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

Question

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.

Answer 1

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)