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Files & Processes

Read and set Unix permissions (the rwx bits and their octal form) and manage running programs with ps, kill, jobs, and background control.

Two things every shell user must control: who may touch a file, and what is running. The first is the permission model; the second is process and job control. Both look cryptic until you see the structure underneath.

Toggle the read/write/execute bits below for the owner, group, and others, and watch the symbolic string (rwxr-x---) and the octal form (750) update together.

The permission bits

Every file has three permission classes — user (owner), group, and other — each with three bits: read, write, ex*ecute. ls -l shows them as a 9-character string after the file type:

$ ls -l deploy.sh
-rwxr-x---  1 learner staff  deploy.sh
 │└┬┘└┬┘└┬┘
 │ u  g  o
 type

Here the owner has rwx, the group has r-x, others have nothing.

Octal: each digit is a class

Each class is a single octal digit, summing r=4, w=2, x=1:

$\text{r}=4,\quad \text{w}=2,\quad \text{x}=1 \;\Rightarrow\; \text{rwx} = 4+2+1 = 7$

So rwxr-x--- is 7 5 0. Common values worth memorizing:

644 — rw-r--r--: a normal file (owner edits, everyone reads).
755 — rwxr-xr-x: a script or directory anyone may run/enter.
600 — rw-------: private file (owner only). Good for secrets and keys.

For a directory, x means “may enter / traverse it” rather than “execute”.

chmod and chown

chmod 755 deploy.sh        # octal: set all nine bits at once
chmod +x deploy.sh         # symbolic: add execute for all classes
chmod u+x,go-w deploy.sh   # add x for user; remove w for group/other
chmod -R 644 docs/         # recurse into a directory tree

chown alice file.txt       # change the owner
chown alice:staff file.txt # change owner and group

Octal sets everything absolutely; symbolic (+, -, = with u/g/o/a) adjusts relative to what’s there.

Processes

A running program is a process with a numeric PID:

ps aux                 # every process: user, PID, CPU, memory, command
ps aux | grep nginx    # find a specific one
top                    # live, sorted view (q to quit)
kill 4321              # ask process 4321 to terminate (signal TERM)
kill -9 4321           # force it (signal KILL) — last resort
pkill -f "node app"    # kill by matching the command line

Jobs: background and foreground

The shell manages programs you start from it as jobs:

sleep 60 &        # run in the background; prints [1] 4321 (job# and PID)
jobs              # list this shell's jobs
fg %1             # bring job 1 back to the foreground
bg %1             # resume a stopped job in the background

Press Ctrl-Z to suspend the foreground job, then bg to keep it running while you get your prompt back. Append & to launch a long task without blocking the terminal.

Takeaways