Condition And Loop

The Terminal

Using Ctrl keys

Ctrl + n : same as Down arrow.
Ctrl + p : same as Up arrow.
Ctrl + r : begins a backward search through command history.(keep pressing Ctrl + r to move backward)
Ctrl + s : to stop output to terminal.
Ctrl + q : to resume output to terminal after Ctrl + s.
Ctrl + a : move to the beginning of line.
Ctrl + e : move to the end of line.
Ctrl + d : if you've type something, Ctrl + d deletes the character under the cursor, else, it escapes the current shell.
Ctrl + k : delete all text from the cursor to the end of line.
Ctrl + x + backspace : delete all text from the beginning of line to the cursor.
Ctrl + t : transpose the character before the cursor with the one under the cursor, press Esc + t to transposes the two words before the cursor.
Ctrl + w : cut the word before the cursor; then Ctrl + y paste it
Ctrl + u : cut the line before the cursor; then Ctrl + y paste it
Ctrl + x + Ctrl + e : launch editor define by $EDITOR
Ctrl + _ : undo typing.
Ctrl + l : equivalent to clear.

Change case

Esc + u
# converts text from cursor to the end of the word to uppercase.  
Esc + l
# converts text from cursor to the end of the word to lowercase. 
Esc + c
# converts letter under the cursor to uppercase.

Run history number (e.g. 53)


Run last command


Run last command and change some parameter using caret substitution (e.g. last command: echo 'aaa' -> rerun as: echo 'bbb')

#last command: echo 'aaa'

#echo 'bbb'

#Notice that only the first aaa will be replaced, if you want to replace all 'aaa', use ':&' to repeat it:

Run past command that began with (e.g. cat filename)

# or
# run cat filename again

Bash globbing

# '*' serves as a "wild card" for filename expansion.
/b?n/?at      #/bin/cat

# '?' serves as a single-character "wild card" for filename expansion.
/etc/pa*wd    #/etc/passwd 

# ‘[]’ serves to match the character from a range.
ls -l [a-z]*   #list all files with alphabet in its filename.

# ‘{}’ can be used to match filenames with more than one patterns
ls {*.sh,*.py}   #list all .sh and .py files

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Types of grep

grep = grep -G # Basic Regular Expression (BRE)
fgrep = grep -F # fixed text, ignoring meta-charachetrs
egrep = grep -E # Extended Regular Expression (ERE)
pgrep = grep -P # Perl Compatible Regular Expressions (PCRE)
rgrep = grep -r # recursive

Grep and count number of empty lines

grep -c "^$"

Grep and return only integer

grep -o '[0-9]*'
grep -oP '\d'

Grep integer with certain number of digits (e.g. 3)

grep ‘[0-9]\{3\}’
# or
grep -E ‘[0-9]{3}’
# or
grep -P ‘\d{3}’

Grep only IP address

grep -Eo '[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}'
# or
grep -Po '\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}'

Grep whole word (e.g. 'target')

grep -w 'target'
#or using RE
grep '\btarget\b'

Grep returning lines before and after match (e.g. 'bbo')

# return also 3 lines after match
grep -A 3 'bbo'
# return also 3 lines before match
grep -B 3 'bbo'

# return also 3 lines before and after match
grep -C 3 'bbo'

Grep string starting with (e.g. 'S')

grep -o 'S.*'

Extract text between words (e.g. w1,w2)

grep -o -P '(?<=w1).*(?=w2)'

Grep lines without word (e.g. bbo)

grep -v bbo filename

Grep lines not begin with string (e.g. #)

grep -v '^#' file.txt

Grep variables with space within it (e.g. bbo="some strings")

grep "$boo" filename
#remember to quote the variable!  

Grep only one/first match (e.g. bbo)

grep -m 1 bbo filename

Grep and return number of matching line(e.g. bbo)

grep -c bbo filename

Count occurrence (e.g. three times a line count three times)

grep -o bbo filename |wc -l 

Case insensitive grep (e.g. bbo/BBO/Bbo)

grep -i "bbo" filename 

COLOR the match (e.g. bbo)!

grep --color bbo filename 

Grep search all files in a directory(e.g. bbo)

grep -R bbo /path/to/directory 
# or
grep -r bbo /path/to/directory 

Search all files in directory, do not ouput the filenames (e.g. bbo)

grep -rh bbo /path/to/directory 

Search all files in directory, output ONLY the filenames with matches(e.g. bbo)

grep -rl bbo /path/to/directory

Grep OR (e.g. A or B or C or D)

grep 'A\|B\|C\|D' 

Grep AND (e.g. A and B)

grep 'A.*B' 

Regex any singer character (e.g. ACB or AEB)

grep 'A.B' 

Regex with or without a certain character (e.g. color or colour)

grep ‘colou?r’

Grep all content of a fileA from fileB

grep -f fileA fileB 

Grep a tab

grep $'\t' 

Grep variable from variable

$echo "$long_str"|grep -q "$short_str"
if [ $? -eq 0 ]; then echo 'found'; fi
#grep -q will output 0 if match found  
#remember to add space between []!

Grep strings between a bracket()

grep -oP '\(\K[^\)]+'

Grep number of characters with known strings in between(e.g. AAEL000001-RA)

grep -o -w "\w\{10\}\-R\w\{1\}"
# \w word character [0-9a-zA-Z_] \W not word character 

Skip directory (e.g. bbo)

grep -d skip 'bbo' /path/to/files/*
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Remove the first line

sed 1d filename

Remove the first 100 lines (remove line 1-100)

sed 1,100d filename

Remove lines with string (e.g. bbo)

sed "/bbo/d" filename
- case insensitive:
sed "/bbo/Id" filename

Remove lines whose nth character not equal to a value (e.g. 5th character not equal to 2)

sed -E '/^.{5}[^2]/d'
#aaaa2aaa (you can stay)
#aaaa1aaa (delete!)

Edit infile (edit and save)

sed -i "/bbo/d" filename

When using variable (e.g. $i), use double quotes " "

# e.g. add >$i to the first line (to make a bioinformatics FASTA file)  
sed "1i >$i"  
# notice the double quotes! in other examples, you can use a single quote, but here, no way!   
# '1i' means insert to first line

Using environment variable and end-of-line pattern at the same time.

