pgrep

Overview

pgrep lists process IDs matching selection criteria (name, full command line, user, terminal, etc.). pkill uses the same selectors but sends a signal instead of printing PIDs. Together they replace brittle ps | grep pipelines for operations and scripts. On Ubuntu both come from procps.

Syntax

pgrep [options] pattern
pkill [options] pattern

Common Options

Shared selection flags (both tools):

Option	Description
-u USER, --uid	Match effective user (name or UID)
-U USER, --euid	Match real user (see man for distinction)
-g PGID / -G GID	Process group / group membership
-t TERM	Controlling terminal (e.g. pts/0)
-P PPID	Children of PPID
-f, --full	Match against full command line, not only comm
-x, --exact	Exact match of the name (or full line with -f)
-n / -o	Newest / oldest match only
-v	Inverse match
-c	Count matches (pgrep)
-d DELIM	PID delimiter (default newline; , useful for htop -p)
-a	Show PID and full command (pgrep)
-l	Show PID and process name (pgrep)
-i	Case-insensitive
-r	Ignore descendants of pgrep/pkill itself (where supported)

pkill-specific:

Option	Description
-SIGNAL / -s SIGNAL	Signal name or number (default TERM)
-e, --echo	Print what will be signaled (newer procps)
-q	Quiet

Safety

• Broad patterns (pkill python, pkill java) are classic outage sources — prefer -x, -f with a specific string, and -u.
• Escalate signals: TERM → wait → KILL; pkill -9 should be rare.
• Managed services: prefer systemctl stop/reload over pkill nginx.
• Always inventory with pgrep -a before pkill on production.

Key Use Cases

1. Resolve PIDs for scripts without parsing ps
2. Signal a homogeneous worker pool
3. Count processes for simple health checks
4. Feed PID lists to top/htop/gdb

Examples with Explanations

Example: list matching PIDs

pgrep sshd
pgrep -a nginx
pgrep -l python3

-a shows full command lines — best pre-kill inspection.

Example: exact name match

pgrep -x sshd
pgrep -x nginx

Avoids partial matches against longer comm names when you want precision.

Example: full command line

pgrep -af 'gunicorn.*myapp'
pgrep -f 'python3 /opt/app/worker.py'

-f is essential when every process is python3 or java.

Example: filter by user

pgrep -u www-data -a php-fpm
pgrep -u "$USER" -f train.py

Reduces collateral damage on shared systems.

Example: count for monitoring

pgrep -c -x sidekiq
pgrep -c -u redis redis-server

Exit status is still 0/1 for found/not found; combine carefully in scripts.

Example: newest / oldest only

pgrep -n -x myapp     # newest
pgrep -o -x myapp     # oldest (often the master)

Useful when a master/worker model embeds the same name.

Example: children of a parent

pgrep -P "$(pgrep -xo master-process)"

List direct children by parent PID.

Example: delimiter for other tools

htop -p "$(pgrep -d, nginx)"
top -p "$(pgrep -d, -x myapp)"

Comma-separated PID lists fit htop -p / top -p.

Example: pkill graceful then force

pkill -TERM -x myapp
sleep 3
pkill -KILL -x myapp 2>/dev/null

Same discipline as kill: TERM first, KILL only for stragglers.

Example: pkill with full-line match

pkill -u deploy -f 'uvicorn app.main:app'

Targets one service invocation pattern under one user.

Example: avoid ps | grep

# fragile:
ps aux | grep '[n]ginx'
# better:
pgrep -a nginx

Character-class grep tricks are obsolete for this job.

Understanding Output

• Default: one PID per line.
• Exit status 0 if any match, 1 if none, 2 on syntax/usage error, 3 fatal error (procps conventions).
• pkill exit status follows the same “matched or not” idea — check man page when scripting strict failure modes.

Notes & Pitfalls

• Pattern is an extended regex by default — dots and brackets matter; quote patterns.
• Without -f, matching uses the process name (comm), often truncated to 15 characters historically — use -f for long names.
• Race: process can exit between pgrep and kill; handle missing PIDs.
• pkill/pgrep may see processes in other sessions; still restricted by permissions for signaling.
• In containers, you only see PIDs in your PID namespace.

Related Commands

• pkill — signal instead of list (same selectors)
• kill / killall — by PID or simple name
• pidof — simpler name → PID (fewer filters)
• ps — rich table and custom columns
• systemctl — prefer for supervised units

Additional Resources

• man pgrep
• man pkill