands files such as:

## global definitions
global:
  debug: yes
  verbose: no
  debugging:
    detailed: no
    header: "debugging started"

## output
output:
   file: "yes"

Which, when parsed using:

parse_yaml sample.yml

will output:

global_debug="yes"
global_verbose="no"
global_debugging_detailed="no"
global_debugging_header="debugging started"
output_file="yes"

it also understands yaml files, generated by ruby which may include ruby symbols, like:

---
:global:
  :debug: 'yes'
  :verbose: 'no'
  :debugging:
    :detailed: 'no'
    :header: debugging started
  :output: 'yes'

and will output the same as in the previous example.

typical use within a script is:

eval $(parse_yaml sample.yml)

parse_yaml accepts a prefix argument so that imported settings all have a common prefix (which will reduce the risk of namespace collisions).

parse_yaml sample.yml "CONF_"

yields:

CONF_global_debug="yes"
CONF_global_verbose="no"
CONF_global_debugging_detailed="no"
CONF_global_debugging_header="debugging started"
CONF_output_file="yes"

Note that previous settings in a file can be referred to by later settings:

## global definitions
global:
  debug: yes
  verbose: no
  debugging:
    detailed: no
    header: "debugging started"

## output
output:
   debug: $global_debug

Another nice usage is to first parse a defaults file and then the user settings, which works since the latter settings overrides the first ones:

eval $(parse_yaml defaults.yml)
eval $(parse_yaml project.yml)