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Update and rename sources/tech/20191021 How to program with Bash- Syntax and tools.md to translated/tech/20191021 How to program with Bash- Syntax and tools.md
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[#]: collector: (lujun9972)
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[#]: translator: (jdh8383)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (How to program with Bash: Syntax and tools)
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[#]: via: (https://opensource.com/article/19/10/programming-bash-part-1)
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[#]: author: (David Both https://opensource.com/users/dboth)
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How to program with Bash: Syntax and tools
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======
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Learn basic Bash programming syntax and tools, as well as how to use
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variables and control operators, in the first article in this three-part
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series.
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![bash logo on green background][1]
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A shell is the command interpreter for the operating system. Bash is my favorite shell, but every Linux shell interprets the commands typed by the user or sysadmin into a form the operating system can use. When the results are returned to the shell program, it sends them to STDOUT which, by default, [displays them in the terminal][2]. All of the shells I am familiar with are also programming languages.
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Features like tab completion, command-line recall and editing, and shortcuts like aliases all contribute to its value as a powerful shell. Its default command-line editing mode uses Emacs, but one of my favorite Bash features is that I can change it to Vi mode to use editing commands that are already part of my muscle memory.
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However, if you think of Bash solely as a shell, you miss much of its true power. While researching my three-volume [Linux self-study course][3] (on which this series of articles is based), I learned things about Bash that I'd never known in over 20 years of working with Linux. Some of these new bits of knowledge relate to its use as a programming language. Bash is a powerful programming language, one perfectly designed for use on the command line and in shell scripts.
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This three-part series explores using Bash as a command-line interface (CLI) programming language. This first article looks at some simple command-line programming with Bash, variables, and control operators. The other articles explore types of Bash files; string, numeric, and miscellaneous logical operators that provide execution-flow control logic; different types of shell expansions; and the **for**, **while**, and **until** loops that enable repetitive operations. They will also look at some commands that simplify and support the use of these tools.
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### The shell
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A shell is the command interpreter for the operating system. Bash is my favorite shell, but every Linux shell interprets the commands typed by the user or sysadmin into a form the operating system can use. When the results are returned to the shell program, it displays them in the terminal. All of the shells I am familiar with are also programming languages.
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Bash stands for Bourne Again Shell because the Bash shell is [based upon][4] the older Bourne shell that was written by Steven Bourne in 1977. Many [other shells][5] are available, but these are the four I encounter most frequently:
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* **csh:** The C shell for programmers who like the syntax of the C language
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* **ksh:** The Korn shell, written by David Korn and popular with Unix users
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* **tcsh:** A version of csh with more ease-of-use features
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* **zsh:** The Z shell, which combines many features of other popular shells
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All shells have built-in commands that supplement or replace the ones provided by the core utilities. Open the shell's man page and find the "BUILT-INS" section to see the commands it provides.
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Each shell has its own personality and syntax. Some will work better for you than others. I have used the C shell, the Korn shell, and the Z shell. I still like the Bash shell more than any of them. Use the one that works best for you, although that might require you to try some of the others. Fortunately, it's quite easy to change shells.
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All of these shells are programming languages, as well as command interpreters. Here's a quick tour of some programming constructs and tools that are integral parts of Bash.
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### Bash as a programming language
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Most sysadmins have used Bash to issue commands that are usually fairly simple and straightforward. But Bash can go beyond entering single commands, and many sysadmins create simple command-line programs to perform a series of tasks. These programs are common tools that can save time and effort.
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My objective when writing CLI programs is to save time and effort (i.e., to be the lazy sysadmin). CLI programs support this by listing several commands in a specific sequence that execute one after another, so you do not need to watch the progress of one command and type in the next command when the first finishes. You can go do other things and not have to continually monitor the progress of each command.
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### What is "a program"?
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The Free On-line Dictionary of Computing ([FOLDOC][6]) defines a program as: "The instructions executed by a computer, as opposed to the physical device on which they run." Princeton University's [WordNet][7] defines a program as: "…a sequence of instructions that a computer can interpret and execute…" [Wikipedia][8] also has a good entry about computer programs.
