Shell Cookbook

狼狼

Introduction

The default shell environment for most GNU/Linux system is Bash ( Bourne Again Shell ).
For any scripting languague in a Linux environment , a script start with a special line called shebang.
Shebang is a line for which #! is prefixed to the environment path. /bin/bash is the interpreter command path for Bash.


Execution of a script can be done in two ways. Either we can run the script as a command-line argument for sh or run a self execution with execution permission. for example:

• sh script.sh
  

If a script is run as command-line argument for sh, the shebang in the script is of no use.


The execution permission for the script can be set as follows:

• chmod +x script.sh
  

For self execution script.

• ./script.sh
  

In Bash, each command or command sequence is delimited by using a semicolon or a new line.

• cmd1 ; cmd2
  

or

• cmd1
  
• cmd2
  

Printing in the terminal



echo, printf,

• echo $PATH #/usr/local/bin:/usr/sbin:/usr/bin
  
• echo "$PATH" #/user/local/bin:.....
  
• echo '$PATH' #$PATH
  

Note: when using echo with single quotes, the variable inside the quotes will not be interpreted by Bash, but will be display as is .

• echo -e "1\t2\t3"
  

escaping new line in echo.

• printf "%-5s -%-10s %-4s\n" No Name Mark
  

Playing with variable and environment variable



For every process , environment variables in its runtime can be viewed by:

• cat /proc/$PID/environ
  

Set the PID with the process ID of the relevent process.
for example. assume that an application called gedit is running . We can obtain the progress ID of gedit with pgrep command as follows.

• pgrep gedit
  

1250

• cat /proc/1250/environ | tr '\0' '\n'
  

• cat /proc/1250/environ | tr '\0' '\n'
  

Printing the content of a variable is done using by prefixing $ with the variable name as follows:

• var="hello"  #Assignment of value to variable var.
  
• echo $var
  

or

• echo ${var}
  

There some default system environment variable:
SHELL, USER, HOME, PATH, UID and so on.
for example:

• echo $SHELL # /bin/bash
  

Finding length of string



Get the length of a variable value as follow:

• length=${#var}
  

for example:

• var=12345
  
• echo ${#var} # 5
  

Check for super user

• if [ $UID -ne 0 ]; then
  
• echo "Non root user. please run as root."
  
• else
  
• echo "Root user"
  
• fi
  

Doing math calculations with the shell



The bash shell environment can perform basic arithmatic operations using the commands  let, ( ( ) ) and [ ]. The two utilities expr and bc are very helpful performing advanced operations.


While using let, we use variable names without the $ prefix, for example:

• n1=2;
  
• n2=3;
  
• let result=n1+n2;
  
• let result++;
  
• echo $result;
  

The [ ]    operator can be used similar to the let command as follows:

• result=$[ n1 + n2 ]; # No space between = and $
  

(( )) can be alse used.

• result=$(( n1 + n2 ));
  

expr can be alse used for basic operation.

• result=`expr 3 + 4`
  
• result=$(expr $n1 + 5);
  

All of above methods do not support floating point numbers, and operate on integer only.


bc the precision calculator is an advanced utility for mathematical operations. It has a wide range of options.

• echo "4 * 0.56" | bc
  
• 
  
• result=`echo "$n1 * 1.5" | bc`
  

Arrays

• array_var=(1 2 3 4)
  
• array_var[0]="test1"
  
• array_var[1]="test2"
  
• array_var[2]="test3"
  
• varay_var[3]="test4"
  
• 
  
• echo ${array_var[0]};
  
• #echo $array_var[0];  # the usage is error.
  
• 
  
• echo ${array_var[@]}; # print array length  or  ${array_var
  
• }
  

Function and Arguments

A function can be defined as follows:

• function fname(){
  
• statements;
  
• }
  

or

• fname(){
  
• statements;
  
• }
  

$1 is the first argument
$n is the n argument
"$@“ expands as "$1" "$2" "$3" and so on
"$*"   expands as "$1c$2c$3", where c is the first character of IFS
”$@" is the most use one.

Store command output

cmd_out=$(COMMAND)
or
cmd_out=`ls | cat -n `


for example:

• cmd_out=$(ls | cat -n)
  
• cmd_out=`ls | cat -n`
  

Spawning a separate process with subshell



Subshell are separate processes. A subshell can be defined using the ( ) operator as follows:

• pwd;
  
• (cd /bin; ls)
  
• pwd;
  

Read input from Teriminal

read -n number-of-chars variable_name


Read a password in non-echoed mode:

• read -s var
  
• echo $var;
  

Display a message with read using:

• read -p "Enter input:" var
  
• echo $var
  

for var in {a..z}
do
echo $var;
done




or

• for((i=0; i/dev/null
  
• if [ $? -eq 0 ];
  
• then
  
• echo "Renaming $f to $newname"
  
• let count++;
  
• fi
  
• done