# Use backslash for end-of-line $ pattern, and double quotes for expressing the variable
sed -e "\$s/\$/\n+--$3-----+/"

Delete/remove empty lines

sed '/^\s*$/d' 
# or
sed '/^$/d' 

Delete/remove last line

sed '$d' 

Delete/remove last character from end of file

sed -i '$ s/.$//' filename

Add string to beginning of file (e.g. "[")

sed -i '1s/^/[/' file

Add string at certain line number (e.g. add 'something' to line 1 and line 3)

sed -e '1isomething -e '3isomething'

Add string to end of file (e.g. "]")

sed '$s/$/]/' filename

Add newline to the end

sed '$a\'

Add string to beginning of every line (e.g. bbo)

sed -e 's/^/bbo/' file

Add string to end of each line (e.g. "}")

sed -e 's/$/\}\]/' filename

Add \n every nth character (e.g. every 4th character)

sed 's/.\{4\}/&\n/g' 

Concatenate/combine/join files with a seperator and next line (e.g separate by ",")

sed -s '$a,' *.json > all.json

Substitution (e.g. replace A by B)

sed 's/A/B/g' filename 

Substitution with wildcard (e.g. replace a line start with aaa= by aaa=/my/new/path)

sed "s/aaa=.*/aaa=\/my\/new\/path/g"

Select lines start with string (e.g. bbo)

sed -n '/^@S/p' 

Delete lines with string (e.g. bbo)

sed '/bbo/d' filename 

Print/get/trim a range of line (e.g. line 500-5000)

sed -n 500,5000p filename

Print every nth lines

sed -n '0~3p' filename

# catch 0: start; 3: step

Print every odd # lines

sed -n '1~2p' 

Print every third line including the first line

sed -n '1p;0~3p' 

Remove leading white space and tabs

sed -e 's/^[ \t]*//'
# Notice a whitespace before '\t'!!

Remove only leading white space

sed 's/ *//'

# notice a whitespace before '*'!!

Remove ending commas

sed 's/,$//g' 

Add a column to the end

sed "s/$/\t$i/"
# $i is the valuable you want to add  

# To add the filename to every last column of the file  
for i in $(ls);do sed -i "s/$/\t$i/" $i;done

Add extension of filename to last column

for i in T000086_1.02.n T000086_1.02.p;do sed "s/$/\t${i/*./}/" $i;done >T000086_1.02.np

Remove newline\ nextline

sed ':a;N;$!ba;s/\n//g'

Print a particular line (e.g. 123th line)

sed -n -e '123p'

Print a number of lines (e.g. line 10th to line 33 rd)

sed -n '10,33p' <filename

Change delimiter

sed 's=/=\\/=g'

Replace with wildcard (e.g A-1-e or A-2-e or A-3-e....)

sed 's/A-.*-e//g' filename

Remove last character of file

sed '$ s/.$//'

Insert character at specified position of file (e.g. AAAAAA --> AAA#AAA)

sed -r -e 's/^.{3}/&#/' file

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Set tab as field separator

awk -F $'\t'  

Output as tab separated (also as field separator)

awk -v OFS='\t' 

Pass variable

awk -v a="$a" -v b="$b" "$1==a && $10=b" filename 

Print line number and number of characters on each line

awk '{print NR,length($0);}' filename 

Find number of columns

awk '{print NF}' 

Reverse column order

awk '{print $2, $1}' 

Check if there is a comma in a column (e.g. column $1)

awk '$1~/,/ {print}'  

Split and do for loop

awk '{split($2, a,",");for (i in a) print $1"\t"a[i]}' filename 

Print all lines before nth occurrence of a string (e.g stop print lines when bbo appears 7 times)

awk -v N=7 '{print}/bbo/&& --N<=0 {exit}'

Print filename and last line of all files in directory

ls|xargs -n1 -I file awk '{s=$0};END{print FILENAME,s}' file

Add string to the beginning of a column (e.g add "chr" to column $3)

awk 'BEGIN{OFS="\t"}$3="chr"$3' 

Remove lines with string (e.g. bbo)

awk '!/bbo/' file 

Remove last column

awk 'NF{NF-=1};1' file

Usage and meaning of NR and FNR

# For example there are two files:
# fileA:  
# a  
# b  
# c  
# fileB:  
# d  
# e  
awk 'print FILENAME, NR,FNR,$0}' fileA fileB 
# fileA    1    1    a  
# fileA    2    2    b  
# fileA    3    3    c  
# fileB    4    1    d  
# fileB    5    2    e  

AND gate

# For example there are two files: 
# fileA:  
# 1    0  
# 2    1  
# 3    1  
# 4    0  
# fileB:  
# 1    0  
# 2    1  
# 3    0  
# 4    1      

awk -v OFS='\t' 'NR=FNR{a[$1]=$2;next} NF {print $1,((a[$1]=$2)? $2:"0")}' fileA fileB 
# 1    0  
# 2    1  
# 3    0  
# 4    0  

Round all numbers of file (e.g. 2 significant figure)

awk '{while (match($0, /[0-9]+\[0-9]+/)){
    \printf "%s%.2f", substr($0,0,RSTART-1),substr($0,RSTART,RLENGTH)
    \$0=substr($0, RSTART+RLENGTH)

Give number/index to every row

awk '{printf("%s\t%s\n",NR,$0)}'

Break combine column data into rows

# For example, seperate the following content:
# David    cat,dog  
# into  
# David    cat  
# David    dog  

awk '{split($2,a,",");for(i in a)print $1"\t"a[i]}' file
# Detail here: http://stackoverflow.com/questions/33408762/bash-turning-single-comma-separated-column-into-multi-line-string

Average a file (each line in file contains only one number)

awk '{s+=$1}END{print s/NR}'

Print field start with string (e.g Linux)

awk '$1 ~ /^Linux/'

Sort a row (e.g. 1 40  35  12  23  --> 1 12    23  35  40)

awk ' {split( $0, a, "\t" ); asort( a ); for( i = 1; i <= length(a); i++ ) printf( "%s\t", a[i] ); printf( "\n" ); }'

Subtract previous row values (add column6 which equal to column4 minus last column5)

awk '{$6 = $4 - prev5; prev5 = $5; print;}'
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Set tab as delimiter (default:space)

xargs -d\t

Display 3 items per line

echo 1 2 3 4 5 6| xargs -n 3
# 1 2 3  
# 4 5 6

Prompt before execution

echo a b c |xargs -p -n 3

Print command along with output

xargs -t abcd
# bin/echo abcd  
# abcd

With find and rm

find . -name "*.html"|xargs rm
# when using a backtick
rm `find . -name "*.html"`

Delete fiels with whitespace in filename (e.g. "hello 2001")

find . -name "*.c" -print0|xargs -0 rm -rf

Show limits

xargs --show-limits

Move files to folder

find . -name "*.bak" -print 0|xargs -0 -I {} mv {} ~/old

# or
find . -name "*.bak" -print 0|xargs -0 -I file mv file ~/old

Move first 100th files to a directory (e.g. d1)

ls |head -100|xargs -I {} mv {} d1


time echo {1..5} |xargs -n 1 -P 5 sleep

# a lot faster than: 
time echo {1..5} |xargs -n1 sleep

Copy all files from A to B

find /dir/to/A -type f -name "*.py" -print 0| xargs -0 -r -I file cp -v -p file --target-directory=/path/to/B

