Practical Linux Commands

Practical Linux Commands


Basic shell Commands
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Command  : Syntax Examples [ Explanation ]
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cd        :  cd foo [ change directory ]
ls        :  ls -lt [ list files witrh time modified and detail ]
ls   -i   :  ls -i  [ Show inode details ]
cp        :  cp foo.sh ~/folder/foo.sh [ copy source to destination ]
mv        :  mv foo.sh ~/folder/foo.sh [ move source to destination ]
rm        :  rm filename [ remove file , -rf recursively no ask ]
wc        :  wc filename [ word count, -l, -w, -c ]
cal       :  cal  [ shows calendar ]
cat       :  cat file  [ show file contents ]
tee       :  cat file1 | tee file 2 [ tee pipeline ] 
more      :  cat file  | more  [ file perusal filter for screen viewing ]
less      :  cat file  | less  [ opposite of more ]
sort      :  sort data.txt  [ sort contents of a file or files, used with uniq ]
touch     :  touch filename [ change file timestamps or create an empty file ]
file      :  file filename  [ determine the filetype ]
mkdir     :  mkdir directory [ create a directory ]
rmdir     :  rmdir directory [ remove an empty directory ]
type      :  type man [ shows info of a shell command ]
which     :  which ls [ locate the folder of a command executable ] 
whereis   :  whereis ls [ locate the folder of a command executable + the man pages ] 
printenv  :  printenv [ Print all of part of the environment variables ]
head      :  head -n 5 [ Shows the first 5 lines from the beggining of a file ] 
tail      :  tail -n 5 [ Shows the last 5 lines from the end of a file ]
date      :  date [ Shows the current time  and date ]
who       :  who  [ Shows who is logged on
whoami    :  whoami  [ Print the effective userid ]
lastlog   :  lastlog [ reports the most recent login of all users or of a given user ]
id        :  id      [ Print real and effective user and group IDs ]
ln        :  ln -s Target link_name [ make symbolic links between files ]
screen    :  screen [ screen manager with VT100/ANSI terminal emulation ]


Environment Variables Check
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Command       : [ Explanation ]
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echo $$        :  [ Prints the process id of the running program ]
echo $0        :  [ Returns the name of the running process/program ]
echo $@        :  [ Returns the values of the parameters given to a command ]
echo $#        :  [ Returns the number of parameters isued to a command ]
echo $1        :  [ Returns the 1st argument provided in a shell script ]
echo $?        :  [ Return the exit code of the last command 0:Succesfull/1:Unsuccesful ]
printenv SHELL :  [ Print the environment variable named SHELL ]
export         :  [ Export a variable to a forked process ]

Disk Status
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Command       : [ Explanation ]
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du -sch .[!.]* * |sort -h  : Disk Usage of Hidden Files and Folders

Searching & Editing
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find Command                   : [ Explanation ]
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find Command
find . -user bob               : [ Find all the files belonging to a specific user ie. bob ]
find /var/www -group developer : [ Search files belong to a specific group ie. developer ]
find / -mtime 50               : [ Find files modified 50 days back ]
find / -atime 50               : [ Find files accessed in the last 50 days ]
find / -mmin -60               : [ Find files modified in the last hour    ]
find / -amin -60               : [ Find accessed files in last 1 hour      ]
find / -size 50M               : [ Find all 50M Files ]
find / -size +50M -size -100M  : [ Find all files between 50 and 100MBytes ]
find /tmp -type f -empty       : [ Find all empty files ]
find ~/ -type d -empty         : [ Find all empty directories ] 
find /var/log -mmin 10         : [ Find the logs which have been modified within the last 10 minutes ]
find . -type f -exec ls -sh {} \;|sort -n -r | head -5 : [ Display the 5 largest files in the directory and subdirectories ]
find / -perm /4000 2>/dev/null : [ Find all files having the SetUID bit set ]
find / -perm /2000 2>/dev/null : [ Find all files having the SetGID bit set ]
find / -perm /1000 2>/dev/null : [ Find all files having the sticky bit set ]

Searching & Editing
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grep Command                       : [ Explanation ]
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grep "literal string" filename     : [ Search for the string in the file ]
grep "this" demo*                  : [ Search pattern in multiple files  ]
grep -i "string" *                 : [ Case insensitive grep of a string in multiple files ]
grep -A3 -i "example"              : [ Display 3 lines after match  ]
grep -B3 "example" FILENAME        : [ Display 3 lines before match ] 
grep -C2 "test" FILENAME           : [ Display 2 lines around match ] 
grep -v "go" FILENAME              : [ Invert Match ] 
grep -c "go" FILENAME              : [ Count the Number of the matches ]
grep -l "test" FILES*              : [ Display only filenames with matches ]
grep -o "is.*line" FILENAME        : [ Show only the matched String  ]
grep -n "go" FILENAME              : [ Show line numbers with Output ]
grep -irl bob /var/log             : [ list all the log files containing the user bob ]
grep -E '^#'  /etc/config.file     : [ Use Regular Expression and print the lines with comments of a given file ]
grep -Ev '^#|^$'  /etc/config.file : [ Print all the lines of a conf file instead of the empty and the comments ]

