1 Common usage

See how a Unicode string is encoded internally

$ echo hello | hexdump -C

See how it is encoded in UTF-16: (Assume UTF-8 is the default encoding)

$ echo hello | iconv -f utf-8 -t utf-16 | hexdump -C

Show Unix epoch time

$ date +%s

Show timestamp in RFC 3339 format

$ date --iso-8601=seconds

Show the MIME type of a file

$ file -0 --mime-type 'filename' | cut -d $'\0' -f2 | cut -d' ' -f2

List files recursively, etc.

$ find .

List files by extension:

$ find . -name "*.py"

To calculate the total number of lines of code, use:

$ find . -name "*.py" | xargs grep -v "^$" | wc -l

To delete all files with some extension, use:

$ find . -name "*.o" -delete

Get the size of a directory

In terms of kilobytes:

$ du -s foo/bar | cut -f1

Or: (in human-readable format)

$ du -hs foo/bar | cut -f1

List sub-directories in descending order of size

$ du -hs * | sort -hr

Archive (and compress) a directory

As ZIP format:

$ zip -r src.zip src

As gzip format:

$ tar -czf src.tar.gz src

To archive some files under a certain directory without including the pathname in the resulting archive, change the directory with -C:

$ tar -czf files.tar.gz -C src files

Login to a remote host

e.g., login to 127.0.0.1 as user root, via port 22:

$ ssh -p 22 root@127.0.0.1

Copy files to a remote host

e.g., copy some files to 127.0.0.1 as user root, via port 22:

$ scp -P 22 files root@127.0.0.1:/home/root/bak

2 System maintenance

View ACPI information (battery status, etc.)

$ acpi -i

View SMART information

# smartctl -a /dev/sda

Wipe out a disk

Data removed by rm -rf can be easily recovered by a software tool like TestDisk. To wipe out all data on a device (e.g., /dev/sda) reliably, use:

# dd if=/dev/urandom of=/dev/sda bs=4k

(Warning: Do NOT try the above command until you need it!)

3 Networking

Show all active Internet connections

Show only TCP connections:

$ netstat -ntlp

To include UDP connections:

$ netstat -ntulp

Port scanning

Check for a port number (e.g., 8000):

$ netcat -v -w 3 -z scanme.nmap.org 8000

Check for a range of ports (e.g., 20 - 80):

$ netcat -v -w 3 -z scanme.nmap.org 20-80

Scan all open ports with Nmap:

$ nmap -v -A scanme.nmap.org

Query a specific DNS server (e.g., Google Public DNS)

$ dig @8.8.8.8 www.example.com

Find out your IP address on the public Internet

$ dig +short myip.opendns.com @resolver1.opendns.com

Look for the geolocation of a host

$ geoiplookup google.com

Or: (IPv6)

$ geoiplookup6 google.com

Start an HTTP server for development

$ python3 -m http.server 4000

4 Multimedia

Record an M3U stream

$ ffmpeg -i http://foo.bar/hls.m3u8 -c copy -bsf:a aac_adtstoasc output.mp4