[linux] How to find out which processes are using swap space in Linux?

Yet two more variants:

Because top or htop could be not installed on small systems, browsing /proc stay always possible.

Even on small systems, you will found a shell...

A shell variant! (Not bash only)

This is exactly same than lolotux script, but without any fork to grep, awk or ps. This is a lot quicker!

And as bash is one of the poorest shell regarding performance, a little work was done to ensure this script will run well under dash, busybox and some other. Then, (thanks to Stéphane Chazelas,) become a lot quicker again!

#!/bin/sh 
# Get current swap usage for all running processes
# Felix Hauri 2016-08-05
# Rewritted without fork. Inspired by first stuff from
# Erik Ljungstrom 27/05/2011
# Modified by Mikko Rantalainen 2012-08-09
# Pipe the output to "sort -nk3" to get sorted output
# Modified by Marc Methot 2014-09-18
# removed the need for sudo

OVERALL=0
rifs=`printf ': \t'`
for FILE in /proc/[0-9]*/status ;do
    SUM=0
    while IFS="$rifs" read FIELD VALUE ;do
        case $FIELD in
            Pid )    PID=$VALUE      ;;
            Name )   PROGNAME="$VALUE" ;;
            VmSwap ) SUM=$((SUM=${VALUE% *}))  ;;
        esac
    done <$FILE
    [ $SUM -gt 0 ] &&
        printf "PID: %9d  swapped: %11d KB (%s)\n" $PID $SUM "$PROGNAME"
    OVERALL=$((OVERALL+SUM))
done
printf "Total swapped memory: %14u KB\n" $OVERALL

Don't forgot to double quote "$PROGNAME" ! See Stéphane Chazelas's comment:

read FIELD PROGNAME < <(
    perl -ne 'BEGIN{$0="/*/*/../../*/*"} print if /^Name/' /proc/self/status
)
echo $FIELD "$PROGNAME"

Don't try echo $PROGNAME without double quote on sensible system, and be ready to kill current shell before!

And a perl version

As this become a not so simple script, time is comming to write a dedicated tool by using more efficient language.

#!/usr/bin/perl -w

use strict;
use Getopt::Std;
my ($tot,$mtot)=(0,0);
my %procs;

my %opts;
getopt('', \%opts);

sub sortres {
    return $a <=> $b                                          if $opts{'p'};
    return $procs{$a}->{'cmd'} cmp $procs{$b}->{'cmd'}        if $opts{'c'};
    return $procs{$a}->{'mswap'} <=> $procs{$b}->{'mswap'}    if $opts{'m'};
    return $procs{$a}->{'swap'} <=> $procs{$b}->{'swap'};
};

opendir my $dh,"/proc";

for my $pid (grep {/^\d+$/} readdir $dh) {
    if (open my $fh,"</proc/$pid/status") {
        my ($sum,$nam)=(0,"");
        while (<$fh>) {
            $sum+=$1 if /^VmSwap:\s+(\d+)\s/;
            $nam=$1 if /^Name:\s+(\S+)/;
        }
        if ($sum) {
            $tot+=$sum;
            $procs{$pid}->{'swap'}=$sum;
            $procs{$pid}->{'cmd'}=$nam;
            close $fh;
            if (open my $fh,"</proc/$pid/smaps") {
                $sum=0;
                while (<$fh>) {
                    $sum+=$1 if /^Swap:\s+(\d+)\s/;
                };
            };
            $mtot+=$sum;
            $procs{$pid}->{'mswap'}=$sum;
        } else { close $fh; };
    };
};
map {
    printf "PID: %9d  swapped: %11d (%11d) KB (%s)\n",
        $_, $procs{$_}->{'swap'}, $procs{$_}->{'mswap'}, $procs{$_}->{'cmd'};
} sort sortres keys %procs;
printf "Total swapped memory: %14u (%11u) KB\n", $tot,$mtot;

could by run with one of

-c  sort by command name
-p  sort by pid
-m  sort by swap values
by default, output is sorted by status's vmsize