# v: verbose|  
# p: keep detail (e.g. owner)

With sed

ls |xargs -n1 -I file sed -i '/^Pos/d' filename

Add the file name to the first line of file

ls |sed 's/.txt//g'|xargs -n1 -I file sed -i -e '1 i\>file\' file.txt

Count all files

ls |xargs -n1 wc -l

Turn output into a single line

ls -l| xargs

Count files within directories

echo mso{1..8}|xargs -n1 bash -c 'echo -n "$1:"; ls -la "$1"| grep -w 74 |wc -l' --
# "--" signals the end of options and display further option processing

Count lines in all file, also count total lines

ls|xargs wc -l

Xargs and grep

cat grep_list |xargs -I{} grep {} filename

Xargs and sed (replace all old ip address with new ip address under /etc directory)

grep -rl '' /etc | xargs sed -i 's/'
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List all sub directory/file in the current directory

find .

List all files under the current directory

find . -type f

List all directories under the current directory

find . -type d

Edit all files under current directory (e.g. replace 'www' with 'ww')

find . -name '*.php' -exec sed -i 's/www/w/g' {} \;

# if there are no subdirectory
replace "www" "w" -- *
# a space before *

Find and output only filename (e.g. "mso")

find mso*/ -name M* -printf "%f\n"

Find and delete file with size less than (e.g. 74 byte)

find . -name "*.mso" -size -74c -delete

# M for MB, etc

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condition and loop

If statement

# if and else loop for string matching
if [[ "$c" == "read" ]]; then outputdir="seq"; else outputdir="write" ; fi  

# Test if myfile contains the string 'test':
if grep -q hello myfile; then …

# Test if mydir is a directory, change to it and do other stuff:
if cd mydir; then
  echo 'some content' >myfile
  echo >&2 "Fatal error. This script requires mydir."

# if variable is null
if [ ! -s "myvariable" ]
#True of the length if "STRING" is zero.

# Test if file exist
if [ -e 'filename' ]
  echo -e "file exists!"

# Test if file exist but also including symbolic links:
if [ -e myfile ] || [ -L myfile ]
  echo -e "file exists!"

# Test if the value of x is greater or equal than 5
if [ "$x" -ge 5 ]; then …

# Test if the value of x is greater or equal than 5, in bash/ksh/zsh:
if ((x >= 5)); then …

# Use (( )) for arithmetic operation
if ((j==u+2))

# Use [[ ]] for comparison
if [[ $age -gt 21 ]]

More if commands

For loop

for i in $(ls); do echo file $i;done
for i in *; do echo file $i; done

# Press any key to continue each loop
for i in $(cat tpc_stats_0925.log |grep failed|grep -o '\query\w\{1,2\}');do cat ${i}.log; read -rsp $'Press any key to continue...\n' -n1 key;done

# Print a file line by line when a key is pressed, 
oifs="$IFS"; IFS=$'\n'; for line in $(cat myfile); do ...; done
while read -r line; do ...; done <myfile

#If only one word a line, simply
for line in $(cat myfile); do echo $line; read -n1; done

#Loop through an array
for i in "${arrayName[@]}"; do echo $i;done

While loop,

# Column subtraction of a file (e.g. a 3 columns file)
while read a b c; do echo $(($c-$b));done < <(head filename)
#there is a space between the two '<'s

# Sum up column subtraction
i=0; while read a b c; do ((i+=$c-$b)); echo $i; done < <(head filename)

# Keep checking a running process (e.g. perl) and start another new process (e.g. python) immediately after it. (BETTER use the wait command! Ctrl+F 'wait')
while [[ $(pidof perl) ]];do echo f;sleep 10;done && python timetorunpython.py

switch (case in bash)

read type;
case $type in
    echo 'how'
    echo 'are'
    echo 'you'

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Some handy environment variables

$0   :name of shell or shell script.  
$1, $2, $3, ... :positional parameters.  
$#   :number of positional parameters.  
$?   :most recent foreground pipeline exit status.  
$-   :current options set for the shell.  
$$   :pid of the current shell (not subshell).  
$!   :is the PID of the most recent background command.  

$DESKTOP_SESSION     current display manager
$EDITOR   preferred text editor.
$LANG   current language.
$PATH   list of directories to search for executable files (i.e. ready-to-run programs)
$PWD    current directory
$SHELL  current shell   
$USER   current username
$HOSTNAME   current hostname

Variable substitution within quotes

# foo=bar
 echo "'$foo'"
# double/single quotes around single quotes make the inner single quotes expand variables

Get the length of variable

var="some string"
echo ${#var}  
# 11

Get the first character of the variable

echo "${var:0:1}"

# or
echo ${var%%"${var#?}"}

Remove the first or last string from variable

var="some string"
echo ${var:2}
#me string

Replacement (e.g. remove the first leading 0 )

echo ${var[@]#0}

Replacement (e.g. replace 'a' with ',')


Replace all (e.g. replace all 'a' with ',')

#with grep
 test="god the father"
 grep ${test// /\\\|} file.txt
 # turning the space into 'or' (\|) in grep

To change the case of the string stored in the variable to lowercase (Parameter Expansion)

echo ${var,,}

Expand and then execute variable/argument

eval "$cmd"
echo "$bar" # foo
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Arithmetic Expansion in Bash (Operators: +, -, *, /, %, etc)

echo $(( 10 + 5 ))  #15
echo $(( x++ )) #1 , notice that it is still 1, since it's post-incremen
echo $(( x++ )) #2
echo $(( ++x )) #4 , notice that it is not 3 since it's pre-incremen
echo $(( x-- )) #4
echo $(( x-- )) #3
echo $(( --x )) #1
echo $(( x ** y )) #8

Print out the prime factors of a number (e.g. 50)

factor 50

Sum up input list (e.g. seq 10)

seq 10|paste -sd+|bc

Sum up a file (each line in file contains only one number)

awk '{s+=$1} END {print s}' filename

Column subtraction

cat file| awk -F '\t' 'BEGIN {SUM=0}{SUM+=$3-$2}END{print SUM}'

Simple math with expr

expr 10+20 #30
expr 10\*20 #600
expr 30 \> 20 #1 (true)

More math with bc

# Number of decimal digit/ significant figure 
echo "scale=2;2/3" | bc  

# Exponent operator  
echo "10^2" | bc  

# Using variables   
echo "var=5;--var"| bc  
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Find out the time require for executing a command

time echo hi

Wait for some time (e.g 10s)

sleep 10

Log out your account after a certain period of time (e.g 10 seconds)

#once you set this variable, logout timer start running!