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Therefore, a program can consist of one or more instructions that perform a specific, related task. A computer program instruction is also called a program statement. For sysadmins, a program is usually a sequence of shell commands. All the shells available for Linux, at least the ones I am familiar with, have at least a basic form of programming capability, and Bash, the default shell for most Linux distributions, is no exception.
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While this series uses Bash (because it is so ubiquitous), if you use a different shell, the general programming concepts will be the same, although the constructs and syntax may differ somewhat. Some shells may support some features that others do not, but they all provide some programming capability. Shell programs can be stored in a file for repeated use, or they may be created on the command line as needed.
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### Simple CLI programs
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The simplest command-line programs are one or two consecutive program statements, which may be related or not, that are entered on the command line before the **Enter** key is pressed. The second statement in a program, if there is one, might be dependent upon the actions of the first, but it does not need to be.
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There is also one bit of syntactical punctuation that needs to be clearly stated. When entering a single command on the command line, pressing the **Enter** key terminates the command with an implicit semicolon (**;**). When used in a CLI shell program entered as a single line on the command line, the semicolon must be used to terminate each statement and separate it from the next one. The last statement in a CLI shell program can use an explicit or implicit semicolon.
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### Some basic syntax
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The following examples will clarify this syntax. This program consists of a single command with an explicit terminator:
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```
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[student@studentvm1 ~]$ echo "Hello world." ;
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Hello world.
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```
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That may not seem like much of a program, but it is the first program I encounter with every new programming language I learn. The syntax may be a bit different for each language, but the result is the same.
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Let's expand a little on this trivial but ubiquitous program. Your results will be different from mine because I have done other experiments, while you may have only the default directories and files that are created in the account home directory the first time you log into an account via the GUI desktop.
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```
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[student@studentvm1 ~]$ echo "My home directory." ; ls ;
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My home directory.
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chapter25 TestFile1.Linux dmesg2.txt Downloads newfile.txt softlink1 testdir6
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chapter26 TestFile1.mac dmesg3.txt file005 Pictures Templates testdir
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TestFile1 Desktop dmesg.txt link3 Public testdir Videos
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TestFile1.dos dmesg1.txt Documents Music random.txt testdir1
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```
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That makes a bit more sense. The results are related, but the individual program statements are independent of each other. Notice that I like to put spaces before and after the semicolon because it makes the code a bit easier to read. Try that little CLI program again without an explicit semicolon at the end:
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```
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`[student@studentvm1 ~]$ echo "My home directory." ; ls`
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```
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There is no difference in the output.
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### Something about variables
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Like all programming languages, the Bash shell can deal with variables. A variable is a symbolic name that refers to a specific location in memory that contains a value of some sort. The value of a variable is changeable, i.e., it is variable.
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Bash does not type variables like C and related languages, defining them as integers, floating points, or string types. In Bash, all variables are strings. A string that is an integer can be used in integer arithmetic, which is the only type of math that Bash is capable of doing. If more complex math is required, the [**bc** command][9] can be used in CLI programs and scripts.
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Variables are assigned values and can be used to refer to those values in CLI programs and scripts. The value of a variable is set using its name but not preceded by a **$** sign. The assignment **VAR=10** sets the value of the variable VAR to 10. To print the value of the variable, you can use the statement **echo $VAR**. Start with text (i.e., non-numeric) variables.
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Bash variables become part of the shell environment until they are unset.
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Check the initial value of a variable that has not been assigned; it should be null. Then assign a value to the variable and print it to verify its value. You can do all of this in a single CLI program:
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```
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[student@studentvm1 ~]$ echo $MyVar ; MyVar="Hello World" ; echo $MyVar ;
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Hello World
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[student@studentvm1 ~]$
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```
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_Note: The syntax of variable assignment is very strict. There must be no spaces on either side of the equal (**=**) sign in the assignment statement._
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The empty line indicates that the initial value of **MyVar** is null. Changing and setting the value of a variable are done the same way. This example shows both the original and the new value.
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As mentioned, Bash can perform integer arithmetic calculations, which is useful for calculating a reference to the location of an element in an array or doing simple math problems. It is not suitable for scientific computing or anything that requires decimals, such as financial calculations. There are much better tools for those types of calculations.
