$ /Applications/Mathematica.app/Contents/SystemFiles/Links/WSTPServer/wstpserver

WSTPServer: Logger (0, 2020-09-29 16:50:00): Information: 
    Using "/Applications/Mathematica.app/Contents/MacOS/WolframKernel" as a default kernel path

WSTPServer: Logger (0, 2020-09-29 16:50:05): Information: 
    Listening for new connections on port 31415

...

$ "C:\Program Files\Wolfram Research\Mathematica\12.2\SystemFiles\Links\WSTPServer\wstpserver.exe"

WSTPServer: Logger (0, 2020-09-29 16:50:00): Information: 
    Using "C:\Program Files\Wolfram Research\Mathematica\12.2\wolfram.exe" as a default kernel path

WSTPServer: Logger (0, 2020-09-29 16:50:05): Information: 
    Listening for new connections on port 31415

...

$ /usr/local/Wolfram/Mathematica/12.2/SystemFiles/Links/WSTPServer/wstpserver

WSTPServer: Logger (0, 2020-09-29 16:50:00): Information: 
    Using "/usr/local/Wolfram/Mathematica/12.2/Executables/WolframKernel" as a default kernel path

WSTPServer: Logger (0, 2020-09-29 16:50:05): Information: 
    Listening for new connections on port 31415

...

$ wolframscript -wstpserver -code "StringReverse[\"hello\"]"
olleh

$ wolframscript -wstpserver -startprofile sampleprofile -code "x = 2 + 2"
4

$ wolframscript -wstpserver -continueprofile sampleprofile -code "x"
4

$ wolframscript -wstpserver 31416 -continueprofile sampleprofile -code "x"
4

-linkmode connect -linkname port[@host] -linkprotocol TCPIP -linkoptions 4

/************************************
    include directives
*************************************/

#include "stdio.h" /* printf */

#include "wstp.h" /* WSENV, WSLINK, WSInitialize, WSOpenArgcArgv, ... */

/************************************
    define directives
*************************************/

#define WSTP_ENVIRONMENT_ERROR 1
#define WSTP_OPEN_ERROR 2
#define WSTP_ACTIVATION_ERROR 3
#define WSTP_INPUTNAMEPACKET_ERROR 4
#define WSTP_GENERAL_ERROR 5

/************************************
    connecting to a link server
*************************************/

/* entry point */
int main(int argc, char **argv)
{
    WSENV env;
    WSLINK link;

    int error;

    /* initialize a WSTP environment */
    env = WSInitialize(NULL);
    if(env == NULL)
    {
        (void) printf("%s\n", "Could not initialize a WSTP environment");
        return WSTP_ENVIRONMENT_ERROR;
    }

    /* open a link to a WSTPServer listening on port 31415 on the local machine */
    link = WSOpenString(env, "-linkmode connect -linkname 31415 -linkprotocol TCPIP -linkoptions 4", &error);
    if(link == NULL || error)
    {
        (void) printf("%s\n", "Could not open a link to WSTPServer");
        return WSTP_OPEN_ERROR;
    }

    /* activate the link to WSTPServer */
    error = WSActivate(link);
    if(!error)
    {
        (void) printf("%s\n", "Could not activate the link to WSTPServer");
        return WSTP_ACTIVATION_ERROR;
    }

    /* receive InputNamePacket packet */
    if(WSNextPacket(link) != INPUTNAMEPKT)
    {
        (void) printf("%s\n", "Could not receive InputNamePacket packet");
        return WSTP_INPUTNAMEPACKET_ERROR;
    }
    (void) WSNewPacket(link);

    /* send over the link:
        EvaluatePacket[2 + 2] */
    if(
        !(
            WSPutFunction(link, "EvaluatePacket", 1) &&
                WSPutFunction(link, "Plus", 2) &&
                    WSPutInteger(link, 2) &&
                        WSPutInteger(link, 2) &&
                            WSFlush(link)
        )
    )
    {
        (void) printf("%s\n", "General WSTP error occurred");
        return WSTP_GENERAL_ERROR;
    }

    /* read off packets until ReturnPacket */
    while(
        (WSNextPacket(link) != RETURNPKT) &&
            (WSError(link) == WSEOK)
    ) { }
    if(WSError(link) != WSEOK)
    {
        (void) printf("%s\n", "General WSTP error occurred");
        return WSTP_GENERAL_ERROR;
    }

