Linux Shell Script Cheatsheet #1

Color console

reset=0, black=30, red=31, green=32, yellow=33, blue=34, magenta=35, cyan=36, and white=37

Coloured background
reset = 0, black = 40, red = 41, green = 42, yellow = 43, blue = 44, magenta = 45, cyan = 46, and white=47

Usage
echo -e "\e[1;XXm Green Background \e[0m"

replace XX with color code



Printf formatting

printf  "%-5s %-10s %-4.2f\n" 1 Sarath 80.3456 

- left align, 5 is the width 

Obtain process id of a running application

pgrep gedit

Get env variables for process

cat /proc/12501/environ  | tr '\0' '\n

tr command will replace null with newline

Setting env/user variables 

export HTTP_PROXY=http://192.168.0.2:3128

Length of variable

length=${#var}

Identify Current shell 

echo $SHELL or echo $0

Current user id

$UID

The UID for the root user is 0.

Change the prompt

Modify the $PS1 in the bashrc file in the home directory

Eg: PS1='\[`[ $? = 0 ] && X=2 || X=1; tput setaf $X`\]\h\[`tput sgr0`\]:$PWD\n\$ ' 

Delete line on vi 

dd 

Calculations in shell script (Integers)

let result=no1+no2
echo $result

Not let command need to pass variables without $ sign

Or 

result=$[ $no1 + no2 ]

result=$(( $no1 + no2 )) 

using 4 sign inside is permitted for these options 

Or

result=`expr 3 + 4`
result=$(expr $no1 + 5)

Floating point calculations 

echo "4 * 0.56" | bc 

result=`echo "$no * 1.5" | bc` 

echo "scale=2;3/8" | bc 

File Descriptors

0 - stdin

1 - stdout

2 - stderr

> truncate the file 

>> append to a file

Redirecting to specific descriptor Eg:-

1 is default

2> or 2>> 

Transfer stderr and stdout to single file

cmd 2>&1 output.txt

OR

cmd &> output.txt

Ignore output

redirect to /dev/null 

Tee command direct stdout to text and also display on screen 

cat a* | tee –a out.txt | cat –n.

-a append the data  

Regular and Associative array

Regular arrays are arrays which can use only integers as its array 
index. But associative arrays are arrays which can take a string as its 
array index.  

declare -A ass_array

Alias

Shortcut for an actual command

alias rm='cp $@ ~/backup; rm $@' 

Get return value of last command

$? will give the return value of the command 

Functions can be exported like variables

Back quote ( ` ) ~ key 

Can be used to store output of a command 

cmd_output=`ls | cat -n` 

Note:- single quote (') will save the command itself.

Subshell 

executing commands in ( )  will not affect the current shell.

 
pwd;
(cd /bin; ls);
pwd;

Read command 

Accepts user input.  

Shortcut conditions 

[ condition ] && action; # action executes if condition is true.
[ condition ] || action; # action executes if condition is false. 

Filesystem tests 

-f is a filepath or file

-x is an executable

-d is a directory

-e is an existing file

-w is a writable file

-r is a readable file

-L is a symbolic link

Eg:-

fpath="/etc/passwd"
if [ -e $fpath ]; then
echo File exists; 
else
echo Does not exist; 
fi

String comparisions: 

Best to use [[double square brackets]] while comparing 

[[ a = b ]] //Comparision. Space is needed

[[a=b]] //Assignment 

-z : Empty string

-n : non empty string

test command

Square brackets can be eliminated by appending command to test command 

if  test $var -eq 0 ; then echo "True"; fi