Set how long you want to run a command

#This will run the command 'sleep 10' for only 1 second.
timeout 1 sleep 10

Set when you want to run a command (e.g 1 min from now)

at now + 1min  #time-units can be minutes, hours, days, or weeks
warning: commands will be executed using /bin/sh
at> echo hihigithub >~/itworks
at> <EOT>   # press Ctrl + D to exit
job 1 at Wed Apr 18 11:16:00 2018

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Download the content of this README.md (the one your are viewing now)

curl https://raw.githubusercontent.com/mohatb/LinuxCheatSheet/master/welcome | pandoc -f markdown -t man | man -l -
# or w3m (a text based web browser and pager)
curl https://raw.githubusercontent.com/mohatb/LinuxCheatSheet/master/welcome | pandoc | w3m -T text/html

# or using emacs (in emac text editor) 
emacs --eval '(org-mode)' --insert <(curl https://raw.githubusercontent.com/mohatb/LinuxCheatSheet/master/welcome | pandoc -t org)

# or using emacs (on terminal, exit using Ctrl + x then Ctrl + c) 
emacs -nw --eval '(org-mode)' --insert <(curl https://raw.githubusercontent.com/mohatb/LinuxCheatSheet/master/welcome | pandoc -t org)

Download all from a page

wget -r -l1 -H -t1 -nd -N -np -A mp3 -e robots=off http://example.com

# -r: recursive and download all links on page  
# -l1: only one level link  
# -H: span host, visit other hosts  
# -t1: numbers of retries  
# -nd: don't make new directories, download to here  
# -N: turn on timestamp  
# -nd: no parent  
# -A: type (separate by ,)  
# -e robots=off: ignore the robots.txt file which stop wget from crashing the site, sorry example.com

Upload a file to web and download (https://transfer.sh/)

#  Upload a file (e.g. filename.txt):
curl --upload-file ./filename.txt https://transfer.sh/filename.txt
# the above command will return a URL, e.g: https://transfer.sh/tG8rM/filename.txt 

# Next you can download it by:
curl https://transfer.sh/tG8rM/filename.txt -o filename.txt

Download file if necessary

if [ ! -s $data ];then
    echo "downloading test data..."
    wget $url

Wget to a filename (when a long name)

wget -O filename "http://example.com"

Wget files to a folder

wget -P /path/to/directory "http://example.com"

Instruct curl to follow any redirect until it reaches the final destination:

curl -L google.com
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Random generate password (e.g. generate 5 password each of length 13)

sudo apt install pwgen
pwgen 13 5
#sahcahS9dah4a xieXaiJaey7xa UuMeo0ma7eic9 Ahpah9see3zai acerae7Huigh7

Random pick 100 lines from a file

shuf -n 100 filename

Random order (lucky draw)

for i in a b c d e; do echo $i; done| shuf

Echo series of random numbers between a range (e.g. shuffle numbers from 0-100, then pick 15 of them randomly)

shuf -i 0-100 -n 15

Echo a random number

echo $RANDOM

Random from 0-9

echo $((RANDOM % 10))

Random from 1-10

echo $(((RANDOM %10)+1))
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X11 GUI applications! Here are some GUI tools for you if you get bored by the text-only environment.

Enable X11 forwarding,in order to use graphical application on servers

ssh -X user_name@ip_address

# or setting through xhost
# --> Install the following for Centos:  
# xorg-x11-xauth  
# xorg-x11-fonts-*  
# xorg-x11-utils  

Little xwindow tools


Open pictures/images from ssh server

1. ssh -X user_name@ip_address
2. apt-get install eog
3. eog picture.png

Watch videos on server

1. ssh -X user_name@ip_address
2. sudo apt install mpv
3. mpv myvideo.mp4

Use gedit on server (GUI editor)

1. ssh -X user_name@ip_address
2. apt-get install gedit
3. gedit filename.txt

Open PDF file from ssh server

1. ssh -X user_name@ip_address
2. apt-get install evince
3. evince filename.pdf

Use google-chrome browser from ssh server

1. ssh -X user_name@ip_address
2. apt-get install libxss1 libappindicator1 libindicator7
3. wget https://dl.google.com/linux/direct/google-chrome-stable_current_amd64.deb
4. sudo apt-get install -f
5. dpkg -i google-chrome*.deb
6. google-chrome
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Eliminate the zombie

# A zombie is already dead, so you cannot kill it. You can eliminate the zombie by killing its parent.
# First, find PID of the zombie
ps aux| grep 'Z'
# Next find the PID of zombie's parent
pstree -p -s <zombie_PID>
# Then you can kill its parent and you will notice the zombie is gone.
sudo kill 9 <parent_PID>

Show memory usage

free -c 10 -mhs 1
# print 10 times, at 1 second interval

Display CPU and IO statistics for devices and partitions.

# refresh every second
iostat -x -t 1

Display bandwidth usage on an network interface (e.g. enp175s0f0)

iftop -i enp175s0f0

Tell how long the system has been running and number of users


Check if it's root running

if [ "$EUID" -ne 0 ]; then
        echo "Please run this as root"
        exit 1

Change shell of a user (e.g. bonnie)

chsh -s /bin/sh bonnie 
# /etc/shells: valid login shells

Change root / fake root / jail (e.g. change root to newroot)

chroot /home/newroot /bin/bash

# To exit chroot

Display file status (size; access, modify and change time, etc) of a file (e.g. filename.txt)

stat filename.txt

Snapshot of the current processes

ps aux

Display a tree of processes


Find maximum number of processes

cat /proc/sys/kernel/pid_max

Print or control the kernel ring buffer


Show IP address

$ip add show

# or

Print previous and current SysV runlevel


# or
who -r

Change SysV runlevel (e.g. 5)

init 5
telinit 5

Display all available services in all runlevels,

chkconfig --list
# update-rc.d equivalent to chkconfig in ubuntu

Check system version

cat /etc/*-release

Linux Programmer's Manuel: hier- description of the filesystem hierarchy

man hier

Control the systemd system and service manager

# e.g. check the status of cron service
systemctl status cron.service

# e.g. stop cron service
systemctl stop cron.service

List job

jobs -l

Run a program with modified priority (e.g. ./test.sh)