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Here's a simple calculation:
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```
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[student@studentvm1 ~]$ Var1="7" ; Var2="9" ; echo "Result = $((Var1*Var2))"
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Result = 63
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```
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What happens when you perform a math operation that results in a floating-point number?
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```
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[student@studentvm1 ~]$ Var1="7" ; Var2="9" ; echo "Result = $((Var1/Var2))"
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Result = 0
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[student@studentvm1 ~]$ Var1="7" ; Var2="9" ; echo "Result = $((Var2/Var1))"
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Result = 1
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[student@studentvm1 ~]$
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```
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The result is the nearest integer. Notice that the calculation was performed as part of the **echo** statement. The math is performed before the enclosing echo command due to the Bash order of precedence. For details see the Bash man page and search "precedence."
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### Control operators
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Shell control operators are one of the syntactical operators for easily creating some interesting command-line programs. The simplest form of CLI program is just stringing several commands together in a sequence on the command line:
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```
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`command1 ; command2 ; command3 ; command4 ; . . . ; etc. ;`
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```
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Those commands all run without a problem so long as no errors occur. But what happens when an error occurs? You can anticipate and allow for errors using the built-in **&&** and **||** Bash control operators. These two control operators provide some flow control and enable you to alter the sequence of code execution. The semicolon is also considered to be a Bash control operator, as is the newline character.
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The **&&** operator simply says, "if command1 is successful, then run command2. If command1 fails for any reason, then command2 is skipped." That syntax looks like this:
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```
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`command1 && command2`
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```
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Now, look at some commands that will create a new directory and—if it's successful—make it the present working directory (PWD). Ensure that your home directory (**~**) is the PWD. Try this first in **/root**, a directory that you do not have access to:
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```
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[student@studentvm1 ~]$ Dir=/root/testdir ; mkdir $Dir/ && cd $Dir
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mkdir: cannot create directory '/root/testdir/': Permission denied
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[student@studentvm1 ~]$
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```
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The error was emitted by the **mkdir** command. You did not receive an error indicating that the file could not be created because the creation of the directory failed. The **&&** control operator sensed the non-zero return code, so the **touch** command was skipped. Using the **&&** control operator prevents the **touch** command from running because there was an error in creating the directory. This type of command-line program flow control can prevent errors from compounding and making a real mess of things. But it's time to get a little more complicated.
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The **||** control operator allows you to add another program statement that executes when the initial program statement returns a code greater than zero. The basic syntax looks like this:
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```
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`command1 || command2`
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```
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This syntax reads, "If command1 fails, execute command2." That implies that if command1 succeeds, command2 is skipped. Try this by attempting to create a new directory:
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```
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[student@studentvm1 ~]$ Dir=/root/testdir ; mkdir $Dir || echo "$Dir was not created."
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mkdir: cannot create directory '/root/testdir': Permission denied
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/root/testdir was not created.
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[student@studentvm1 ~]$
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```
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This is exactly what you would expect. Because the new directory could not be created, the first command failed, which resulted in the execution of the second command.
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Combining these two operators provides the best of both. The control operator syntax using some flow control takes this general form when the **&&** and **||** control operators are used:
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```
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`preceding commands ; command1 && command2 || command3 ; following commands`
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```
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This syntax can be stated like so: "If command1 exits with a return code of 0, then execute command2, otherwise execute command3." Try it:
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```
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[student@studentvm1 ~]$ Dir=/root/testdir ; mkdir $Dir && cd $Dir || echo "$Dir was not created."
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mkdir: cannot create directory '/root/testdir': Permission denied
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/root/testdir was not created.
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[student@studentvm1 ~]$
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```
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Now try the last command again using your home directory instead of the **/root** directory. You will have permission to create this directory:
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```
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[student@studentvm1 ~]$ Dir=~/testdir ; mkdir $Dir && cd $Dir || echo "$Dir was not created."
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[student@studentvm1 testdir]$
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```
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The control operator syntax, like **command1 && command2**, works because every command sends a return code (RC) to the shell that indicates if it completed successfully or whether there was some type of failure during execution. By convention, an RC of zero (0) indicates success, and any positive number indicates some type of failure. Some of the tools sysadmins use just return a one (1) to indicate a failure, but many use other codes to indicate the type of failure that occurred.
