Commands

$ chmod 644 file
$ chmod g+r file

Change a file’s permission bits.

Two notations, and both are worth knowing. Symbolic (g+r) changes only what you name and leaves the rest alone. Numeric (644) sets all nine bits at once. Reach for symbolic when adjusting one thing, numeric when you want a known end state.

$ chmod 600 ~/.ssh/id_ed25519

Careful: Removing your own read or execute bit is easy and immediately confusing. Numeric notation overwrites every bit, not just the one you were thinking about.

File permissions and links umask ls man page

$ curl -sSI https://example.com

Make an HTTP request and show what comes back.

The fastest way to prove a service answers, and to see the headers it sets. -I fetches headers only, -s silences the progress meter, and -S puts errors back after -s removed them.

$ curl -sS -o /dev/null -w '%{http_code} %{time_total}s\n' https://example.com

The OSI model dig ss man page

$ dig +short example.com

Query DNS and show the answer.

Use it rather than ping when the question is about name resolution. +short gives just the answer; @resolver asks a specific server, which is how you tell “DNS is broken” apart from “this resolver is broken”.

$ dig +short @9.9.9.9 example.com

Network protocols and ports curl man page

$ free -h

Show physical memory and swap use.

Reads its numbers from /proc/meminfo. The available column is the one to look at — Linux deliberately uses spare memory for cache, so a small free figure is normal rather than a problem.

Linux abstraction layers ps man page

$ grep -r pattern path

Find lines matching a pattern.

Most often the middle of a pipeline rather than a command on its own. -r recurses, -n gives line numbers, -i ignores case, and -v inverts the match.

$ ps -ef | grep sshd

Streams, redirection, and pipes ps man page

$ ip -br address
$ ip route

Show and configure addresses, routes, and interfaces.

The modern replacement for ifconfig and route. -br gives a brief one-line-per-interface view, -4 and -6 restrict it to one family, and ip -s link adds error and drop counters — a link can negotiate fine and still be marginal.

$ ip -6 neigh show

IPv4 addressing IPv6 addressing ss dig man page

$ ln -s target linkname

Create a link to a file or directory.

With -s it makes a symbolic link. The argument order is the thing that catches people: the target that already exists comes first, the name being created second.

File permissions and links ls man page

$ ls -l
$ ls -la

List directory contents.

-l is the form worth reading closely: ten characters of mode, then links, owner, group, size, time, and name. -a includes dot files, which is most of a home directory.

$ ls -ld /bin

File permissions and links chmod ln man page

$ man 5 passwd
$ man -k "disk space"

Read the manual page for a command, file format, or system call.

Sections matter — the same name can appear in several. Section 1 is user commands, 5 is file formats, 8 is administration. man -k searches the descriptions when you do not know the name. Press / to search within a page.

The shell and the command line ls man page

$ ping -c2 host

Send ICMP echo requests to test reachability.

Proves a host answers ICMP. It says nothing about whether a service is listening — use ss or curl for that. Plenty of hosts drop ICMP by policy, so silence is not proof of absence.

Careful: A quiet ping is weak evidence in both directions. Do not conclude a host is down from it.

IPv4 addressing ss curl ip man page

$ ps -ef
$ ps -ef --forest

List running processes.

--forest draws the parent/child tree, which makes the result of fork() visible — every process descends from the first one. Usually piped into grep to find a particular service.

$ ps -ef --forest | head -20

Linux abstraction layers grep free man page

$ ss -tln

Show sockets — what is listening, and what is connected.

The replacement for netstat. -t is TCP, -l listening only, -n skips name resolution so the output is fast and unambiguous. Compare services bound to loopback against those bound to a wildcard: the first group is reachable only from the machine itself.

$ ss -tlnp

Network protocols and ports ip ping man page

$ tar czf archive.tar.gz dir/
$ tar tvf archive.tar.gz

Bundle many files into one archive, optionally compressed.

The flags are positional and old-fashioned: c create, x extract, t list, v verbose, f naming the file, which must come last of the letters. Run tar tvf before extracting so you know whether the archive expands into a directory or scatters files where you stand.

$ zcat archive.tar.gz | tar xvf -

Careful: Ownership is restored by numeric ID and needs root to reproduce. ACLs, extended attributes, and SELinux contexts each need asking for explicitly.

Archives and compression ls man page

$ tcpdump -i eth0 udp port 4789

Capture and print packets crossing an interface.

The instrument for questions no status command answers — what is actually on the wire, and whether an encapsulated frame really carries what you think. Needs root, and a filter expression, or it will drown you.

Careful: Captures may contain credentials and payload data. Use purpose-built fixtures for examples you intend to share.

Software-defined networking ip ss man page

$ umask 022
$ umask 077

Set the permission bits removed from newly created files.

Subtractive, and from different bases: 666 for files and 777 for directories. 022 gives 644 and 755; 077 gives 600 and 700. It applies to the shell that runs it, so persistence means putting it in a startup file.

File permissions and links chmod man page