This action tag, as described previously, executes one or more commands, which can either be internal GPS commands (written in any of the scripting language supported by GPS), or external commands provided by executables found on the PATH.
The command line for each of these commands can either be hard-coded in the
customization file, or be the result of previous commands executed as part of
the same action. As GPS executes each command from the action in turn, it
saves its output on a stack as needed. If a command line contains a special
construct %1, %2... then these constructs will be replaced
by the result of respectively the last command executed, the previous from last
command, and so on. They are replaced by the returned value of the command, not
by any output it might have done to some of the consoles in GPS.
Every time you execute a new command, it pushes the previous %1, %2... parameters one step further on the stack, so that they become respectively %2, %3... and the output of that command becomes %1.
The result value of the previous commands is substituted exactly as is. However, if the output is surrounded by quotes, they are ignored when a substitution takes place, so you need to put them back if they are needed. The reason for this behavior is so that for scripting languages that systematically protect their output with quotes (simple or double), these quotes are sometimes in the way when calling external commands.
<?xml version="1.0" ?>
<quotes>
<action name="test quotes">
<shell lang="python">'-a -b -c'</shell>
<external> echo with quotes: "%1"</external>
<external> echo without quotes: %2</external/>
</action>
</quotes>
If one of the commands in the action raises an error, the execution of the action is stopped immediately, and no further command is performed.