Build basic script

When creating a shell script, you must specify the shell to be used in the first line of the file. Its format is:

#!/bin/bash

environment variable

Example:

#!/bin/bash
#display user information from the system
echo "User info for userid: $USER"
echo UID: $UID
echo HOME: $HOME

User variable

The user variable can be any text string composed of letters, numbers or underscores, with a length of no more than 20. User variables are case sensitive

Example:

#!/bin/bash
#testing variales
days=10
guest="Katie"
echo "$guest checked in $days days ago"
days=5
guest="Jessica"
echo "$guest checked in $days days ago"

The dollar sign is required when referencing a variable, but not when assigning a value

Example:

#!/bin/bash
#assigning a variable value to another variable
value1=10
value2=$value1
echo The resulting value is $value2

Command replacement

Enclose the entire command with backquotes ` or $()

Example:

#!/bin/bash
testing=$(date)
#testing=`date`
echo "The date and time are: " $testing

Example:

#!/bin/bash
#copy the /usr/bin directory listing to a log file
today=$(date +%y%m%d)
ls /usr/bin -al > log.$today

Perform mathematical operations

expr command

Operator	describe
ARG1 | ARG2	If ARG1 is neither null nor zero, return ARG1; Otherwise, return to ARG2
ARG1 & ARG2	If no parameter is null or zero, return ARG1; Otherwise, return 0
ARG1 < ARG2	If ARG1 is less than ARG2, return 1, otherwise return 0
ARG1 <= ARG2	If ARG1 is less than or equal to ARG2, return 1, otherwise return 0
ARG1 = ARG2	If ARG1 is equal to ARG2, return 1, otherwise return 0
ARG1 != ARG2	If ARG1 is not equal to ARG2, return 1; Otherwise, return 0
ARG1 >= ARG2	If ARG1 is greater than or equal to ARG2, return 1; Otherwise, return 0
ARG1 > ARG2	If ARG1 is greater than ARG2, return 1; Otherwise, return 0
ARG1 + ARG2	Returns the arithmetic sum of ARG1 and ARG2
ARG1 - ARG2	Returns the arithmetic difference between ARG1 and ARG2
ARG1 * ARG2	Returns the arithmetic product of ARG1 and ARG2
ARG1 / ARG2	Returns the arithmetic quotient of ARG1 divided by ARG2
ARG1 % ARG2	Returns the arithmetic remainder of ARG1 divided by ARG2

Example:

#!/bin/bash
#An example of using the expr command
var1=10
var2=20
var3=$(expr $var2 / $var1)
echo The result is $var3

Use square brackets

In bash, when a mathematical operation result symbol is given to a variable, the mathematical expression can be enclosed by dollar sign and square brackets $[operation]

Example:

#!/bin/bash
var1=100
var2=50
var3=45
var4=$[$var1*($var2-$var3)]
echo The final result is $var4

bash shell mathematical operators only support integer operations

Example:

#!/bin/bash
var1=100
var2=45
var3=$[$var1/$var2]
echo The final result is $var3

Floating point solution

bc bash calculator

Example:

varriable=$(echo "options;expression" | bc)

scale is reserved for decimal places

Example:

#!/bin/bash
var1=$(echo "scale=4;3.44/5"|bc)
echo The answer is $var1

Example:

#!/bin/bash
var1=100
var2=45
var3=$(echo "scale=4;$var1/$var2"|bc)
echo The answer for this is $var3

Example:

#!/bin/bash
var1=20
var2=3.14159
var3=$(echo "scale=4;$var1*$var1"|bc)
var4=$(echo "scale=4;$var3*$var2"|bc)
echo The final result is $var4

The above method can solve the shorter operation, and the longer operation can be solved by the following methods:

    variable=$(bc<<EOF
    options
    statements
    expressions
    EOF
	)

The EOF text string identifies the start and end of inline redirection data

Example:

#!/bin/bash
var1=10.46
var2=43.67
var3=33.2
var4=71

var5=$(bc<<EOF
scale=4
a1=($var1*$var2)
b1=($var3*$var4)
a1+b1
EOF
	)
echo The final answer for this mess is $var5

Exit script

The exit status code of a successful command is 0. If there is an error at the end of a command, the exit status code is a positive number

LINUX exit status code

Status code	describe
0	The command ended successfully
1	General unknown error
2	Unsuitable shell command
126	The command is not executable
127	No command found
128	Invalid exit parameter
128+x	Serious error related to linux signal x
130	Command terminated by ctrl+c
255	Exit status code outside the normal range

Returns the specified exit code

Example:

#!/bin/bash
#testing the exit status
var1=10
var2=30
var3=$[$var1+$var2]
echo The answer is $var3
exit 5

exit returns the variables used in the parameters of the command

Example:

#!/bin/bash
var1=10
var2=30
var3=$[$var1+$var2]
exit $var3

The maximum return value can only be 255. If it exceeds the range of 0-255, the modular operation result will be obtained

Example:

#!/bin/bash
var1=10
var2=30
var3=$[$var1*$var2]
exit $var3