Aleksandar's computer workshop

Let's see what Aleksandar was fixing today.
My findings, tips & tricks related to computers, internet, programming and other stuff I was working with.

Linux command line reference for common operations

All these commands have been tested both on Fedora and Ubuntu.

Command Description
apropos whatis Show commands pertinent to string.
man -t man | ps2pdf - > man.pdf make a pdf of a manual page
  which command Show full path name of command
  time command See how long a command takes
time cat Start stopwatch. Ctrl-d to stop.
nice info Run a low priority command (The "info" reader in this case)
renice 19 -p $$ Make shell (script) low priority. Use for non interactive tasks
dir navigation
cd - Go to previous directory
cd Go to $HOME directory
  (cd dir && command) Go to dir, execute command and return to current dir
pushd . Put current dir on stack so you can popd back to it
alias l='ls -l --color=auto' quick dir listing
ls -lrt List files by date.
ls /usr/bin | pr -T9 -W$COLUMNS Print in 9 columns to width of terminal
  find -name '*.[ch]' | xargs grep -E 'expr' Search 'expr' in this dir and below.
  find -type f -print0 | xargs -r0 grep -F 'example' Search all regular files for 'example' in this dir and below
  find -maxdepth 1 -type f | xargs grep -F 'example' Search all regular files for 'example' in this dir
  find -maxdepth 1 -type d | while read dir; do echo $dir; echo cmd2; done Process each item with multiple commands (in while loop)
find -type f ! -perm -444 Find files not readable by all (useful for web site)
find -type d ! -perm -111 Find dirs not accessible by all (useful for web site)
locate -r 'file[^/]*\.txt' Search cached index for names. This re is like glob *file*.txt
look reference Quickly search (sorted) dictionary for prefix
grep --color reference /usr/share/dict/words Highlight occurances of regular expression in dictionary
archives and compression
  gpg -c file Encrypt file
  gpg file.gpg Decrypt file
  tar -c dir/ | bzip2 > dir.tar.bz2 Make compressed archive of dir/
  bzip2 -dc dir.tar.bz2 | tar -x Extract archive (use gzip instead of bzip2 for tar.gz files)
  tar -c dir/ | gzip | gpg -c | ssh user@remote 'dd of=dir.tar.gz.gpg' Make encrypted archive of dir/ on remote machine
  find dir/ -name '*.txt' | tar -c --files-from=- | bzip2 > dir_txt.tar.bz2 Make archive of subset of dir/ and below
  find dir/ -name '*.txt' | xargs cp -a --target-directory=dir_txt/ --parents Make copy of subset of dir/ and below
  ( tar -c /dir/to/copy ) | ( cd /where/to/ && tar -x -p ) Copy (with permissions) copy/ dir to /where/to/ dir
  ( cd /dir/to/copy && tar -c . ) | ( cd /where/to/ && tar -x -p ) Copy (with permissions) contents of copy/ dir to /where/to/
  ( tar -c /dir/to/copy ) | ssh -C user@remote 'cd /where/to/ && tar -x -p' Copy (with permissions) copy/ dir to remote:/where/to/ dir
  dd bs=1M if=/dev/sda | gzip | ssh user@remote 'dd of=sda.gz' Backup harddisk to remote machine
rsync (Use the --dry-run option for testing)
  rsync -P rsync://rsync.server.com/path/to/file file Only get diffs. Do multiple times for troublesome downloads
  rsync --bwlimit=1000 fromfile tofile Locally copy with rate limit. It's like nice for I/O
  rsync -az -e ssh --delete ~/public_html/ remote.com:'~/public_html' Mirror web site (using compression and encryption)
  rsync -auz -e ssh remote:/dir/ . && rsync -auz -e ssh . remote:/dir/ Synchronize current directory with remote one
ssh (Secure SHell)
  ssh $USER@$HOST command Run command on $HOST as $USER (default command=shell)
ssh -f -Y $USER@$HOSTNAME xeyes Run GUI command on $HOSTNAME as $USER
  scp -p -r $USER@$HOST: file dir/ Copy with permissions to $USER's home directory on $HOST
  ssh -g -L 8080:localhost:80 root@$HOST Forward connections to $HOSTNAME:8080 out to $HOST:80
  ssh -R 1434:imap:143 root@$HOST Forward connections from $HOST:1434 in to imap:143
wget (multi purpose download tool)
(cd cli && wget -nd -pHEKk http://gsmblog.com/) Store local browsable version of a page to the current dir
  wget -c http://www.example.com/large.file Continue downloading a partially downloaded file
  wget -r -nd -np -l1 -A '*.jpg' http://www.example.com/dir/ Download a set of files to the current directory
  wget ftp://remote/file[1-9].iso/ FTP supports globbing directly
wget -q -O- http://gsmblog.com/ | grep 'a href' | head Process output directly
  echo 'wget url' | at 01:00 Download url at 1AM to current dir
  wget --limit-rate=20k url Do a low priority download (limit to 20KB/s in this case)
  wget -nv --spider --force-html -i bookmarks.html Check links in a file
