Shell appendix

$ bash < ex1

Can read in

 ex1

program in and execute

Example: taking script name as parameter

Its general form is:

 $ bash script name [parameter]

For example:

 $ bash ex2 /usr/meng /usr/zhang

The execution process is the same as the previous method, but the advantage of this method is that it can have parameters after the script name, thereby passing the parameter values ​​​​to the commands in the program, so that a Shell script can handle multiple situations, just like a function When calling, the corresponding actual parameters can be passed according to the specific problem.

Example: Execute with .

If you start with the current shell (starting with

 ·

means) to execute a Shell script, you can use the following simple form:

 $ · ex3 [parameter]

Example: Direct execution

Set the shell script's permissions to executable, and then execute it directly at the prompt.

Specific methods:

 $ chmod a+x ex4$ ./ex4

This requirement specifies the specific Shell that executes the script at the beginning of the Shell script, for example

 /bin/bash

:

 #!/bin/bash

Shell execution principle

The Shell receives the command (script name) entered by the user and analyzes it. If the file is marked executable but is not a compiled program, the shell considers it to be a shell script. The shell will read the contents, interpret them and execute them. So, from the user's perspective, executing a shell script is similar to executing a regular executable file.

Therefore, user-developed shell scripts can reside under a directory on the command search path (usually

 /bin

,

 /usr/bin

etc.), use it like a normal command. In this way, you develop your own new commands. This method is more convenient if you plan to use the prepared shell script repeatedly.

variable assignment

You can assign the execution result of a command to a variable. There are two forms of command substitution: one is to use backticks to quote the command, its general form is:

 command list

.

Example: Get the current working directory and store it in a variable

For example: store the full path name of the current working directory in the variable dir and enter the following command line:

 $ dir=`pwd`

Another form is:

 $(command list)

. The above command line can also be rewritten as:

 $dir=$(pwd)

array

 Bash

Only one-dimensional arrays are provided, and the size of the array is not limited. Similar to C language, the subscripts of array elements are numbered starting from 0. To obtain elements in an array, use subscripts. The subscript can be an integer or an arithmetic expression, and its value should be greater than or equal to 0. Users can use assignment statements to assign values ​​to array variables.

Example: Assigning values ​​to array elements

The general form of assigning values ​​to array elements is:

 Array name [subscript] = value

,For example:

 $ city[0]=Beijing$ city[1]=Shanghai$ city[2]=Tianjin

Can also be used

 declare

The command explicitly declares an array, the general form is:

 $ declare -a array name

Example: Access an array element

The general format for reading array element values ​​is:

 ${array name[subscript]}

,For example:

 $ echo ${city[0]}Beijing

Example: array combination assignment

Each element of an array can be assigned element by element using the above method, or it can be assigned in combination. The general form of defining an array and assigning it an initial value is:

 Array name = (value 1 value 2 ... value n)

Each value is separated by spaces. For example:

 $ A=(this is an example of shell script)$ echo ${A[0]} ${A[2]} ${A[3]} ${A[6]}this an example script$ echo ${ A[8]}

Since there are 7 initial values ​​in the value table, so

 A

The number of elements is also 7.

 A[8]

Exceeds assigned array

 A

range, it is considered to be a new element. Since there is no value assigned in advance, its value is an empty string.

If the subscript of the array element is not given, the array name represents the array element with subscript 0, such as

 city

is equivalent to

 city[0]

.

Example: List all contents in an array

use

 *

or

 @

When subscripted, all elements in the array will be replaced.

 $ echo ${A[*]}this is an example of shell script

Example: Get the number of array elements

 $ echo ${#A[*]}7

Parameter passing

If you want to write a Shell to find the sum of two numbers, how can you implement it? In order to introduce the use of parameter passing, write a script like this:

 $ cat > addlet sum=$1+$2echo $sum

After saving, execute:

 $ chmod a+x ./add$ ./add 5 1015

It can be seen that 5 and 10 are passed to

 $1

and

 $2

, this is the shell's own preset parameter order. In fact, you can also define the variables first and then pass them in.

For example, modify the above script to get:

 let sum=$X+$Yecho $sum

Execute again:

 $ X=5 Y=10 ./add15

It can be found that the correct results can also be obtained.

