Linux Shell Programming

 

Shell itself is a program written in C language, which serves as a bridge between users using Linux. Shell is both a command language and a programming language. As a command language, it interactively interprets and executes user-input commands. As a programming language, it defines various variables and parameters, it also provides many control structures that are available in advanced languages, including loops and branches. Although it is not part of the core of Linux, it calls most of the core functions of the system to execute programs, create files, and coordinate the running of various programs in parallel. Therefore, shell is the most important utility for users. It is the key to make good use of the Linux system by thoroughly understanding and mastering the features of shell. It can be said that the proficiency of shell reflects the user's proficiency in Linux. I. What is shell? After a user logs on to Linux, the system initialization program init runs a shell program for each user. So what is shell? To be exact, shell is a command line interpreter that provides users with an interface system-level program that sends requests to the Linux kernel to run the program, you can use shell to start, suspend, stop, or even write some programs. When you use Linux, you can use commands to complete the required work. A command is a basic unit of dialog between a user and a shell. It consists of multiple characters and ends with a line break. Shell explains the commands entered by the user, just as command.com in DOS does. The difference is that in DOS, command.com has only one, in Linux, there are several popular shells, each of which has its own merits. Generally, bash is used as the default shell in Linux. 2. Several Popular Shells Currently, popular shell types include ash, Bash, KSh, CSH, and zsh. You can use the following command to view your own shell types: # Echo $ Shell $ Shell is an environment variable that records the shell type used by the user. You can run the following command: # Shell-name To another shell. Here, shell-name is the name of the shell you want to use, such as ash. This command starts another shell for the user. This shell is called a lower-level shell or a sub-shell after the shell is first logged on. Run the following command: $ Exit You can exit this sub-shell. The reason for using different shells is that they all have their own characteristics. The following is a brief introduction: 1. The Ash shell is compiled by kenth almqualified. in Linux, it is a small shell that occupies the least system resources. It contains only 24 internal commands, so it is inconvenient to use. 2. Bash is the default shell used in Linux. It is jointly completed by Brian Fox and Chet Ramey. It is the abbreviation of Bourne again shell and contains a total of 40 internal commands. Linux uses it as the default shell because it has the following features: (1) You can use a function similar to doskey under DOS, and use the direction keys to view and quickly enter and modify commands. (2) The command starting with a string is automatically provided by searching for matching. (3) It includes its own help function. You only need to enter help at the prompt to get the help. 3. ksh KSh is short for Korn shell. It is written by Eric Gisin and contains 42 Internal commands. The biggest advantage of this shell is that it is almost completely compatible with the commercial release ksh, so that you can try the performance of the commercial version without spending money to buy the commercial version. 4. CSH is a large Linux kernel. It is compiled by a total of 47 authors represented by William joy and contains 52 Internal commands. This shell actually points to a shell like/bin/tcsh, that is, CSH is actually tcsh. 5. zsh is one of the largest shell in Linux. It is completed by Paul Falstad and has 84 Internal commands. For general purposes, there is no need to install such a shell. 3. Shell Program Design (basic) In fact, the interactive interpretation and execution of user input commands as a command language is only one aspect of the shell function, shell can also be used for program design, it provides a variety of methods to define variables and parameters as well as program control structures. Using shell programming is similar to a batch processing file in DOS, called a shell script, or a shell program or shell command file. 1. Shell basic syntax the basic syntax of shell is mainly about how to input commands to run the program and how to communicate between programs through shell parameters. (1) Input/Output redirection In Linux, each process has three special file description pointers: standard input (0), standard output, file description pointer is 1), standard error output (standard error, file description pointer is 2 ). These three special file description pointers enable the process to normally receive input from the standard input terminal and display the output from the standard terminal, linux also provides users with the ability to use common files or pipelines to replace these standard input and output devices. In shell, you can use ">" and "<" to redirect input and output. For example: Command> file: redirects the command output to a file. Command> & file: redirects the standard error output of the command to a file. Command> file: append the standard output result to the file. Command >>> & file: append the structure of the standard output and standard error output to the file. Command (2) pipeline Pipe Pipe can also replace the standard input output and standard error output, so that the output of a program can be sent to the input of another program. The syntax is as follows: Command1 | command2 [| command3. ..] It can also be sent to the pipeline together with the standard error output: Command1 | & command2 [| & command3. ..] (3) foreground and background In shell, a new process can be executed by using the symbols ";" and "&" after the command respectively in the foreground and background. The syntax is as follows: Command Generate a foreground process. The next command can only be entered after the command is run. Command & Generates a background process that runs in the background and can input other commands. 