# nice value is adjustable from -20 (most favorable) to +19
# the nicer the application, the lower the priority 
# Default niceness: 10; default priority: 80

nice -10 ./test.sh

Export PATH

export PATH=$PATH:~/path/you/want

Make file executable

chmod +x filename
# you can now ./filename to execute it

Print system information

uname -a

# Check system hardware-platform (x86-64)
uname -i

Surf the net

links www.google.com

Add user, set passwd

useradd username
passwd username

Edit variable for bash, (e.g. displaying the whole path)

1. joe ~/.bash_profile 
2. export PS1='\u@\h:\w\$' 
# $PS1 is a variable that defines the makeup and style of the command prompt 
3. source ~/.bash_profile

Edit environment setting (e.g. alias)

1. joe ~/.bash_profile
2. alias pd="pwd" //no more need to type that 'w'!
3. source ~/.bash_profile

Print all alias

alias -p

Unalias (e.g. after alias ls='ls --color=auto')

unalias ls

Set and unset shell options

# print all shell options

# to unset (or stop) alias
shopt -u expand_aliases

# to set (or start) alias
shopt -s expand_aliases

List environment variables (e.g. PATH)

echo $PATH
# list of directories separated by a colon

List all environment variables for current user


Unset environment variable (e.g. unset variable 'MYVAR')

unset MYVAR

Show partition format


Inform the OS of partition table changes


Soft link program to bin

ln -s /path/to/program /home/usr/bin
# must be the whole path to the program

Show hexadecimal view of data

hexdump -C filename.class

Jump to different node

rsh node_name

Check port (active internet connection)

netstat -tulpn

Print resolved symbolic links or canonical file names

readlink filename

Find out the type of command and where it link to (e.g. python)

type python
# python is /usr/bin/python
# There are 5 different types, check using the 'type -f' flag
# 1. alias    (shell alias)
# 2. function (shell function, type will also print the function body)
# 3. builtin  (shell builtin)
# 4. file     (disk file)
# 5. keyword  (shell reserved word) 

# You can also use `which`
which python
# /usr/bin/python

List all functions names

declare -F

List total size of a directory

du -hs .

# or
du -sb

Copy directory with permission setting

cp -rp /path/to/directory

Store current directory

pushd . 
# then pop
#or use dirs to display the list of currently remembered directories. 
dirs -l 

Show disk usage

df -h 

# or
du -h 

du -sk /var/log/* |sort -rn |head -10

Show current runlevel


Switch runlevel

init 3 

telinit 3 

Permanently modify runlevel

1. edit /etc/init/rc-sysinit.conf 

Become root


Become somebody

su somebody

Report user quotes on device

repquota -auvs

Get entries in a number of important databases

getent database_name

# (e.g. the 'passwd' database)
getent passwd
# list all user account (all local and LDAP)  

# (e.g. fetch list of grop accounts)
getent group
# store in database 'group'

Change owner of file

chown user_name filename
chown -R user_name /path/to/directory/
# chown user:group filename

Mount and unmount

# e.g. Mount /dev/sdb to /home/test
mount /dev/sdb /home/test

# e.g. Unmount /home/test
umount /home/test

List current mount detail

# or

List current usernames and user-numbers

cat /etc/passwd

Get all username

getent passwd| awk '{FS="[:]"; print $1}'

Show all users

compgen -u

Show all groups

compgen -g

Show group of user

group username

Show uid, gid, group of user

id username

Check if it's root

if [ $(id -u) -ne 0 ];then
    echo "You are not root!"
# 'id -u' output 0 if it's not root

Find out CPU information

more /proc/cpuinfo

# or

Set quota for user (e.g. disk soft limit: 120586240; hard limit: 125829120)

setquota username 120586240 125829120 0 0 /home

Show quota for user

quota -v username

Display current libraries from the cache

ldconfig -p

Print shared library dependencies (e.g. for 'ls')

ldd /bin/ls

Check user login


Edit path for all users

joe /etc/environment
# edit this file

Show and set user limit

ulimit -u

Which ports are listening for TCP connections from the network

nmap -sT -O localhost
#notice that some companies might not like you using nmap

Print out number of cores/ processors

nproc --all

Check status of each core

1. top  
2. press '1'

Show jobs and PID

jobs -l

List all running services

service --status-all

Schedule shutdown server

shutdown -r +5 "Server will restart in 5 minutes. Please save your work."

Cancel scheduled shutdown

shutdown -c

Broadcast to all users

wall -n hihi

Kill all process of a user

pkill -U user_name

Kill all process of a program

kill -9 $(ps aux | grep 'program_name' | awk '{print $2}')

Set gedit preference on server

# You might have to install the following:

apt-get install libglib2.0-bin; 
# or
yum install dconf dconf-editor;  
yum install dbus dbus-x11;  

# Check list  
gsettings list-recursively

# Change some settings 
gsettings set org.gnome.gedit.preferences.editor highlight-current-line true
gsettings set org.gnome.gedit.preferences.editor scheme 'cobalt'
gsettings set org.gnome.gedit.preferences.editor use-default-font false
gsettings set org.gnome.gedit.preferences.editor editor-font 'Cantarell Regular 12'

Add user to a group (e.g add user 'nice' to the group 'docker', so that he can run docker without sudo)

sudo gpasswd -a nice docker

Pip install python package without root

1. pip install --user package_name
2. You might need to export ~/.local/bin/ to PATH: export PATH=$PATH:~/.local/bin/

Removing old linux kernels (when /boot almost full...)

1. uname -a  #check current kernel, which should NOT be removed
2. sudo apt-get purge linux-image-X.X.X-X-generic  #replace old version

Change hostname

sudo hostname your-new-name

# if not working, do also:  
hostnamectl set-hostname your-new-hostname
# then check with:
# Or check /etc/hostname  

# If still not working..., edit:  
#add HOSTNAME="your-new-hostname"  

List installed packages

apt list --installed

# or on Red Hat: 
yum list installed

Check which file make the device busy on umount

lsof /mnt/dir

When sound not working

killall pulseaudio
# then press Alt-F2 and type in pulseaudio  

When sound not working

killall pulseaudio

List information about SCSI devices


Tutorial for setting up your own DNS server


Tutorial for creating a simple daemon


Tutorial for using your gmail to send email


Using telnet to test open ports, test if you can connect to a port (e.g 53) of a server (e.g

telnet 53

change network maximum transmission unit (mtu) (e.g. change to 9000)

ifconfig eth0 mtu 9000

get pid of a running process (e.g python)

pidof python

# or  
ps aux|grep python


# Start ntp:

# Check ntp:  
ntpq -p

Remove unnecessary files to clean your server

sudo apt-get autoremove
sudo apt-get clean
sudo rm -rf ~/.cache/thumbnails/*

# Remove old kernal:
sudo dpkg --list 'linux-image*'
sudo apt-get remove linux-image-OLDER_VERSION

Increase/ resize root partition (root partition is an LVM logical volume)

lvextend -L +130G /dev/rhel/root -r
# Adding -r will grow filesystem after resizing the volume.

