Why doesn't "pgrep firefox" work on Ubuntu 20.04

Question

As the title suggests, the following command on Ubuntu 20.04:

pgrep firefox

... returns no results.

Meanwhile, the following command:

ps aux |grep firefox

... returns 6 results, 5 of which should match pgrep firefox. What's wrong with the original pgrep command on Ubuntu?

ubuntu     81646  1.9  7.9 3063696 300208 ?      Sl   13:12   0:29 /usr/lib/firefox/firefox -new-window
ubuntu     81891  0.2  3.8 2473448 144796 ?      Sl   13:12   0:03 /usr/lib/firefox/firefox -contentproc -childID 1 -isForBrowser -prefsLen 1 -prefMapSize 222536 -parentBuildID 20200720193547 -appdir /usr/lib/firefox/browser 81646 true tab
ubuntu     82020  0.1  3.7 2438972 141396 ?      Sl   13:12   0:02 /usr/lib/firefox/firefox -contentproc -childID 3 -isForBrowser -prefsLen 897 -prefMapSize 222536 -parentBuildID 20200720193547 -appdir /usr/lib/firefox/browser 81646 true tab
ubuntu     82051  0.0  2.8 2401248 107520 ?      Sl   13:12   0:01 /usr/lib/firefox/firefox -contentproc -childID 4 -isForBrowser -prefsLen 6935 -prefMapSize 222536 -parentBuildID 20200720193547 -appdir /usr/lib/firefox/browser 81646 true tab
ubuntu     83658  0.0  2.0 2387704 78496 ?       Sl   13:12   0:00 /usr/lib/firefox/firefox -contentproc -childID 6 -isForBrowser -prefsLen 10362 -prefMapSize 222536 -parentBuildID 20200720193547 -appdir /usr/lib/firefox/browser 81646 true tab
ubuntu     90700  0.0  0.0  17664   664 pts/0    S+   13:37   0:00 grep --color=auto firefox

Running the same command on Ubuntu 18.04 and macOS works just fine.

Answer 1

pgrep matches against the process name, as reported by ps without -f while pgrep -f matches against the argument list as reported by ps -f or your non-standard ps aux.

firefox changes the names of its threads to identify what each does.

Try (assuming the default value of $IFS):

ps -L -o comm= -p $(pgrep -wf firefox)

Which lists the names of all of the threads from processes with an argument list that contains firefox, and you'll see a lot of things like:

MainThread
gmain
gdbus
Gecko_IOThread
Timer
Netlink Monitor
Socket Thread
Permission
JS Watchdog
JS Helper
[...]

So for firefox, you may want to match on argument list (argument list from the last command they or their parents executed if any) with pgrep -f firefox (add -w if you want to list all threads) rather than process/thread name. Or pgrep -f '^[^ ]*firefox' to match only on the first word of that arg list (pgrep joins the arguments with spaces before matching, like in ps -f).

In any case, any process can set or change their process name or argument list to anything they want¹, so finding processes that way is never foolproof.

If you wanted to match on process that are currently running the /usr/lib/firefox/firefox executable, on Linux and with zsh:

print -rC1 /proc/<1->/exe(Ne['[[ $REPLY -ef /usr/lib/firefox/firefox ]]']:h:t)

Or for the threads:

print -rC1 /proc/<1->/task/*/exe(Ne['[[ $REPLY -ef /usr/lib/firefox/firefox ]]']:h:t)

(note that if you don't have superuser privileges, you may not be able to get that information for processes not running as you).

Or possibly slightly more portably with lsof:

lsof -Fd /usr/lib/firefox/firefox | sed '/^p/h;/^ftxt/!d;g;s/^p//'

Or for threads:

lsof -K -FKd /usr/lib/firefox/firefox | sed '/^K/h;/^ftxt/!d;g;s/^K//'

---

^{¹ though note that process name length is often limited (15 bytes on Linux), and depending on OS and OS version, not all of the arg list is easily made available by the system for inspection.}

Answer 2

Why doesn't pgrep firefox work on Ubuntu 20.04

Because it's a bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1649010.

A workaround is:

pgrep "firefox|MainThread"

Why this works?

• When Firefox 79 on Ubuntu is running, it writes a value of MainThread to /proc/<pid>/comm (thanks to @steeldriver for pointing this out in the comments/questions section above)
• pgrep then uses uses the value from /proc/<pid>/comm, to find match the command
• The | pipe delimiter allows pgrep to match multiple values. Keeping firefox AND MainThread is recommended so that this command still succeeds on older versions -- such as Ubuntu 18.04 + Firefox 78 -- where pgrep firefox is still working fine.

Why might this be inaccurate?

• WARNING: Any process with a process name of MainThread that is NOT firefox will also report as running using this technique. Use with caution.
• WARNING: Multiple processes may be active. See this answer to find out why: https://unix.stackexchange.com/a/607242/190347
• NOTE: For an exact match pgrep -x "firefox|MainThread" is recommended

What other options do we have?

• As @steeldriver points out, one can infer that pgrep -f firefox is a suitable replacement. It suffers a similar fate as MainThread where it may match commands which contain firefox in them (such as a directory name, file name, etc). For this reason, I find it more likely to match rogue processes -- and thus more likely to throw a false-positive -- than the MainThread string. This is a hunch based on experience, not documented evidence however a quick search on GitHub shows that MainThread is not a common project name (58 results, as opposed to firefox matching 21,000 repositories)
• Alternately, using a combination of commands/techniques to filter out false-positives may be preferred over all others (such as finding a union of results from both commands)
• Ask. Knowledge of the bug came after asking Mozilla directly why this is happening: https://bugzilla.mozilla.org/show_bug.cgi?id=1662232