2. shell program variables and parameters Like an advanced programming language, shell also provides instructions and the ability to use variables. For shell, the values of all variables are a string, and the shell program uses $ VaR to reference the value of the variable named var. Shell has the following basic types of variables: (1) environment variables defined by Shell At the beginning of execution, shell has defined some variables related to the system's working environment. You can also redefine these variables. Common shell environment variables include: Home: the full path name used to save the registration directory. Path: used to save the directory path names separated by colons. Shell searches these directories in the order given in the PATH variable. The first executable file with the same name as the command will be executed. Term: the type of the terminal. UID: The identifier of the current user. The value is a string consisting of digits. PWD: the absolute path name of the current working directory. The value of this variable varies with the use of the CD command. PS1: Primary prompt. for privileged users, the default primary prompt is "#". For common users, the default primary prompt is "$ ". PS2: When the shell receives the user's input command, if the user enters "/" at the end of the input line, and then press enter, or, when the shell determines that the command entered by the user is not over when the user presses the Enter key, the pop-up prompt is displayed, prompting the user to continue entering the rest of the command. The default pop-up prompt is "> ". (2) User-Defined variables You can define your own variables according to the following syntax rules: Variable name = variable value Note that "$" should not be added before the variable name when defining the variable, and "$" should be added before the variable name When referencing the variable content "; when assigning values to variables, spaces must not be left on both sides of the equal sign. If the variable itself contains spaces, the entire string should be enclosed in double quotation marks. When writing a shell program, we recommend that you use uppercase letters to indicate all variable names to distinguish them from command names. Sometimes we want to explain a variable and set it to a specific value without changing its value. This can be ensured by the following command: Readly variable name At any time, the created variables are only local variables of the Current Shell, so they cannot be used by other commands or Shell programs run by the shell, the Export command provides a local variable to other commands executed by shell in the format: Export variable name You can also use the Export command when assigning values to variables: Export variable name = variable value Variables described in export can be accessed in all commands or programs run after shell. (3) location parameters A location parameter is a variable that is determined by its location in the command line that calls the shell program. It is a parameter entered after the program name. Location parameters are separated by spaces. Shell replaces $1 with the first location parameter in the program file, and the second with $2, and so on. $0 is a special variable whose content is the file name of the current shell program. Therefore, $0 is not a location parameter, $0 is not included when all the current location parameters are displayed. (4) predefined Variables Predefined variables are similar to environment variables and are also defined at the beginning of shell. The difference is that users can only use these variables according to shell definitions, you cannot redefine it. All predefined variables are composed of the $ operator and another symbol. Common shell predefined variables include: $ #: Number of location parameters $ *: Content of parameters at all locations $? : Status returned after the command is executed $: Process ID of the current process $! : The last process number running in the background $0: name of the currently executed process Where, "$ ?" This command is used to check whether the execution of the previous command is correct (in Linux, the exit status of the command is 0, indicating that the command is correctly executed, and any non-0 value indicates that the command has an error ). The most common use of the '$' variable is to use it as the name of a temporary file to ensure that the temporary file will not be repeated. (5) parameter substitution Variables Shell provides the parameter replacement capability so that you can assign different values to variables based on different conditions. There are four types of parameters for parameter substitution. These