Create a UEFI Bootable USB drive (e.g. /dev/sdc1)

sudo dd if=~/path/to/isofile.iso of=/dev/sdc1 oflag=direct bs=1048576

Locate and remove a package

sudo dpkg -l | grep <package_name>
sudo dpkg --purge <package_name>

Create a ssh tunnel

ssh -f -L 9000:targetservername:8088 root@ -N
#-f: run in background; -L: Listen; -N: do nothing  
#the 9000 of your computer is now connected to the 8088 port of the targetservername through  
#so that you can see the content of targetservername:8088 by entering localhost:9000 from your browser.

Get process ID of a process (e.g. sublime_text)

pidof sublime_text

#pgrep, you don't have to type the whole program name
pgrep sublim

#top, takes longer time
top|grep sublime_text

Some benchmarking tools for your server

aio-stress - AIO benchmark.
bandwidth - memory bandwidth benchmark.
bonnie++ - hard drive and file system performance benchmark.
dbench -  generate I/O workloads to either a filesystem or to a networked CIFS or NFS server.
dnsperf - authorative and recursing DNS servers.
filebench - model based file system workload generator.
fio - I/O  benchmark.
fs_mark - synchronous/async file creation benchmark.
httperf - measure web server performance.
interbench - linux interactivity  benchmark.
ioblazer - multi-platform storage stack micro-benchmark.
iozone - filesystem benchmark.
iperf3 - measure TCP/UDP/SCTP performance.
kcbench - kernel compile benchmark, compiles a kernel and measures the time it takes.
lmbench - Suite of simple, portable benchmarks.
netperf - measure network performance, test unidirectional throughput, and end-to-end latency.
netpipe - network protocol independent performance evaluator.
nfsometer - NFS performance framework.
nuttcp - measure network performance.
phoronix-test-suite - comprehensive automated testing and benchmarking platform.
seeker - portable disk seek benchmark.
siege - http load tester and benchmark.
sockperf - network benchmarking utility over socket API.
spew - measures I/O performance and/or generates I/O load.
stress - workload generator for POSIX systems.
sysbench - scriptable database and system performance benchmark.
tiobench - threaded IO benchmark.
unixbench - the original BYTE UNIX benchmark suite, provide a basic indicator of the performance of a Unix-like system.
wrk - HTTP benchmark.

Show a listing of last logged in users.


Show a listing of current logged in users, print information of them


Show who is logged on and what they are doing


Print the user names of users currently logged in to the current host.


Stop tailing a file on program terminate

tail -f --pid=<PID> filename.txt
# replace <PID> with the process ID of the program.

List all enabled services

systemctl list-unit-files|grep enabled

Back To Top


Collect and summarize all hardware info of your machine

lshw -json >report.json
# Other options are: [ -html ]  [ -short ]  [ -xml ]  [ -json ]  [ -businfo ]  [ -sanitize ] ,etc

Finding Out memory device detail

sudo dmidecode -t memory

Print detail of CPU hardware

dmidecode -t 4
#          Type   Information
#          0   BIOS
#          1   System
#          2   Base Board
#          3   Chassis
#          4   Processor
#          5   Memory Controller
#          6   Memory Module
#          7   Cache
#          8   Port Connector
#          9   System Slots
#         11   OEM Strings
#         13   BIOS Language
#         15   System Event Log
#         16   Physical Memory Array
#         17   Memory Device
#         18   32-bit Memory Error
#         19   Memory Array Mapped Address
#         20   Memory Device Mapped Address
#         21   Built-in Pointing Device
#         22   Portable Battery
#         23   System Reset
#         24   Hardware Security
#         25   System Power Controls
#         26   Voltage Probe
#         27   Cooling Device
#         28   Temperature Probe
#         29   Electrical Current Probe
#         30   Out-of-band Remote Access
#         31   Boot Integrity Services
#         32   System Boot
#         34   Management Device
#         35   Management Device Component
#         36   Management Device Threshold Data
#         37   Memory Channel
#         38   IPMI Device
#         39   Power Supply

Count the number of Segate hard disks

lsscsi|grep SEAGATE|wc -l
# or
sg_map -i -x|grep SEAGATE|wc -l

Get UUID of a disk (e.g. sdb)

blkid /dev/sdb

Print detail of all hard disks

#where ROTA means rotational device / spinning hard disks (1 if true, 0 if false)

List all PCI (Peripheral Component Interconnect) devices

# List information about NIC
lspci | egrep -i --color 'network|ethernet'

List all USB devices


Linux modules

# Show the status of modules in the Linux Kernel

# Add and remove modules from the Linux Kernel

# or
# Remove a module 

# Insert a module

Controlling IPMI-enabled devices (e.g. BMC)

# Remotely finding out power status of the server
ipmitool -U <bmc_username> -P <bmc_password> -I lanplus -H <bmc_ip_address> power status

# Remotely switching on server 
ipmitool -U <bmc_username> -P <bmc_password> -I lanplus -H <bmc_ip_address> power on

# Turn on panel identify light (default 15s)  
ipmitool chassis identify 255

# Found out server sensor temperature
ipmitool sensors |grep -i Temp

# Reset BMC
ipmitool bmc reset cold

# Prnt BMC network
ipmitool lan print 1

# Setting BMC network
ipmitool -I bmc lan set 1 ipaddr
ipmitool -I bmc lan set 1 netmask
ipmitool -I bmc lan set 1 defgw ipaddr
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Display IP address

ip a

Display route table

ip r

Display ARP cache (ARP cache displays the MAC addresses of device in the same network that you have connected to)

ip n

Add transient IP addres (reset after reboot) (e.g. add to device eno16777736)

ip address add dev eno16777736

Persisting network configuration changes

sudo vi /etc/sysconfig/network-scripts/ifcfg-enoxxx
# then edit the fields: BOOTPROT, DEVICE, IPADDR, NETMASK, GATEWAY, DNS1 etc

Refresh NetworkManager

sudo nmcli c reload

Restart all interfaces

sudo systemctl restart network.service

To view hostname, OS, kernal, architecture at the same time!


