Shell脚本概述 Shell Script

What is Bash Shell?

The default shell for many Linux distros is the GNU Bourne-Again Shell (bash). Bash is succeeded by Bourne shell (sh).

When first launching the shell, it will use a startup script located in the .bashrc or .bash_profile file.

shell prompt

• When a shell is used interactively, it displays a $ when it is waiting for a command from the user.

• If shell is running as root, the prompt is changed to #.

What is a Bash script?

A Bash script is a series of commands written in a file. The program executes them line by line.

You can run the script any number of times.

How to define a bash script?

• file extension of .sh
• script starts with a shebang, Shebang is a combination of bash # and bang ! followed the bash shell path
  • sample: #! /bin/bash
• execution permission x

Create the first Bash script

• create the script touch hello_world.sh

• find your bash which bash

• write the command

  #! /bin/bash
  echo "Hello World"

• provide the execution right to your user chmod u+x hello_world.sh

• run the script in the following two ways

  1. ./hello_world.sh
  2. bash hello_world.sh

The basic syntax

Define variable

We can define the variable by using variable_name=value. To get the value of a variable, add $ before the variable.

#! /bin/bash
greeting=Hello
name=Tux
echo $greeting $name

Run it

Arithmetic Expressions

Operator	Usage
+	addition
-	subtraction
	multiplication
/	division
*	exponentiation
%	modulus

We can execute the expressions by running the following cmd.

Note the space, they are part of the syntax

expr 1 + 1

Here are some samples

We can calculate the expressions and store the result in a variable like below

var=$((expression))

example

#!/bin/bash

var=$((3+9))
echo $var

Decimal calculations

As shown in the sample like below, bc stands for bash calculator, scale defines the number of decimal places required in the output

echo "scale=2;22/7" | bc

Read user input

read variable_name

example

#!/bin/bash

echo "Enter a numner"
read a

echo "Enter a numner"
read b

var=$((a+b))
echo $var

execution result

If else

condition checking

if [[ condition ]]
then
    statement
elif [[ condition ]]; then
    statement 
else
    do this by default
fi

Numeric Comparison Logical operators

Operation	Syntax	Explanation
Equality	num1 -eq num2	is num1 equal to num2
Greater than equal to	num1 -ge num2	is num1 greater than equal to num2
Greater than	num1 -gt num2	is num1 greater than num2
Less than equal to	num1 -le num2	is num1 less than equal to num2
Less than	num1 -lt num2	is num1 less than num2
Not Equal to	num1 -ne num2	is num1 not equal to num2

sample

read x
read y

if [ $x -gt $y ]
then
echo X is greater than Y
elif [ $x -lt $y ]
then
echo X is less than Y
elif [ $x -eq $y ]
then
echo X is equal to Y
fi

To make powerful comparison, we can use AND -a and OR -o as well.

The below statement translates to: if a is greater than 40 and b is less than 6.

if [ $a -gt 40 -a $b -lt 6 ]

Empty String

https://stackoverflow.com/questions/13509508/check-if-string-is-neither-empty-nor-space-in-shell-script

Desc	Syntax
not empty	[[ -n "${str// /}" ]]
str empty	if [ "$str" = "" ];then echo NULL fi
str empty v2	if [ ! "$str" ];then echo NULL fi
str not null	! -z "$str"
str not space	"$str" != " "
str not null and not space	! -z "$str" -a "$str" != " "

Example

str1="Hello World"
str2=" "
str3=""

if [ ! -z "$str1" -a "$str1" != " " ]; then
        echo "Str1 is not null or space"
fi

if [ ! -z "$str2" -a "$str2" != " " ]; then
        echo "Str2 is not null or space"
fi

if [ ! -z "$str3" -a "$str3" != " " ]; then
        echo "Str3 is not null or space"
fi

only str1 is not null or space

For loop

looping the numbers

#!/bin/bash

for i in {1..5}
do
    echo $i
done

looping the strings

#!/bin/bash

for X in cyan magenta yellow  
do
    echo $X
done

While loop

#!/bin/bash
i=1
while [[ $i -le 10 ]] ; do
   echo "$i"
  (( i += 1 ))
done

Reading file

#!/bin/bash

LINE=1

while read -r CURRENT_LINE
    do
        echo "$LINE: $CURRENT_LINE"
    ((LINE++))
done < "user_input.sh"

Get the output of commands

If you need to include the output of a complex command in your script, you can write the statement inside back ticks.

Syntax

• var= commands

Sample

#!/bin/bash

var=`df -h | grep home`                                                                                                130 ↵
echo $var

Get the arguement from command line

$@ represents the position of the parameters, starting from one.

#!/bin/bash

for x in $@
do
    echo "Entered arg is $x"
done

echo $0 # the sh itself
echo $1 # the first argument

Cronjob

Check the crontab expr here: https://crontab.guru/

Here, * represents minute(s) hour(s) day(s) month(s) weekday(s), respectively.

# Cron job example
* * * * * sh /path/to/script.sh

check the existing crons

crontab -l

Find cmd

find . -type f -name "*.sh"

• . represents the current directory. You can change the path accordingly.
• -type f indicates that the file type we are looking for is a text based file.
• *.sh tells to match all files ending with .sh.

3

Todo

environment variables

date

print

Reference

1. Shell Scripting for Beginners – How to Write Bash Scripts in Linux