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The Bash shell variable **$?** contains the RC from the last command. This RC can be checked very easily by a script, the next command in a list of commands, or even the sysadmin directly. Start by running a simple command and immediately checking the RC. The RC will always be for the last command that ran before you looked at it.
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```
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[student@studentvm1 testdir]$ ll ; echo "RC = $?"
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total 1264
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drwxrwxr-x 2 student student 4096 Mar 2 08:21 chapter25
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drwxrwxr-x 2 student student 4096 Mar 21 15:27 chapter26
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-rwxr-xr-x 1 student student 92 Mar 20 15:53 TestFile1
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<snip>
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drwxrwxr-x. 2 student student 663552 Feb 21 14:12 testdir
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drwxr-xr-x. 2 student student 4096 Dec 22 13:15 Videos
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RC = 0
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[student@studentvm1 testdir]$
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```
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The RC, in this case, is zero, which means the command completed successfully. Now try the same command on root's home directory, a directory you do not have permissions for:
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```
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[student@studentvm1 testdir]$ ll /root ; echo "RC = $?"
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ls: cannot open directory '/root': Permission denied
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RC = 2
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[student@studentvm1 testdir]$
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```
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In this case, the RC is two; this means permission was denied for a non-root user to access a directory to which the user is not permitted access. The control operators use these RCs to enable you to alter the sequence of program execution.
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### Summary
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This article looked at Bash as a programming language and explored its basic syntax as well as some basic tools. It showed how to print data to STDOUT and how to use variables and control operators. The next article in this series looks at some of the many Bash logical operators that control the flow of instruction execution.
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/19/10/programming-bash-part-1
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作者：[David Both][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/dboth
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/bash_command_line.png?itok=k4z94W2U (bash logo on green background)
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[2]: https://opensource.com/article/18/10/linux-data-streams
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[3]: http://www.both.org/?page_id=1183
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[4]: https://opensource.com/19/9/command-line-heroes-bash
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[5]: https://en.wikipedia.org/wiki/Comparison_of_command_shells
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[6]: http://foldoc.org/program
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[7]: https://wordnet.princeton.edu/
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[8]: https://en.wikipedia.org/wiki/Computer_program
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[9]: https://www.gnu.org/software/bc/manual/html_mono/bc.html
