Linux Command Line Reference for Common Operations （2）

My previous reference for practical Linux commands was surprisingly popular
with over 3.5 million hits in nearly 5 years. So I've decided to start compiling
another list of somewhat more involved/esoteric commands.
Examples marked with • are valid/safe to paste without modification into a terminal, so
you may want to keep a terminal window open while reading this so you can cut & paste.

Command		Description
•	grep . /proc/sys/net/ipv4/*	List the contents of flag files
•	set | grep $USER	Search current environment
•	tr '\0' '\n' < /proc/$$/environ	Display the startup environment for any process
•	echo $PATH | tr : '\n'	Display the $PATH one per line
•	kill -0 $$ && echo process exists and can accept signals	Check for the existence of a process (pid)
•	find /etc -readable | xargs less -K -p'*ntp' -j $((${LINES:-25}/2))	Search paths and data with full context. Use n to iterate
Low impact admin
#	apt-get install "package" -o Acquire::http::Dl-Limit=42 \ -o Acquire::Queue-mode=access	Rate limit apt-get to 42KB/s
	echo 'wget url' | at 01:00	Download url at 1AM to current dir
#	apache2ctl configtest && apache2ctl graceful	Restart apache if config is OK
•	nice openssl speed sha1	Run a low priority command (openssl benchmark)
•	chrt -i 0 openssl speed sha1	Run a low priority command (more effective than nice)
•	renice 19 -p $$; ionice -c3 -p $$	Make shell (script) low priority. Use for non interactive tasks
Interactive monitoring
•	watch -t -n1 uptime	Clock with system load
•	htop -d 5	Better top (scrollable, tree view, lsof/strace integration, ...)
•	iotop	What's doing I/O
#	watch -d -n30 "nice ps_mem.py | tail -n $((${LINES:-12}-2))"	What's using RAM
#	iftop	What's using the network. See also iptraf
#	mtr www.pixelbeat.org	ping and traceroute combined
Useful utilities
•	pv < /dev/zero > /dev/null	Progress Viewer for data copying from files and pipes
•	wkhtml2pdf http://.../linux_commands.html linux_commands.pdf	Make a pdf of a web page
•	timeout 1 sleep 3	run a command with bounded time. See also timeout
Networking
•	python -m SimpleHTTPServer	Serve current directory tree at http://$HOSTNAME:8000/
•	openssl s_client -connect www.google.com:443 </dev/null 2>&0 | openssl x509 -dates -noout	Display the date range for a site's certs
•	curl -I www.pixelbeat.org	Display the server headers for a web site
#	lsof -i tcp:80	What's using port 80
#	httpd -S	Display a list of apache virtual hosts
•	vim scp://user@remote//path/to/file	Edit remote file using local vim. Good for high latency links
•	curl -s http://www.pixelbeat.org/pixelbeat.asc | gpg --import	Import a gpg key from the web
•	tc qdisc add dev lo root handle 1:0 netem delay 20msec	Add 20ms latency to loopback device (for testing)
•	tc qdisc del dev lo root	Remove latency added above
Notification
•	echo "DISPLAY=$DISPLAY xmessage cooker" | at "NOW +30min"	Popup reminder
•	notify-send "subject" "message"	Display a gnome popup notification
	echo "mail -s 'go home' P@draigBrady.com < /dev/null" | at 17:30	Email reminder
	uuencode file name | mail -s subject P@draigBrady.com	Send a file via email
	ansi2html.sh | mail -a "Content-Type: text/html" P@draigBrady.com	Send/Generate HTML email
Better default settings (useful in your .bashrc)
#	tail -s.1 -f /var/log/messages	Display file additions more responsively
•	seq 100 | tail -n $((${LINES:-12}-2))	Display as many lines as possible without scrolling
#	tcpdump -s0	Capture full network packets
Useful functions/aliases (useful in your .bashrc)
•	md () { mkdir -p "$1" && cd "$1"; }	Change to a new directory
•	strerror() { python -c "import os; print os.strerror($1)"; }	Display the meaning of an errno