Set environment variables

export an environment variable:

 $ export opid=True

That's it. If you want it to take effect after logging in, you can add it directly to

 /etc/profile

or

 ~/.bashrc

Inside.

Read variable value from keyboard

can pass

 read

To read a variable value, for example, to wait for the user to enter a value and display it:

 $ read -p Please enter a value: input ; echo You entered a value: $input Please enter a value: 21500 You entered a value: 21500

Set the read-only attribute of a variable

Some important Shell variables should not be modified after assignment, so you can set them to

 readonly

:

 $ oracle_home=/usr/oracle7/bin$ readonly oracle_home

Conditional test command test

grammar:

 test expression

Returns true if the expression is true, otherwise, returns false.

Example: Numeric comparison

First, the common comparison symbols used in numerical comparisons are given:

 -eg =; -ne !=; -gt >; -ge >=; -lt <; -le <=

 $ test var1 -gt var2

Example: Test file properties

Whether the file is readable, writable, and executable depends on whether it is an ordinary file and whether it is a directory:

 -r; -w; -x; -f; -d

 $ test -r filename

Example: character transfer attributes and comparison

The length of the string is zero:

 -z

; non-zero:

 -n

,like:

 $ test -z s1

if string

 s1

The length is zero and returns true.

Example: String comparison

equal

 s1=s2

; not equal

 s1!=s2

There is another way to compare strings (you can compare them in lexicographic order):

 $ if [[ 'abcde' < 'abcdf' ]]; then echo yeah, as expected; fiyeah, as expected, yes

Integer arithmetic or relational operation expr

The operations that can be performed with this command are:

Arithmetic operations:

 + - * / %

;Logical operation

 := ! < <= > >=

like:

 $ i=5;expr $i+5

in addition,

 bc

is a command line calculator that can perform some arithmetic calculations.

Control execution process commands

Example: conditional branch command if

 if

Command example: If the first parameter is a common file name, then print the file in pages; otherwise, if it is a directory name, enter the directory and print all the files in the directory. If it is not a directory, then prompt relevant information .

 if test -f $1then pr $1>/dev/lp0elif test-d $1then (cd $1;pr *>/dev/lp0)else echo $1 is neither a file nor a directoryfi

Example: case command example

 case

The command is a multi-way branch command based on pattern matching. The following will determine which set of commands will be executed next based on the user's keyboard input.

 while [ $reply!=y ] && [ $reply!=Y ] #The loop statement we will learn below do echo nAre you want to continue?(Y/N)c read reply #Read the keyboard case $replay in ( y|Y) break;; #Exit the loop (n|N) echo nnTerminatingn exit 0;; *) echo nnPlease answer y or n continue; #Return directly to the inner loop and start to continue esacdone

Example: loop statement while, until

grammar:

 while/until command list 1do command list 2done

The difference is that after the former executes command table 1, if the exit status is zero, then execute

 do

The following command list 2 will then return to the starting point, and after the latter executes command list 1, it will only perform similar operations if the exit status is non-zero. Same example as above.

Example: Finite loop command for

grammar:

 for variable name in string table do command table done

Example:

 FILE=test1.c myfile1.f pccn.hfor i in $FILEdo cd ./tmp cp $i $i.old echo $idone copied

function

Now let’s take a look at the usage of functions in Shell. Let’s look at an example first: write a function and then call it to display

 Hello, World!

 $ cat > show# Function definition function show{ echo $1$2;}H=Hello,W=World!# Call the function and pass in two parameters H and Wshow $H $W

Demo:

 $ chmod 770 show$./showHello,World!

Did you see something fishy?

 $ show $H $W

We can directly follow the function name with the actual parameters.

The order of actual parameters corresponds to the "virtual parameters"

 $1,$2,$3

…

Note: If you want to pass in a parameter, what should you do if there are spaces in the middle of the parameter? Give it a try first.

to display

 Hello World

(There is a space between the two words)

 function show{ echo $1}HW=Hello Worldshow $HW

If directly

 show $HW

, definitely not possible, because

 $1

Only received

 Hello

, so the results only show

 Hello

, the reason is that string variables must be used

Contain it.

postscript

If you are interested, continue learning!

There are many powerful things waiting for you, such as

 cut

,

 expr

,

 sed

,

 awk

etc.