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I'm experimenting with the top(1) program, and I'm trying to figure out how to display only specified program names. For example, if I invoke a command line like

$ ssh localhost sleep 15

I'd like to be able to filter top's display to show only processes whose program names are sshd and sleep.

When top is running, if I press 'O' (uppercase oh == case-sensitive filter) and add a filter COMMAND=sshd, then I see what I expect: top displays only those processes whose program names start with sshd. If I delete that filter (press '=') and create a new filter COMMAND=sleep, I see the processes whose programs begin with sleep.

However, if I create two filters where filter 1 is COMMAND=sshd and filter 2 is COMMAND=sleep, then top doesn't display anything at all, even when there are running sshd and sleep processes. So apparently this method of filter creation produces an AND relationship: show if COMMAND=sshd && COMMAND=sleep.

So how do I create a filter (or filter set) that produces the desired OR relationship: show if COMMAND=sshd || COMMAND=sleep?

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With open source you can look at the code and see if it might be easy to do what you want. On my old rpm-based Fedora, I could easily download and unpack the source with

$ rpm -qf /usr/bin/top
procps-ng-3.3.10-11.fc24.x86_64
$ dnf download --source procps-ng
$ rpm -i procps-ng-3.3.10-11.fc24.src.rpm 
$ rpmbuild -bp ~/rpmbuild/SPECS/procps-ng.spec --nodeps
$ cd ~/rpmbuild/BUILD/procps-ng-3.3.10/

and the code is in top/top.c. Looking for the word filter eventually shows that function other_selection() is implementing the o or O command with a small C struct in which sel = strcasestr or sel = strstr is being set for input such as COMMAND=somestring. You can see the gitlab code for this release. It has changed since, but is similar.

A means to implement an or between 2 strings looked a little difficult, but a simple alternative seems to accept a regular expression instead of just doing a string comparison. You can then just change the assignment to sel = myfunction and write a function that takes 2 strings and does a regexec() instead of strstr().

If you prefer not to download and compile the program, you can write a small shim to load in front of the C library that overrides the strstr() function with your own. However, I found this function was used in other places by top, so I preferred to override strcasestr(), which seems to be only used for the o command. Create a file shim_strcasestr.c with the following:

/* capture calls to a routine and replace with your code
 * https://unix.stackexchange.com/a/463461/119298
 * gcc -Wall -O2 -fpic -shared -ldl -o shim_strcasestr.so shim_strcasestr.c */
#define _GNU_SOURCE /* needed to get RTLD_NEXT defined in dlfcn.h */
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <dlfcn.h>
#include <regex.h>
#define NUMMATCH 1 /* max num matching capture groups in pattern */
char *strcasestr(const char *haystack, const char *needle){
    static char *(*real_strcasestr)(const char *haystack, const char *needle) = NULL;
    regex_t myexpn;
    regmatch_t matches[NUMMATCH] = {0};
    if (!real_strcasestr) {
        real_strcasestr = dlsym(RTLD_NEXT, "strcasestr");
        char *error = dlerror();
        if (error != NULL) {
            fprintf(stderr, "%s\n", error);
            exit(1);
        }
    }
    if(needle[0]!='/') return real_strcasestr(haystack,needle);
    int rc = regcomp(&myexpn, needle+1, REG_EXTENDED);
    if(rc!=0)return NULL;
    rc = regexec(&myexpn, haystack, NUMMATCH, matches, 0);
    regfree(&myexpn);
    if(rc==REG_NOMATCH)return NULL;
    return (char*)haystack;
}

and compile it as shown in the comment. You can then run it as LD_PRELOAD=./shim_strcstr.so /usr/bin/top and when you type the o key, you can enter, for example, COMMAND=/sshd|sleep and get the desired result. I added a check for an initial / in the string so you can still get the original anycase matching if you omit it. Obviously, this code can be optimised to cache the regcomp() result.

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