    /* clean up */
    (void) WSClose(link);
    (void) WSDeinitialize(env);

    return 0;
}

{
#    Lines that begin with # are comments
#    "AllowStealingKernels":false,
#    "AllowSilentKernelReplacement":false,
#    "EnableAutomaticKernelConnection":false,
#    "SendInputNamePacketUponKernelConnection":true,
    "Pools":
        {
            "KernelPoolOne":
                {
#                    "Default":true,
#                    Only uncomment one of the "KernelPath" lines below
#                    Make sure to change the value to reflect the actual location of your Wolfram kernel binary
#                    "KernelPath":"C:\\Program Files\\Wolfram Research\\Mathematica\\13.2\\wolfram",
#                    "KernelPath":"/usr/local/Wolfram/Mathematica/13.2/Executables/WolframKernel",
#                    "KernelPath":"/Applications/Mathematica.app/Contents/MacOS/WolframKernel",
#                    "InitializationCode":"x=2+2",
#                    "InitializationFile":"/path/to/your/initialization/file.wl",
#                    "KernelOptions":"-yourkerneloption1 -yourkerneloption2",
#                    "KeepAlive":true,
#                    "MinimumKernelNumber":2,
#                    "ParallelKernels":false,
#                    "ReservePreviouslyConnectedKernels":false
                },
#            "KernelPoolTwo":
#                {
#                    "ParallelKernelDefault":true,
#                    "ParallelKernels":true,
#                    Only uncomment one of the "KernelPath" lines below
#                    Make sure to change the value to reflect the actual location of your Wolfram kernel binary
#                    "KernelPath":"C:\\Program Files\\Wolfram Research\\Mathematica\\13.2\\wolfram",
#                    "KernelPath":"/usr/local/Wolfram/Mathematica/13.2/Executables/WolframKernel",
#                    "KernelPath":"/Applications/Mathematica.app/Contents/MacOS/WolframKernel"
#                }
        }
}

{
    "result":
        {
            # An object containing configured pools and their properties
            "Pools":
                {
                    string: # The name of a particular pool
                        # An object containing the properties of a particular pool and their values
                        {
                            "InitializationCode":string,
                            "InitializationFile":string,
                            "MaximumKernelNumber":number,
                            "MinimumKernelNumber":number,
                            "KeepAlive":Boolean,
                            "EntryPool":Boolean,
                            "KernelPath":string,
                            "PreferFreshKernel":Boolean,
                            "ReservePreviouslyConnectedKernels":Boolean,
                            "ParallelKernels":Boolean,
                            "ParallelKernelEntryPool":Boolean
                        },
                        ...
                }
            }
        }
}

# wstpserver.conf: place in the current working directory
{
    "Pools":
        {
            "IProvideParallelKernels":
                {
                    "Parallel":true,
                    "MinimumKernelNumber":4
                }
        }
}

# wstpserver.conf: place in the current working directory
{
    "Pools":
        {
            "IDefineX":
                {
                    "InitializationCode":"x=4"
                }
        }
}

$ wolframscript -wstpserver -kernelpool AlternativeThree

In[1]:= WSTPServer`GetPoolName[]

Out[1]= IDefineX

In[2]:= x

Out[2]= 4

In[3]:= 

# wstpserver.conf: place in the current working directory
{
    "Pools":
        {
            "IBuildUpState":
                {
                    "KeepAlive":true
                }
        }
}

$ wolframscript -wstpserver -startprofile

In[1]:= WSTPServer`GetKernelID[]

Out[1]= a806-2645-9651

In[2]:= 

In[10]:= WSTPServer`DisconnectKernel[]
$

$ wolframscript -wstpserver -continueprofile

In[11]:= WSTPServer`GetKernelID[]

Out[11]= a806-2645-9651

In[12]:= 

# wstpserver.conf: place in the current working directory
{
    "Pools":
        {
                "IHaveSpecificProperties":
                {
                    "KeepAlive":true,
                    "ReservePreviouslyConnectedKernels":false
                }
        }
}

# sampleprofile
KEEPALIVE=TRUE # I want a kernel with "KeepAlive" set to true
RESERVE=TRUE # I want a kernel with "ReservePreviouslyConnectedKernels" set to false

$ wolframscript -wstpserver -startprofile sampleprofile

In[1]:= 

In[1]:= x = 2 + 2

Out[1]:= 4

In[10]:= WSTPServer`DisconnectKernel[]
$ 

$ wolframscript -wstpserver -continueprofile sampleprofile

In[11]:= x

Out[11]:= 4

$ wolframscript -wstpserver -continueprofile

In[11]:= WSTPServer`GetKernelID[]

Out[11]= 825c-6cbd-129b

In[12]:=