variables are usually associated with a location parameter and determine the value of the variable based on whether the specified location parameter has been set to a class, their syntax and functions are as follows. A. Variable =$ {parameter-word}: If a parameter is set, replace the value of the parameter with the value of the parameter. Otherwise, replace the value with word. That is, the value of this variable is equal to the value of a parameter. If this parameter is not set, the variable is equal to the value of word. B. Variable =$ {parameter = word}: If a parameter is set, replace the value with the value of the parameter. Otherwise, set the variable to word and replace the parameter value with word. Note that location parameters cannot be used in this way, because location parameters cannot be assigned values in Shell programs. C. Variable =$ {parameter? Word}: If a parameter is set, replace the value of the variable with the value of the parameter. Otherwise, word is displayed and exited from shell. If word is omitted, standard information is displayed. This variable must be equal to the value of a parameter. If this parameter is not set, a message is displayed and then exited. Therefore, this method is often used for Error Indication. D. Variable =$ {parameter + word}: If the parameter is set, replace the variable with word. Otherwise, do not replace the variable. In all these four forms, the "parameter" can be either a location parameter or another variable, but there are many situations where the location parameter is used. Next we will take Bash as an example to introduce the advanced part of shell program design: the process control, debugging method and running method of shell program. By the way, we will also introduce the Internal commands of bash. 4. Process Control for Shell Program Design Like other advanced programming languages, shell provides commands used to control program execution processes, including conditional branches and loop structures. You can use these commands to create very complex programs. Unlike traditional languages, shell is used to specify the condition value instead of a Boolean expression but a command and string. 1. Test command The test command is used to check whether a condition is true. It can be used to test values, characters, and files. The test characters and corresponding functions are as follows: (1) numerical test: -EQ: true if it is equal -Ne: true if not equal -GT: true if the value is greater -Ge: true if the value is greater than or equal -Lt: true if the value is smaller -Le: true if the value is less than or equal (2) string test: =: Equal to true ! =: True if not equal -Z string: the pseudo-String Length is true. -N string: the string length is true if it is not pseudo. (3) file test: -E file name: true if the file exists -R file name: true if the file exists and is readable -W file name: true if the file exists and can be written. -X file name: true if the file exists and can be executed -S file name: true if the file exists and contains at least one character -D file name: true if the file exists and is a directory -F file name: true if the file exists and is a normal file -C file name: true if the file exists and is a special character file -B file name: true if the file exists and is a special file. In addition, Linux also provides , Or ("-O), non ("-a ") three logical operators are used to connect test conditions, the priority is:"!" Highest, followed by "-a", and "-o. Bash can also perform simple arithmetic operations in the following format: $ [Expression] Example: var1 = 2 Var2 = $[var1*10 + 1] Then, the value of var2 is 21. 2. If Condition Statement The condition branch in the shell program is implemented through the IF Condition Statement. The general format is: If condition command string Then Command string when the condition is true Else Command string when the condition is false Fi 3. For Loop The for loop executes a command sequence for all possible values of a variable. The values assigned to the variable can be provided in the form of a numerical list in the program, or in the form of a positional parameter outside the program. The general format of the For Loop is: For variable name [In Value List] Do Several command lines Done The variable name can be any string selected by the user. If the variable name is Var, the value given after in replaces $ VaR in the circular command list in sequence. If in is omitted, the value of VaR is the location parameter. Every possible value assignment to a variable will execute the command list between do and done. 4. While and until Loops Both the while and until commands use the return status value of the command to control the loop. The general format of the while loop is: While Several command lines 1 Do Several command lines 2 Done As long as the return status of the last command in "Several command line 1" in while is true, the while loop continues to execute "Several command line 2" between do... done ". The until command is another loop structure, which is similar to the while command in the following format: Until Several command lines 1 Do Several command lines 2 Done The difference between an until loop and a while loop is that a while loop continues to run when the condition is true, while an until loop continues to run when the condition is false. Shell also provides two commands, true and false, to create an infinite loop structure. Their return states are always 0 or not 0. 