pgrep Command: Finding a PID by Name, the -f Option, and How pidof and pkill Differ

To find the number (PID) of a running process by its name, account, or command line, use the pgrep command. On an AlmaLinux 9 server, we copied the sleep program under different names, started three of them, and compared the processes each option found. Searching by name returned only those three processes, while searching the whole command line with -f also returned the timeout that launched them.

The output below comes from the root account on an AlmaLinux 9.8 server and a regular account on a Debian 12 server. The targets were copies of sleep named pgdemo-a and pgdemo-b, started with timeout 300 so they would exit on their own after 5 minutes. Process numbers (PIDs) and the Debian account name (user1) were replaced with example values, and the procps-ng version on the AlmaLinux server is shown only up to the major and minor version.

1️⃣ Basic pgrep Command Usage

If you write part of a name after pgrep, it prints the numbers of the processes whose names contain those characters, one per line. Searching for pgdemo returned three numbers: two pgdemo-a processes and one pgdemo-b.

pgrep pgdemo

2143
2152
2155

Writing the name up to pgdemo-a left only the two pgdemo-a processes.

pgrep pgdemo-a

2143
2152

The search text is read as an extended regular expression, and without options pgrep looks only at process names.

When no process matches, pgrep prints nothing and exits, so check the result with the exit code. After searching for the nonexistent name pgdemo-zz, running [echo $?] returned 1.

pgrep pgdemo-zz

echo $?

1

Running it without any criteria produced the error below and exit code 2. The man page describes the exit codes as 0 (found), 1 (not found), 2 (command-line syntax error), and 3 (fatal error such as running out of memory).

pgrep

pgrep: no matching criteria specified
Try `pgrep --help' for more information.

echo $?

2

By using this exit code as a condition instead of the output, a script can check whether a process is running. Check the version with [pgrep -V]. The Debian 12 server had procps-ng 4.0.2, and the AlmaLinux 9.8 server had procps-ng 3.3.

pgrep -V

pgrep from procps-ng 4.0.2

2️⃣ The pgrep -f Option and -l, -a Output

The number alone does not tell you which process it is, so print the name with -l or the whole command line with -a.

pgrep -l pgdemo

2143 pgdemo-a
2152 pgdemo-a
2155 pgdemo-b

pgrep -a pgdemo

2143 ./pgdemo-a 300
2152 ./pgdemo-a 301
2155 ./pgdemo-b 300

The 300 and 301 at the end of the -a output are the arguments in seconds passed to sleep. These arguments told the two pgdemo-a processes with the same name apart.

With -f, pgrep searches the whole command line instead of the name. Searching for pgdemo-a with -f raised the count from two numbers to four.

pgrep -f pgdemo-a

2141
2143
2150
2152

Adding -a showed that the two extra lines were the timeout processes that received pgdemo-a as an argument. Their name is timeout, but their command line contains pgdemo-a, so they matched -f.

pgrep -af pgdemo-a

2141 timeout 300 ./pgdemo-a 300
2143 ./pgdemo-a 300
2150 timeout 300 ./pgdemo-a 301
2152 ./pgdemo-a 301

When you use -f inside a script, check whether the script itself shows up in the results. pgrep never includes itself in the results, but if the command line of the script that ran pgrep contains the search text, that script is listed too.

Searching for pgdemo-a with chk.sh, a script that passes its argument straight to [pgrep -af], added bash chk.sh pgdemo-a, the script itself, as the last line.

bash chk.sh pgdemo-a

2141 timeout 300 ./pgdemo-a 300
2143 ./pgdemo-a 300
2150 timeout 300 ./pgdemo-a 301
2152 ./pgdemo-a 301
2160 bash chk.sh pgdemo-a

The pattern [p]gdemo-a, which wraps the first letter in brackets, matches the same text as pgdemo-a as a regular expression, but the script's command line contains the brackets literally, so it no longer matches. Passing this pattern to the same script dropped the script line and left only four lines.

bash chk.sh '[p]gdemo-a'

2141 timeout 300 ./pgdemo-a 300
2143 ./pgdemo-a 300
2150 timeout 300 ./pgdemo-a 301
2152 ./pgdemo-a 301

3️⃣ pgrep Matching Options

With -x, pgrep finds only processes whose whole name matches, not part of it. Searching for pgdemo returned nothing, while searching for pgdemo-a returned two.

pgrep -x pgdemo

pgrep -lx pgdemo-a

2143 pgdemo-a
2152 pgdemo-a

-u selects processes by the account that started them. If you list several accounts separated by commas, it finds every process that belongs to any one of them. The three processes started as root all appeared with [-u root], and [-u daemon] returned none.

pgrep -u root -l pgdemo

2143 pgdemo-a
2152 pgdemo-a
2155 pgdemo-b

pgrep -u daemon -l pgdemo

pgrep -u root,daemon -c pgdemo

3

Giving an account that does not exist produced the [invalid user name] error and exit code 2.

pgrep -u no-such-user9 pgdemo

pgrep: invalid user name: no-such-user9

On the Debian 12 server, a regular account also found the single sleep it had started under its own account with the same options.

pgrep -u user1 -l sleep

3047 sleep

pgrep -u user1 -a sleep

3047 sleep 60

When several processes share a name, -n prints only the most recently started one and -o only the oldest one. Of two pgdemo-a processes started 2 seconds apart, -n picked the later one and -o the earlier one.

pgrep -n pgdemo-a

2152

pgrep -o pgdemo-a

2143

The man page says -n and -o cannot be used together. Giving both printed only the usage text, with no error message.

-c prints a count instead of numbers. When nothing matched, it printed 0 and the exit code was 1.

pgrep -c pgdemo

3

pgrep -c pgdemo-zz

0

Here is a summary of the pgrep options.

🔲 pgrep Command Simulator

The simulator below starts with a list of the six pgdemo and timeout lines plus example processes. Tap the chips to compare the lines each option finds in the list on the left, and check how the result shrinks with [pgrep -l pgdemo] after [pkill pgdemo-b].

4️⃣ pgrep vs pidof and pkill

pidof finds only processes whose program name matches exactly, without regular expressions, and prints the numbers on one line separated by spaces. Searching for pgdemo-a printed both numbers on one line, with 2152, the one started later, first.

pidof pgdemo-a

2152 2143

Unlike pgrep, pidof found nothing for the partial name pgdemo, and the exit code was 1. If you need just one number, add -s. [pidof -s pgdemo-a] printed only 2152.

pidof pgdemo

pidof -s pgdemo-a

2152

To see the start time and parent number (PPID) of the numbers you found, join them with commas using pgrep -d and pass them to -p of the ps command.

pgrep -d, pgdemo

2143,2152,2155

ps -fp $(pgrep -d, -x pgdemo-a)

UID          PID    PPID  C STIME TTY          TIME CMD
root        2143    2141  0 15:59 ?        00:00:00 ./pgdemo-a 300
root        2152    2150  0 15:59 ?        00:00:00 ./pgdemo-a 301

2141 and 2150 in the PPID column are the numbers of the timeout processes that started the two pgdemo-a processes.

pkill finds processes with the same criteria as pgrep, but sends them a signal instead of printing their numbers. Without a signal it sends SIGTERM, and you can change the signal as in [pkill -HUP name].

pkill sends the signal as soon as it finds a match, so check the targets first with [pgrep -a] or [pgrep -af] using the same criteria. With -f, a timeout or a script whose command line contains the same text becomes a target too. If you already know the number, use the kill command to send a signal to that number only.

🔢 FAQ & Recommended Content

🔗 Recommended Reading

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