  wget --mirror http://www.example.com/ Efficiently update a local copy of a site (handy from cron)
networking (Note ifconfig, route, mii-tool, nslookup commands are obsolete)
  ethtool eth0 Show status of ethernet interface eth0
  ethtool --change eth0 autoneg off speed 100 duplex full Manually set ethernet interface speed
  iwconfig eth1 Show status of wireless interface eth1
  iwconfig eth1 rate 1Mb/s fixed Manually set wireless interface speed
iwlist scan List wireless networks in range
ip link show List network interfaces
  ip link set dev eth0 name wan Rename interface eth0 to wan
  ip link set dev eth0 up Bring interface eth0 up (or down)
ip addr show List addresses for interfaces
  ip addr add 1.2.3.4/24 brd + dev eth0 Add (or del) ip and mask (255.255.255.0)
ip route show List routing table
  ip route add default via 1.2.3.254 Set default gateway to 1.2.3.254
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
host pixelbeat.org Lookup DNS ip address for name or vice versa
hostname -i Lookup local ip address (equivalent to host `hostname`)
whois pixelbeat.org Lookup whois info for hostname or ip address
netstat -tupl List internet services on a system
netstat -tup List active connections to/from system
windows networking (Note samba is the package that provides all this windows specific networking support)
smbtree Find windows machines. See also findsmb
  nmblookup -A 1.2.3.4 Find the windows (netbios) name associated with ip address
  smbclient -L windows_box List shares on windows machine or samba server
  mount -t smbfs -o fmask=666,guest //windows_box/share /mnt/share Mount a windows share
  echo 'message' | smbclient -M windows_box Send popup to windows machine (off by default in XP sp2)
text manipulation (Note sed uses stdin and stdout, so if you want to edit files, append <oldfile >newfile)
  sed 's/string1/string2/g' Replace string1 with string2
  sed 's/\(.*\)1/\12/g' Modify anystring1 to anystring2
  sed '/ *#/d; /^ *$/d' Remove comments and blank lines
  sed ':a; /\\$/N; s/\\\n//; ta' Concatenate lines with trailing \
  sed 's/[ \t]*$//' Remove trailing spaces from lines
  sed 's/\([\\`\\"$\\\\]\)/\\\1/g' Escape shell metacharacters active within double quotes
seq 10 | sed "s/^/      /; s/ *\(.\{7,\}\)/\1/" Right align numbers
  sed -n '1000p;1000q' Print 1000th line
  sed -n '10,20p;20q' Print lines 10 to 20
  sed -n 's/.*<title>\(.*\)<\/title>.*/\1/ip;T;q' Extract title from HTML web page
  sort -t. -k1,1n -k2,2n -k3,3n -k4,4n Sort IPV4 ip addresses
echo 'Test' | tr '[:lower:]' '[:upper:]' Case conversion
tr -dc '[:print:]' < /dev/urandom Filter non printable characters
history | wc -l Count lines
set operations (Note you can export LANG=C for speed. Also these assume no duplicate lines within a file)
  sort file1 file2 | uniq Union of unsorted files
  sort file1 file2 | uniq -d Intersection of unsorted files
  sort file1 file1 file2 | uniq -u Difference of unsorted files
  sort file1 file2 | uniq -u Symmetric Difference of unsorted files
  join -a1 -a2 file1 file2 Union of sorted files
  join file1 file2 Intersection of sorted files
  join -v2 file1 file2 Difference of sorted files
  join -v1 -v2 file1 file2 Symmetric Difference of sorted files
math
echo '(1 + sqrt(5))/2' | bc -l Quick math (Calculate φ).
echo 'pad=20; min=64; (100*10^6)/((pad+min)*8)' | bc More complex (int) e.g. This shows max FastE packet rate
echo 'pad=20; min=64; print (100E6)/((pad+min)*8)' | python Python handles scientific notation
echo 'pad=20; plot [64:1518] (100*10**6)/((pad+x)*8)' | gnuplot -persist Plot FastE packet rate vs packet size
echo 'obase=16; ibase=10; 64206' | bc Base conversion (decimal to hexadecimal)
echo $((0x2dec)) Base conversion (hex to dec) ((shell arithmetic expansion))
units -t '100m/9.74s' 'miles/hour' Unit conversion (metric to imperial)
units -t '500GB' 'GiB' Unit conversion (SI to IEC prefixes)
units -t '1 googol' Definition lookup
seq 100 | (tr '\n' +; echo 0) | bc Add a column of numbers.
calendar
cal -3 Display a calendar
cal 9 1752 Display a calendar for a particular month year
date -d fri What date is it this friday.
date --date='25 Dec' +%A What day does xmas fall on, this year
date --date '1970-01-01 UTC 2147483647 seconds' Convert number of seconds since the epoch to a date
TZ=':America/Los_Angeles' date What time is it on West coast of US (use tzselect to find TZ)
  echo "mail -s 'get the train' P@draigBrady.com < /dev/null" | at 17:45 Email reminder
echo "DISPLAY=$DISPLAY xmessage cooker" | at "NOW + 30 minutes" Popup reminder
locales
printf "%'d\n" 1234 Print number with thousands grouping appropriate to locale
