Day 7 - Processes and Job Control

This lesson explains how Linux runs programs and how to manage them from the shell. It covers listing and filtering processes, shell job control, signals, foreground and background execution, persistence across logout, priority tuning with nice and renice, and quick inspection methods.

Prerequisites

• Day 1 through Day 6 completed
• Terminal with Bash and a user account with sudo

Listing and filtering processes

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# snapshot of processes for current user
ps -u "$USER" -o pid,ppid,tty,stat,cmd --sort=pid | head -20

# full system view in BSD style
ps aux | head -20

# tree view to see parents and children
ps axjf | head -30

# find by name or user
pgrep -a ssh
pgrep -u "$USER" -l bash

# list PIDs and commands owned by a user\pps -U "$USER" -o pid,stat,comm --sort=pid | column -t | head

top gives an updating view. Press q to quit.

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top

Useful keys inside top:

• P sort by CPU, M sort by memory
• k send signal to a PID, default is SIGTERM
• u filter by user, o add a filter like COMMAND=ssh
• 1 toggle per CPU display

htop is friendlier when installed.

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sudo apt install -y htop 2>/dev/null || sudo dnf install -y htop 2>/dev/null
htop

Job control in the shell

Start a long running command in the background with &.

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sleep 60 &
jobs -l

Suspend a foreground job with Ctrl Z, then continue it.

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# start a long command
sleep 300
# press Ctrl Z to suspend
bg %1            # continue in background
jobs
fg %1            # bring back to foreground

Numbered jobs use the % prefix. jobs -r shows running jobs. jobs -s shows stopped jobs.

Foreground, background, and persistence

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# run immune to hangup and log output to a file
nohup long_task.sh > long_task.log 2>&1 &

# detach an already running job from the shell's job table
disown %1

nohup makes the process ignore SIGHUP and writes output to nohup.out if redirection is not set.

Signals and safe termination

Common signals:

• SIGTERM 15 ask a process to exit cleanly
• SIGKILL 9 force kill without cleanup
• SIGHUP 1 request reload or hangup depending on the program
• SIGINT 2 interrupt, same as Ctrl C

Send signals by PID or by name.

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# by PID
kill -TERM 12345
kill -KILL 12345    # last resort

# by name or pattern
pkill -TERM -u "$USER" sleep
killall -v firefox 2>/dev/null || true

Check that a PID exists.

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kill -0 12345 && echo "running" || echo "not running"

Understanding process states and stats

ps shows a state code in the STAT column.

• R running, S sleeping, D uninterruptible sleep, T stopped, Z zombie
• a trailing + means the process is in the foreground process group of the terminal

Quick CPU and memory usage by PID.

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ps -p 1,2,3 -o pid,stat,%cpu,%mem,cmd

Measure a command's resource use.

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/usr/bin/time -v sleep 2

Monitor output repeatedly.

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watch -n 2 'ps -o pid,%cpu,%mem,cmd -C sshd'

Priority and niceness

Niceness ranges from -20 to 19. Lower is higher priority. Normal users can only increase niceness. Root can decrease it.

Start a program with a given niceness.

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nice -n 10 ./cpu_heavy_task

Change an existing process.

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# show PID then adjust priority
pgrep -a cpu_heavy_task
sudo renice -n 15 -p <PID>

Quick inspection under /proc

Each PID has a directory under /proc.

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# replace 1 with a real PID
PID=1
ls -l /proc/$PID
ls -l /proc/$PID/fd | head
cat /proc/$PID/cmdline | tr '\0' ' ' && echo
cat /proc/$PID/status | head -20
readlink -f /proc/$PID/exe
readlink -f /proc/$PID/cwd

List processes holding a file or port with lsof when installed.

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sudo apt install -y lsof 2>/dev/null || sudo dnf install -y lsof 2>/dev/null
sudo lsof /var/log/syslog 2>/dev/null | head
sudo lsof -i :22 2>/dev/null

pstree shows ancestry as a tree.

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sudo apt install -y psmisc 2>/dev/null || sudo dnf install -y psmisc 2>/dev/null
pstree -ap | head -30

Practical lab

The lab builds confidence with process discovery and control.

1. Start a long lived program and manage it as a job.

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sleep 600 &
jobs -l
fg %1      # bring to foreground
# press Ctrl Z
bg %1

2. Start a CPU heavy workload and adjust its niceness.

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# simple CPU burner
yes > /dev/null &
pgrep -a yes
renice -n 15 -p $(pgrep -n yes)

3. Send signals and observe behavior.

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PID=$(pgrep -n yes)
kill -TERM "$PID"
# if still running after a few seconds
kill -KILL "$PID"

4. Keep a program alive across logout.

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nohup bash -c 'for i in $(seq 1 5); do date; sleep 10; done' > ~/nohup-demo.log 2>&1 &
disown %+

5. Inspect a process in /proc.

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PID=$$   # current shell
cat /proc/$PID/cmdline | tr '\0' ' ' && echo
readlink -f /proc/$PID/exe
ls -l /proc/$PID/fd | head

Troubleshooting

• Operation not permitted when lowering niceness. Only root can set negative niceness values.
• no job control in this shell appears in non interactive shells or when job control is disabled. Use an interactive terminal.
• A background process keeps stopping with Stopped (tty input). Redirect input from /dev/null or a file.
• kill reports no such process. The PID may have already exited. Refresh with pgrep or ps.
• nohup created nohup.out in an unexpected directory. Run the command from the desired working directory or set explicit redirection.

Next steps

Day 8 introduces package management across major distributions. It explains repositories, indexes, signatures, and the commands used to search, install, remove, and update software with apt, dnf, and pacman. It also covers cleaning caches and verifying package provenance.