5. Select case conditions The IF Condition Statement is used to select one of the two options, and the case condition selection provides you with a method to select one from multiple options based on the string or variable value. The format is as follows: Case string in Exp-1) Several command lines 1 ;; Exp-2) Several command lines 2 ;; ...... *) Other command lines Esac By calculating the string value, shell compares the result to the expression exp-1, exp-2, and so on until a matching expression is found, if a match is found, execute the following command until a semicolon (;) is encountered. You can also use shell wildcards ("*", "?" , "[]"). Generally, "*" is used as the final expression of the Case command to execute the "other command line" command when no matching item is found. 6. Unconditional control statements break and continue Break is used to terminate the execution of the current loop immediately, while contiune is used to start the execution of the next loop immediately without executing the statements following the loop. These two statements are valid only when they are placed between do and done. 7. Function Definition You can also define functions in shell. A function is actually composed of several shell commands, so it is similar to a shell program in form. The difference is that it is not a separate process, but a part of the shell program. The basic format of the function definition is: Functionname { Several command lines } The format of the called function is: Functionname param1 param2 ...... Shell functions can complete some routine work and have their own exit States. Therefore, functions can also be used as conditions for control structures such as if and while. Parameters are not required for function definition, but can be included when a function is called. In this case, shell will assign these parameters to the corresponding location parameters $1, $2 ,... and $ *. 9. Signal The trap command is used to capture signals in the shell program. There are three methods to respond to these signals: (1) execute a program to process this signal. (2) default operation for receiving signals (3) Ignore this signal Trap provides three basic forms for the above three methods: The first form of Trap Command will execute the command string in double quotation marks when shell receives signals with the same values in the signal list. Trap 'commands' signal-list Trap "commands" signal-list To restore the default signal, use the second form of trap command: Trap signal-list The third form of trap command allows ignoring signals: Trap "" signal-list Note: (1) The signal 11 (segment violation) cannot be captured, because the shell itself needs to capture the signal for memory dump. (2) In the trap, you can define the processing of the signal 0 (in fact, there is no such signal). The shell program sends this signal when it terminates (such as executing the exit Statement. (3) After capturing the signal specified in the signal-list and executing the corresponding commands, if these commands do not terminate the shell program, the shell program will continue to execute the command after the command received when the signal is received, which will easily cause the shell program to be unable to terminate. In the trap statement, single quotes and double quotation marks are different. When the shell program first encounters a trap statement, it will scan the commands in commands. If commands is enclosed in single quotes, shell will not replace the variables and commands in commands. Otherwise, the variables and commands in commands will be replaced with the specific values at that time. 5. How to run a shell program You can use any editing program to write Shell programs. Because the shell program is interpreted and executed, it does not need to be compiled and assembled into the target program. According to the shell programming Convention, for example, the first line of the program is generally "#! /Bin/bash ", where # indicates that the row is a comment and the exclamation point"!" Tell the shell to run the command after the exclamation mark and use the rest of the file as the input, that is, run/bin/bash and let/bin/bash execute the shell program content. There are three methods to execute a shell program: (1) sh shell program file name The command format for this method is: BASH Shell program file name This is actually to call a new bash command to explain the program, and pass the shell program file name as a parameter to it. The newly started shell will read the specified file and execute the commands listed in the file. When all the commands are completed. The advantage of this method is that the shell debugging function can be used. (3) Use the CHMOD command to make the shell program executable Whether a file can run depends on whether the file content is executable and the file has the execution right. For Shell programs, when you use the editor to generate a file, the system grants 644 (RW-r --) Permission. Therefore, when you need to run this file, you only need to directly type the file name. Among the three methods for running Shell programs, it is best to choose the following method: when a shell program is just created and its correctness is not yet grasped, the first method should be used for debugging. When a shell program has been debugged, use the third method to fix it. Later, you only need to enter the corresponding file name and can be called by another program.