•	plot() { { echo 'plot "-"' "$@"; cat; } | gnuplot -persist; }	Plot stdin. (e.g: • seq 1000 | sed 's/.*/s(&)/' | bc -l | plot)
•	hili() { e="$1"; shift; grep --col=always -Eih "$e|$" "$@"; }	highlight occurences of expr. (e.g: • env | hili $USER)
•	alias hd='od -Ax -tx1z -v'	Hexdump. (usage e.g.: • hd /proc/self/cmdline | less)
•	alias realpath='readlink -f'	Canonicalize path. (usage e.g.: • realpath ~/../$USER)
•	ord() { printf "0x%x\n" "'$1"; }	shell version of the ord() function
•	chr() { printf $(printf '\\%03o\\n' "$1"); }	shell version of the chr() function
Multimedia
•	DISPLAY=:0.0 import -window root orig.png	Take a (remote) screenshot
•	convert -filter catrom -resize '600x>' orig.png 600px_wide.png	Shrink to width, computer gen images or screenshots
	mplayer -ao pcm -vo null -vc dummy /tmp/Flash*	Extract audio from flash video to audiodump.wav
	ffmpeg -i filename.avi	Display info about multimedia file
•	ffmpeg -f x11grab -s xga -r 25 -i :0 -sameq demo.mpg	Capture video of an X display
DVD
	for i in $(seq 9); do ffmpeg -i $i.avi -target pal-dvd $i.mpg; done	Convert video to the correct encoding and aspect for DVD
	dvdauthor -odvd -t -v "pal,4:3,720xfull" *.mpg;dvdauthor -odvd -T	Build DVD file system. Use 16:9 for widescreen input
	growisofs -dvd-compat -Z /dev/dvd -dvd-video dvd	Burn DVD file system to disc
Unicode
•	python -c "import unicodedata as u; print u.name(unichr(0x2028))"	Lookup a unicode character
•	uconv -f utf8 -t utf8 -x nfc	Normalize combining characters
•	printf '\300\200' | iconv -futf8 -tutf8 >/dev/null	Validate UTF-8
•	printf 'ŨTF8\n' | LANG=C grep --color=always '[^ -~]\+'	Highlight non printable ASCII chars in UTF-8
•	fc-match -s "sans:lang=zh"	List font match order for language and style
Development
•	gcc -march=native -E -v -</dev/null 2>&1|sed -n 's/.*-mar/-mar/p'	Show autodetected gcc tuning params. See also gcccpuopt
•	for i in $(seq 4); do { [ $i = 1 ] && wget http://url.ie/6lko -qO-|| ./a.out; } | tee /dev/tty | gcc -xc - 2>/dev/null; done	Compile and execute C code from stdin
•	cpp -dM /dev/null	Show all predefined macros
•	echo "#include <features.h>" | cpp -dN | grep "#define __USE_"	Show all glibc feature macros
	gdb -tui	Debug showing source code context in separate windows
udev
•	udevadm info -a -p $(udevadm info -q path -n /dev/input/mouse0)	List udev attributes of a device, for matching rules etc.
•	udevadm test /sys/class/input/mouse0	See how udev rules are applied for a device
#	udevadm control --reload-rules	Reload udev rules after modification
Extended Attributes (Note you may need to (re)mount with "acl" or "user_xattr" options)
•	getfacl .	Show ACLs for file
•	setfacl -m u:nobody:r .	Allow a specific user to read file
•	setfacl -x u:nobody .	Delete a specific user's rights to file
	setfacl --default -m group:users:rw- dir/	Set umask for a for a specific dir
	getcap file	Show capabilities for a program
	setcap cap_net_raw+ep your_gtk_prog	Allow gtk program raw access to network
•	stat -c%C .	Show SELinux context for file
	chcon ... file	Set SELinux context for file (see also restorecon)
•	getfattr -m- -d .	Show all extended attributes (includes selinux,acls,...)
•	setfattr -n "user.foo" -v "bar" .	Set arbitrary user attributes
BASH specific
•	echo 123 | tee >(tr 1 a) | tr 1 b	Split data to 2 commands (using process substitution)
	meld local_file <(ssh host cat remote_file)	Compare a local and remote file (using process substitution)
Multicore
•	taskset -c 0 nproc	Restrict a command to certain processors
•	find -type f -print0 | xargs -r0 -P$(nproc) -n10 md5sum	Process files in parallel over available processors
	sort -m <(sort data1) <(sort data2) >data.sorted	Sort separate data files over 2 processors