Set hostname (set all transient, static, pretty hostname at once)

hostnamectl set-hostname "mynode"
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Encode strings as Base64 strings

echo test|base64

Do not echo the trailing newline

username=`echo -n "bashterminalcheatsheet"`

Get parent directory of current directory

dirname `pwd`

Copy a file to multiple files (e.g copy fileA to file(B-D))

tee <fileA fileB fileC fileD >/dev/null

Remove newline / nextline

tr --delete '\n' <input.txt >output.txt

Replace newline

tr '\n' ' ' <filename

To uppercase/lowercase

tr /a-z/ /A-Z/

Translate a range of characters (e.g. substitute a-z into a)

echo 'something' |tr a-z a
# aaaaaaaaa

Compare two files (e.g. fileA, fileB)

diff fileA fileB
# a: added; d:delete; c:changed

# or
sdiff fileA fileB
# side-to-side merge of file differences

Compare two files, strip trailing carriage return/ nextline (e.g. fileA, fileB)

 diff fileA fileB --strip-trailing-cr

Number a file (e.g. fileA)

nl fileA

nl -nrz fileA
# add leading zeros

nl -w1 -s ' '
# making it simple, blank separate

Join two files field by field with tab (default join by the first column of both file, and default separator is space)

# fileA and fileB should have the same ordering of lines.
join -t '\t' fileA fileB
# Join using specified field (e.g. column 3 of fileA and column 5 of fileB)
join -1 3 -2 5 fileA fileB

Combine/ paste two or more files into columns (e.g. fileA, fileB, fileC)

paste fileA fileB fileC
# default tab separate

Reverse string

echo 12345| rev

Read .gz file without extracting

zmore filename

# or
zless filename

Run command in background, output error file

some_commands  &>log &

# or
some_commands 2>log &

# or
some_commands 2>&1| tee logfile

# or
some_commands |& tee logfile

# or
some_commands 2>&1 >>outfile
#0: standard input; 1: standard output; 2: standard error

Run multiple commands in background

# run sequentially
(sleep 2; sleep 3) &

# run parallelly 
sleep 2 & sleep 3 &

Run process even when logout (immune to hangups, with output to a non-tty)

# e.g. Run myscript.sh even when log out.
nohup bash myscript.sh

Send mail

echo 'heres the content'| mail -a /path/to/attach_file.txt -s 'mail.subject' me@gmail.com
# use -a flag to set send from (-a "From: some@mail.tld")

Convert .xls to csv

xls2csv filename

Append to file (e.g. hihi)

echo 'hihi' >>filename

Make BEEP sound

speaker-test -t sine -f 1000 -l1

Set beep duration

(speaker-test -t sine -f 1000) & pid=$!;sleep 0.1s;kill -9 $pid

History edit/ delete


history -d [line_number]

Get last history/record filename

head !$

Clean screen


# or

Send data to last edited file

cat /directory/to/file
echo 100>!$

To create an archive of a directory and its contents

tar -czvf name-of-archive.tar.gz /path/to/directory-or-file

#-c: Create an archive.
#-z: Compress the archive with gzip.
#-v: Display progress in the terminal while creating the archive, also known as “verbose” mode. The v is always optional in these commands, but it’s helpful.
#-f: Allows you to specify the filename of the archive.

To Compress multiple directories and files to a single file

tar -czvf archive.tar.gz /home/ubuntu/Downloads /usr/local/stuff /home/ubuntu/Documents/notes.txt

to extract an archive archive.tar.gz to the current directory

tar -xzvf archive.tar.gz

Extract .xf

unxz filename.tar.xz  
# then
tar -xf filename.tar

Install python package

pip install packagename

Delete current bash command


# or

# or
# to make it to history

Add something to history (e.g. "addmetohistory")

# addmetodistory
# just add a "#" before~~

Sleep awhile or wait for a moment or schedule a job

sleep 5;echo hi

Backup with rsync

rsync -av filename filename.bak
rsync -av directory directory.bak
rsync -av --ignore_existing directory/ directory.bak
rsync -av --update directory directory.bak

rsync -av directory user@ip_address:/path/to/directory.bak
# skip files that are newer on receiver (i prefer this one!)

Create Directory path at once

mkdir -p project/{lib/ext,bin,src,doc/{html,info,pdf},demo/stat}
# -p: make parent directory  
# this will create project/doc/html/; project/doc/info; project/lib/ext ,etc

Run command only if another command returns zero exit status (well done)

cd tmp/ && tar xvf ~/a.tar

Run command only if another command returns non-zero exit status (not finish)

cd tmp/a/b/c ||mkdir -p tmp/a/b/c

Extract to a path

tar xvf -C /path/to/directory filename.gz

Use backslash "" to break long command

cd tmp/a/b/c \
> || \
>mkdir -p tmp/a/b/c

Get pwd

VAR=$PWD; cd ~; tar xvf -C $VAR file.tar
# PWD need to be capital letter

List file type of file (e.g. /tmp/)

file /tmp/
# tmp/: directory

Bash script

# remove string before a "."

Python simple HTTP Server

python -m SimpleHTTPServer
# or when using python3:
python3 -m http.server

Read user input

read input
echo $input

Generate sequence 1-10

seq 10

Find average of input list/file

i=`wc -l filename|cut -d ' ' -f1`; cat filename| echo "scale=2;(`paste -sd+`)/"$i|bc

Generate all combination (e.g. 1,2)

echo {1,2}{1,2}
# 1 1, 1 2, 2 1, 2 2

Generate all combination (e.g. A,T,C,G)

set = {A,T,C,G}
group= 5
for ((i=0; i<$group; i++));do
    bash -c "echo "$repetition""

Read file content to variable


Echo size of variable

echo ${#foo}

Echo tab

echo -e ' \t '


declare -a array=()
# or
declare array=()
# or associative array 
declare -A array=()

Send a directory to remote path

scp -r directoryname user@ip:/path/to/send

Split file into smaller file

# Split by line (e.g. 1000 lines/smallfile)
split -d -l 1000 largefile.txt
# Split by byte without breaking lines across files
split -C 10 largefile.txt

Create a large amount of dummy files (e.g 100000 files, 10 bytes each):

#1. Create a big file
dd if=/dev/zero of=bigfile bs=1 count=1000000

#2. Split the big file to 100000 10-bytes files
 split -b 10 -a 10 bigfile

Rename all files (e.g. remove ABC from all .gz files)

rename 's/ABC//' *.gz

Remove file extension (e.g remove .gz from filename.gz)

basename filename.gz .gz

zcat filename.gz> $(basename filename.gz .gz).unpacked

Fork bomb

# Don't try this at home!
# It is a function that calls itself twice every call until you run out of system resources.
# A '# ' is added in front for safety reason, remove it when seriously you are testing it. 
# :(){:|:&};:

Add file extension to all file(e.g add .txt)

rename s/$/.txt/ *
# You can use rename -n s/$/.txt/ * to check the result first, it will only print sth like this:
# rename(a, a.txt)
# rename(b, b.txt)
# rename(c, c.txt)

Use the squeeze repeat option (e.g. /t/t --> /t)

tr -s "/t" < filename

Do not print nextline with echo

echo -e 'text here \c'

Use the last argument


Check last exit code

echo $?

