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

I wish to provide a structured configuration file which is as easy as possible for a non-technical user to edit (unfortunately it has to be a file) and so I wanted to use YAML. I can't find any way of parsing this from a Unix shell script however.

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)

I've written shyaml in python for YAML query needs from the shell command line.

Overview:

$ pip install shyaml      ## installation

Example's YAML file (with complex features):

$ cat <<EOF > test.yaml
name: "MyName !!"
subvalue:
    how-much: 1.1
    things:
        - first
        - second
        - third
    other-things: [a, b, c]
    maintainer: "Valentin Lab"
    description: |
        Multiline description:
        Line 1
        Line 2
EOF

Basic query:

$ cat test.yaml | shyaml get-value subvalue.maintainer
Valentin Lab

More complex looping query on complex values:

$ cat test.yaml | shyaml values-0 | \
  while read -r -d $'\0' value; do
      echo "RECEIVED: '$value'"
  done
RECEIVED: '1.1'
RECEIVED: '- first
- second
- third'
RECEIVED: '2'
RECEIVED: 'Valentin Lab'
RECEIVED: 'Multiline description:
Line 1
Line 2'

A few key points:

• all YAML types and syntax oddities are correctly handled, as multiline, quoted strings, inline sequences...
• \0 padded output is available for solid multiline entry manipulation.
• simple dotted notation to select sub-values (ie: subvalue.maintainer is a valid key).
• access by index is provided to sequences (ie: subvalue.things.-1 is the last element of the subvalue.things sequence.)
• access to all sequence/structs elements in one go for use in bash loops.
• you can output whole subpart of a YAML file as ... YAML, which blend well for further manipulations with shyaml.

More sample and documentation are available on the shyaml github page or the shyaml PyPI page.