Text Manipulation
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Command                        : [ Explanation ]
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echo ${variable #?}            : [ Extract all but the 1st character ]
echo $variable | cut -c1       : [ Extract the 1st Character ]
echo ${variable:0:1}           : [ Extract the 1st Character ]
echo ${variable%${variable#?}} : [ Extract the 1st Character ]

sed 's/Nick/John/g' report.txt        : [ Report every occurence of Nick with John ]
sed -n12,18p file.txt                 : [ Show only lines 12-18 of file.txt ]
sed 12,18d file.txt                   : [ Show all of file.txt except for lines form 12 to 18 ]
sed G file.txt                        : [ Double space file ]
sed '$d' file.txt                     : [ Delete the last line ]
sed '/boom/!s/aaa/bb/'                : [ Unless boom is found replace aaa with bb ]
sed 's/.$//' file.txt                 : [ A way to replace dos2unix ]
sed 's/^[ ^t]*//' file.txt            : [ Delete all spaces in front of every line ]
sed '/./!d' file.txt                  : [ Delete all blank lines from a file ]
sed 's/.\x08//g' file                 : [ Remove nroff overstrikes ]
sed -e :a -e 's/<[^>]*>//g;/</N;//ba' : [ Remove HTML tags ]
sed 's/[^[:digit:]]//g'               : [ Discard everything except integers ]
sed 's/[^[:uppper:],.]//g'            : [ Keep only an upper Character and Comma ]
sed '/^#\|^$/d' filename              : [ Show all the lines of a file instead of the commented and empty ]


Monitor Performance/Resources
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Command                       : [ Explanation ]
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cat /proc/cpuinfo                                  : [ Shows CPU Info ]
free                                               : [ Shows Free Memory ]
ps -eo pid, euser,ruser,comm                       : [ Process ID, Effective, Real UID ]
ps m -o pid,tid,command                            : [ Print the Thread IDs ]
top                                                : [ Display the linux processes ]
htop                                               : [ An interactive process viewer ]
iostat                                             : [ System Input and Output Storage Device Statistics ]
lsscsi                                             : [ Show info about the ssd drivers, sata drives etc. ]
sudo blkid                                         : [ Print block devices information ]
cat /proc/cmdline                                  : [ Show the Bootloader Options ]
strace -e trace=network,read,write executable args : [ Monitor system calls from a user program ]

Bonus - Kill all the processes of the dd command one by one:
ps -C dd | grep -v "PID" | awk '{ print $1'} | while read CMD;do kill $CMD; done


Network Tools
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Command                                            : [ Explanation ]
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lsof -i                                            : [ List Programs currently used or listening to ports ]
dig +short myip.opendns.com @resolver1.opendns.com : [ Return WAN IP ] 
tcpdump -i eth1 -w -lpv -bert > /dev/null          : [ Measure the Network throughput with tcpdump        ]
iptraf                                             : [ Opensource real time network monitor utility       ]
sudo nethogs eth1                                  : [ Net Motinor & Real time Network traffic bandwidth  ]
whois domain.com                                   : [ Client for the whois directory service             ]
host -t ns domain.com                              : [ Shows the DNS Servers for the domain               ]
host -l domain.com ns2.zoneedit.com                : [ Perform DNS zone transfer                          ]
arp                                                : [ Manipulate the system ARP cache                    ]
sudo netstat -tupln                                : [ Show the Tcp and Udp listing ports with PID        ]
ssh -L 9000:test.com:80 sshserver -pxx -N          : [ Perform ssh tunneling of localhost port 9000 to remote port 80 ] 
nc [ip] [port]  (issue --> GET / HTTP/1.1)         : [ Netcat client mode - Connect to a specific ip:port ]
nc -l 3333                                         : [ Create a Server and set it to listen to port 3333  ]
nslookup                                           : [ Featch mail exchanger servers                      ]
> set type=mx 
> bulbsecurity     


nmap 
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Command                 : [ Explanation ]
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nmap -sC -sV [ip]       : [ Scan and retrieve software version & use NSE ]
nmap -SA 192.168.0.1    : [ Scan a host to detect a firewall ]
nmap -SP 192.168.0.*    : [ Find out live hosts in class C IP range ]
nmap -sP 192.168.0.1/24 : [ Find out live hosts in class C IP range ]



Syncing - Backup 
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Command                                                   : [ Explanation ]
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wget -r -nH -nd -np -R index.html* http://test.com/videos : [ Download all files in a directory without the parent structure ]
tar cBvf - directory | ssh remote_host tar xBvpf          : [ Compress a directory and send it to a remote host where it uncompresses itself ]
dd if=/dev/hdd5 | gzip -9 | nc -l 3333                    : [ Server Machine to perform backup ]
nc 192.168.0.1 3333 | pv -b > hdd5partion.img.gz          : [ Connect to the above server and receive the partition image ]
rsync -rsh='ssh -p 3333' -azv bash hostname:~/tmp .       : [ Use a different port than 22 for rsync, copy all bash contents recursively, verbose mode and compress ]
rsync -avz -e "ssh -p $portNumber" user@remoteip:/path/to/files/ /local/path/

rsync -n                                                  : [ Can be used in case you want a dry run, not dry rum :-) ]