View first 50 characters of file

head -c 50 file

Group/combine rows into one row

# e.g.  
# AAAA  
# BBBB  
# CCCC  
# DDDD  
cat filename|paste - -
cat filename|paste - - - -

Fastq to fasta

cat file.fastq | paste - - - - | sed 's/^@/>/g'| cut -f1-2 | tr '\t' '\n' >file.fa

Cut and get last column

cat file|rev | cut -d/ -f1 | rev

Add one to variable/increment/ i++ a numeric variable (e.g. $var)

# or

Clear the contents of a file (e.g. filename)


Unzip tar.bz2 file (e.g. file.tar.bz2)

tar xvfj file.tar.bz2

Unzip tar.xz file (e.g. file.tar.xz)

unxz file.tar.xz
tar xopf file.tar

Output a y/n repeatedly until killed

# 'y':

# or 'n':
yes n

# or 'anything':
yes anything

# For example: 
yes | rm -r large_directory

Create dummy file of certain size instantly (e.g. 200mb)

dd if=/dev/zero of=//dev/shm/200m bs=1024k count=200
# or
dd if=/dev/zero of=//dev/shm/200m bs=1M count=200

# Standard output:  
# 200+0 records in  
# 200+0 records out  
# 209715200 bytes (210 MB) copied, 0.0955679 s, 2.2 GB/s 

Cat to a file

cat >myfile
let me add sth here
exit by control + c

Keep /repeatedly executing the same command (e.g Repeat 'wc -l filename' every 1 second)

watch -n 1 wc -l filename

Print commands and their arguments when execute (e.g. echo expr 10 + 20)

set -x; echo `expr 10 + 20 `

Print some meaningful sentences to you (install fortune first)


Colorful (and useful) version of top (install htop first)


Press any key to continue

read -rsp $'Press any key to continue...\n' -n1 key

Run sql-like command on files from terminal

# download:  
# https://github.com/harelba/q  
# example:
q -d "," "select c3,c4,c5 from /path/to/file.txt where c3='foo' and c5='boo'"

Using Screen for multiple terminal sessions

# Create session and attach:

# Create detached session foo:  
screen -S foo -d -m	

# Detached session foo:  
screen: ^a^d

# List sessions:  
screen -ls	

# Attach last session: 
screen -r	

# Attach to session foo:
screen -r foo	

# Kill session foo:
screen -r foo -X quit

# Scroll:  
Hit your screen prefix combination (C-a / control+A), then hit Escape.  
Move up/down with the arrow keys (↑ and ↓).  

# Redirect output of an already running process in Screen:  
 (C-a / control+A), then hit 'H'  

# Store screen output for Screen:  
Ctrl+A, Shift+H  
# You will then find a screen.log file under current directory.

Using Tmux for multiple terminal sessions

# Create session and attach:

# Attach to session foo:
tmux attach -t foo

# Detached session foo:  

# List sessions:  
tmux ls
# Attach last session: 
tmux attach

# Kill session foo:
tmux kill-session -t foo

# Create detached session foo:  
tmux new -s foo -d

# Send command to all panes in tmux:
:setw synchronize-panes

# Some tmux pane control commands:
#   Panes (splits), Press Ctrl+B, then input the following symbol:
#   %  horizontal split
#   "  vertical split
#   o  swap panes
#   q  show pane numbers
#   x  kill pane
#   space - toggle between layouts

#   Distribute Vertically (rows): 
select-layout even-vertical
#   or
Ctrl+b, Alt+2

# Distribute horizontally (columns): 
select-layout even-horizontal
#   or
Ctrl+b, Alt+1

# Scroll
Ctrl-b then \[ then you can use your normal navigation keys to scroll around.  
Press q to quit scroll mode.  

Cut the last column

cat filename|rev|cut -f1|rev

Pass password to ssh

sshpass -p mypassword ssh root@ "df -h"

Wait for a pid (job) to complete

wait %1 
# or 
wait $PID
wait ${!}
#wait ${!} to wait till the last background process ($! is the PID of the last background process)

Convert pdf to txt

sudo apt-get install poppler-utils
pdftotext example.pdf example.txt

List only directory

ls -ld -- */

Capture/record/save terminal output (capture everything you type and output)

script output.txt
# start using terminal 
# to logout the screen session (stop saving the contents), type exit.

List contents of directories in a tree-like format.

# go to the directory you want to list, and type tree (sudo apt-get install tree)
# output:
# home/
# └── project
#     ├── 1
#     ├── 2
#     ├── 3
#     ├── 4
#     └── 5

# set level directories deep (e.g. level 1)
tree -L 1
# home/
# └── project

Set up virtualenv(sandbox) for python

# 1. install virtualenv.
sudo apt-get install virtualenv
# 2. Create a directory (name it .venv or whatever name your want) for your new shiny isolated environment.
virtualenv .venv
# 3. source virtual bin
source .venv/bin/activate
# 4. you can check check if you are now inside a sandbox.
type pip
# 5. Now you can install your pip package, here requirements.txt is simply a txt file containing all the packages you want. (e.g tornado==4.5.3).
pip install -r requirements.txt

Working with json data

#install the useful jq package
#sudo apt-get install jq
#e.g. to get all the values of the 'url' key, simply pipe the json to the following jq command(you can use .[]. to select inner json, i.e jq '.[].url')
jq '.url'

Editing your history

history -w
vi ~/.bash_history
history -r

Decimal to Binary (e.g get binary of 5)

echo -e ${D2B[5]}
echo -e ${D2B[255]}

Wrap each input line to fit in specified width (e.g 4 integers per line)

echo "00110010101110001101" | fold -w4
# 0011
# 0010
# 1011
# 1000
# 1101

Sort a file by column and keep the original order

sort -k3,3 -s

Right align a column (right align the 2nd column)

cat file.txt|rev|column -t|rev

To both view and store the output

echo 'hihihihi' | tee outputfile.txt
# use '-a' with tee to append to file.

Show non-printing (Ctrl) characters with cat

cat -v filename

Convert tab to space

expand filename

Convert space to tab

unexpand filename

Display file in octal ( you can also use od to display hexadecimal, decimal, etc)

od filename

Reverse cat a file

tac filename

Reverse the result from uniq -c

while read a b; do yes $b |head -n $a ;done <test.txt

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