BLOCK_SIZE=\'1 ls -l get ls to do thousands grouping appropriate to locale
echo "I live in `locale territory`" Extract info from locale database
LANG=en_IE.utf8 locale int_prefix Lookup locale info for specific country.
locale | cut -d= -f1 | xargs locale -kc | less List fields available in locale database
recode (Obsoletes iconv, dos2unix, unix2dos)
recode -l | less Show available conversions (aliases on each line)
  recode windows-1252.. file_to_change.txt Windows "ansi" to local charset (auto does CRLF conversion)
  recode utf-8/CRLF.. file_to_change.txt Windows utf8 to local charset
  recode iso-8859-15..utf8 file_to_change.txt Latin9 (western europe) to utf8
  recode ../b64 < file.txt > file.b64 Base64 encode
  recode /qp.. < file.txt > file.qp Quoted printable decode
  recode ..HTML < file.txt > file.html Text to HTML
recode -lf windows-1252 | grep euro Lookup table of characters
echo -n 0x80 | recode latin-9/x1..dump Show what a code represents in latin-9 charmap
echo -n 0x20AC | recode ucs-2/x2..latin-9/x Show latin-9 encoding
echo -n 0x20AC | recode ucs-2/x2..utf-8/x Show utf-8 encoding
CDs
  gzip < /dev/cdrom > cdrom.iso.gz Save copy of data cdrom
  mkisofs -V LABEL -r dir | gzip > cdrom.iso.gz Create cdrom image from contents of dir
  mount -o loop cdrom.iso /mnt/dir Mount the cdrom image at /mnt/dir (read only)
  cdrecord -v dev=/dev/cdrom blank=fast Clear a CDRW
  gzip -dc cdrom.iso.gz | cdrecord -v dev=/dev/cdrom - Burn cdrom image (use dev=ATAPI -scanbus to confirm dev)
  cdparanoia -B Rip audio tracks from CD to wav files in current dir
  cdrecord -v dev=/dev/cdrom -audio *.wav Make audio CD from all wavs in current dir (see also cdrdao)
  oggenc --tracknum='track' track.cdda.wav -o 'track.ogg' Make ogg file from wav file
disk space
ls -lSr Show files by size, biggest last
du -s * | sort -k1,1rn | head Show top disk users in current dir.
df -h Show free space on mounted filesystems
df -i Show free inodes on mounted filesystems
fdisk -l Show disks partitions sizes and types (run as root)
rpm -q -a --qf '%10{SIZE}\t%{NAME}\n' | sort -k1,1n List all packages by installed size (Bytes) on rpm distros
dpkg-query -W -f='${Installed-Size;10}\t${Package}\n' | sort -k1,1n List all packages by installed size (KBytes) on deb distros
dd bs=1 seek=2TB if=/dev/null of=ext3.test Create a large test file (taking no space).
monitoring/debugging
tail -f /var/log/messages Monitor messages in a log file
strace -c ls >/dev/null Summarise/profile system calls made by command
strace -f -e open ls >/dev/null List system calls made by command
ltrace -f -e getenv ls >/dev/null List library calls made by command
lsof -p $$ List paths that process id has open
lsof ~ List processes that have specified path open
tcpdump not port 22 Show network traffic except ssh.
ps -e -o pid,args --forest List processes in a hierarchy
ps -e -o pcpu,cpu,nice,state,cputime,args --sort pcpu | sed '/^ 0.0 /d' List processes by % cpu usage
ps -e -orss=,args= | sort -b -k1,1n | pr -TW$COLUMNS List processes by mem usage.
ps -C firefox-bin -L -o pid,tid,pcpu,state List all threads for a particular process
ps -p 1,2 List info for particular process IDs
last reboot Show system reboot history
free -m Show amount of (remaining) RAM (-m displays in MB)
watch -n1 'cat /proc/interrupts' Watch changeable data continuously
system information('#' means root access is required)
uname -a Show kernel version and system architecture
head -n1 /etc/issue Show name and version of distribution
cat /proc/partitions Show all partitions registered on the system
grep MemTotal /proc/meminfo Show RAM total seen by the system
grep "model name" /proc/cpuinfo Show CPU(s) info
lspci -tv Show PCI info
lsusb -tv Show USB info
mount | column -t List mounted filesystems on the system (and align output)
# dmidecode -q | less Display SMBIOS/DMI information
# smartctl -A /dev/sda | grep Power_On_Hours How long has this disk (system) been powered on in total
# hdparm -i /dev/sda Show info about disk sda
# hdparm -tT /dev/sda Do a read speed test on disk sda
# badblocks -s /dev/sda Test for unreadable blocks on disk sda
interactive
readline Line editor used by bash, python, bc, gnuplot, ...
screen Virtual terminals with detach capability, ...
mc Powerful file manager that can browse rpm, tar, ftp, ssh, ...
gnuplot Interactive/scriptable graphing
links Web browser
miscellaneous
alias hd='od -Ax -tx1z -v' Handy hexdump. (usage e.g.: • hd /proc/self/cmdline | less)
alias realpath='readlink -f' Canonicalize path. (usage e.g.: • realpath ~/../$USER)
set | grep $USER Search current environment
  touch -c -t 0304050607 file Set file timestamp (YYMMDDhhmm)

