Syntax for Photoshop actions

Actions are intended to be recorded. That being said, xbytor's xtools does offer an action to XML and XML to action conversion script. Writing actions manually as XML would be tortuous though IMHO, as you would have to reverse engineer actions to work out the format.

As Ged mentioned, you should probably look into scripting as an alternative.

xtools here:

https://sourceforge.net/projects/ps-scripts/files/xtools/v2.3/

The above three step action, select all + copy + paste is a 27 line XML file using xbytor's script. In order for the XML to action script to convert the manually written file to an action, you would have to get the XML correct.

<ActionFile key="Action4.atn" file="/private/var/folders/qn/j2dj8gf53ys13x9tqgh59h880000gq/T/TemporaryItems/Action4.atn">
  <ActionSet version="16" name="Action4" expanded="false" count="1">
    <Action key="1" name="Action 4" expanded="false" count="3">
      <ActionItem key="TEXT" expanded="false" enabled="true" withDialog="false" dialogOptions="2" identifier="TEXT" event="set" name="Set" hasDescriptor="true">
        <ActionDescriptor key="set" count="2">
          <DescValueType.REFERENCETYPE key="1853189228" id="1853189228" symname="Null" sym="null">
            <ActionReference key="1853189228" id="1853189228" symname="Null" sym="null" count="1">
              <ReferenceFormType.PROPERTY key="1130917484" id="1130917484" symname="Channel" sym="Chnl" desiredClassString="Channel" desiredClass="Chnl" property="1718838636" propertyName="selection" propertySym="selection"/>
            </ActionReference>
          </DescValueType.REFERENCETYPE>
          <DescValueType.ENUMERATEDTYPE key="1411391520" id="1411391520" symname="To" sym="T   " enumeratedTypeString="Ordinal" enumeratedType="Ordn" enumeratedValueString="All" enumeratedValue="Al  "/>
        </ActionDescriptor>
      </ActionItem>
      <ActionItem key="TEXT" expanded="false" enabled="true" withDialog="false" dialogOptions="2" identifier="TEXT" event="copyEvent" name="Copy" hasDescriptor="false"/>
      <ActionItem key="TEXT" expanded="false" enabled="true" withDialog="false" dialogOptions="2" identifier="TEXT" event="paste" name="Paste" hasDescriptor="true">
        <ActionDescriptor key="paste" count="2">
          <DescValueType.ENUMERATEDTYPE key="1097757761" id="1097757761" symname="AntiAliasedPICTAcquire" sym="AntA" enumeratedTypeString="AntiAlias" enumeratedType="Annt" enumeratedValueString="AntiAliasNone" enumeratedValue="Anno"/>
          <DescValueType.CLASSTYPE key="1098063904" id="1098063904" symname="As" sym="As  " classString="Pixel" class="Pxel"/>
        </ActionDescriptor>
      </ActionItem>
    </Action>
  </ActionSet>
</ActionFile>

Whereas with scripting using ExtendScript/JavaScript, the equivalent lines of "DOM" code would be:

app.activeDocument.selection.selectAll();
app.activeDocument.selection.copy();
app.activeDocument.paste();
app.activeDocument.selection.deselect();