Free disk space checking in Linux

There are plenty of tools with which you can check your disk space.
However, Linux already has a built in function to show you just what you need to know.
Open a terminal window or push (ctrl+alt+F1 to go to console) and type:

df

You will see something like this:

Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/hdb2             12262868   2383044   9256904  21% /
/dev/hdb1               101086     11096     84771  12% /boot
none                     95580         0     95580   0% /dev/shm
 

This one is bit unreadable, because size is represented in 1K-blocks. Lets try to make it clean and more readable:

df -h

Filesystem            Size  Used Avail Use% Mounted on
/dev/hdb2              12G  2.3G  8.9G  21% /
/dev/hdb1              99M   11M   83M  12% /boot
none                   94M     0   94M   0% /dev/shm
 

Now the size is represented by megabytes and gigabytes.

How to find out which version of Linux you are running

To get running version type this in command line:
uname -a

Output:
Linux asterisk1.local 2.6.9-34.0.2.EL #1 Fri Jul 7 19:24:57 CDT
2006 i686 i686 i386 GNU/Linux

To get release version which was used during installation type this:
cat /etc/*-release

Output:
CentOS release 4.3 (Final)

How to change Name Server (DNS Client) Settings under Linux

In order to be able to use conical names you have to use name servers (also called DNS servers) which will convert some name into IP address. I will here show you where are configuration files under Linux

Edit /etc/resolv.conf which can like this:
nameserver 192.168.1.1
nameserver 10.0.0.138

Also you might hardcode some IP's in hosts file. It looks like this:
127.0.0.1 localhost.localdomain localhost
194.109.6.67 ns.xs4all.nl

To have your new settings valid you have to restart the computer or do this in command line which will restart all your network services and devices:

/etc/init.d/network restart

If you are connected with SSH be patient connection will be restored.