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@ -0,0 +1,272 @@
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[#]: collector: (lujun9972)
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[#]: translator: (jdh8383)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (How to program with Bash: Syntax and tools)
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[#]: via: (https://opensource.com/article/19/10/programming-bash-part-1)
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[#]: author: (David Both https://opensource.com/users/dboth)
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怎样用 Bash 编程：语法和工具
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======
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让我们通过本系列文章来学习基本的 Bash 编程语法和工具，以及如何使用变量和控制运算符，这是三篇中的第一篇。
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![bash logo on green background][1]
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Shell 是操作系统的命令解释器，其中 Bash 是我最喜欢的。每当用户或者系统管理员将命令输入系统的时候，Linux 的 shell 解释器就会把这些命令转换成操作系统可以理解的形式。而执行结果返回 shell 程序后，它会将结果输出到 STDOUT（标准输出），默认情况下，这些结果会[显示在你的终端][2]。所有我熟悉的 shell 同时也是门编程语言。
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Bash 是个功能强大的 shell，包含众多便捷特性，比如：tab 补全、命令回溯和再编辑、aliases 别名等。它的命令行默认编辑模式是 Emacs，但是我最喜欢的Bash特性之一是我可以将其更改为 Vi 模式，以使用那些储存在我肌肉记忆中的的编辑命令。
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然而，如果你把 Bash 当作单纯的 shell 来用，则无法体验它的真实能力。我在设计一套包含三卷的 [Linux 自学课程][3]时（这个系列的文章正是基于此课程），了解到许多 Bash 的知识，这些是我在过去 20 年的 Linux 工作经验中所没有掌握的，其中的一些知识就是关于 Bash 的编程用法。不得不说，Bash 是一门强大的编程语言，是一个能够同时用于命令行和 shell 脚本的完美设计。
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本系列文章将要探讨如何使用 Bash 作为命令行界面（CLI）编程语言。第一篇文章简单介绍 Bash 命令行编程、变量以及控制运算符。其他文章会讨论诸如：Bash 文件的类型；字符串、数字和一些逻辑运算符，它们能够提供代码执行流程中的逻辑控制；不同类型的 shell 扩展；通过 **for**、**while** 和 **until** 来控制循环操作。
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### Shell
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Shell 是操作系统的命令解释器，其中 Bash 是我最喜欢的。每当用户或者系统管理员将命令输入系统的时候，Linux 的 shell 解释器就会把这些命令转换成操作系统可以理解的形式。而执行结果返回 shell 程序后，它会将结果输出到终端。所有我熟悉的 shell 同时也是门编程语言。
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Bash 是 Bourne Again Shell 的缩写，因为 Bash shell 是 [基于][4] 更早的 Bourne shell，后者是 Steven Bourne 在 1977 年开发的。另外还有很多[其他的 shell][5] 可以使用，但下面四个是我经常见到的：
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* **csh：** C shell 适合那些习惯了 C 语言语法的开发者。
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* **ksh：** Korn shell，由 David Korn 开发，在 Unix 用户中更流行。
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* **tcsh：** 一个 csh 的变种，增加了一些易用性。
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* **zsh：** Z shell，集成了许多其他流行 shell 的特性。
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所有 shell 都有内置命令，用以补充或替代核心工具集。打开 shell 的 man 说明页，找到“BUILT-INS”那一段，可以查看都有哪些内置命令。
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|
||||
每种 shell 都有它自己的特性和语法风格。我用过 csh、ksh 和 zsh，但我还是更喜欢 Bash。你可以多试几个，寻找更适合你的 shell，尽管这可能需要花些功夫。但幸运的是，切换不同 shell 很简单。
|
||||
|
||||
所有这些 shell 既是编程语言又是命令解释器。下面我们来快速浏览一下 Bash 中集成的编程结构和工具。
|
||||
|
||||
### 做为编程语言的 Bash
|
||||
|
||||
大多数场景下，系统管理员都会使用 Bash 来发送简单明了的命令。但 Bash 不仅可以输入单条命令，很多系统管理员可以编写简单的命令行程序来执行一系列任务，这些程序可以作为通用工具，能节省时间和精力。
|
||||
|
||||
编写 CLI 程序的目的是要提高效率（做一个“懒惰的”系统管理员）。在 CLI 程序中，你可以用特定顺序列出若干命令，逐条执行。这样你就不用盯着显示屏，等待一条命令执行完，再输入另一条，省下来的时间就可以去做其他事情了。
|
||||
|
||||
### 什么是“程序”？
|
||||
|
||||
自由在线计算机词典（[FOLDOC][6]）对于程序的定义是：“由计算机执行的指令，而不是运行它们的物理硬件。”普林斯顿大学的 [WordNet][7] 将程序定义为：“……计算机可以理解并执行的一系列指令……”[维基百科][8]上也有一条不错的关于计算机程序的条目。
|
||||
|
||||
总结下，程序由一条或多条指令组成，目的是完成一个具体的相关任务。对于系统管理员而言，一段程序通常由一系列的 shell 命令构成。Linux 下所有的 shell （至少我所熟知的）都有基本的编程功能，Bash 作为大多数 linux 发行版的默认 shell，也不例外。
|
||||
|
||||
本系列用 Bash 举例（因为它无处不在），假如你使用一个不同的 shell 也没关系，尽管结构和语法有所不同，但编程思想是相通的。有些 shell 支持某种特性而其他 shell 则不支持，但它们都提供编程功能。Shell 程序可以被存在一个文件中被反复使用，或者在需要的时候才创建它们。
|
||||
|
||||
|
||||
### 简单 CLI 程序
|
||||
|
||||
最简单的命令行程序只有一或两条语句，它们可能相关，也可能无关，在按**回车**键之前被输入到命令行。程序中的第二条语句（如果有的话）可能取决于第一条语句的操作，但也不是必须的。
|
||||
|
||||
这里需要特别讲解一个标点符号。当你在命令行输入一条命令，按下**回车**键的时候，其实在命令的末尾有一个隐含的分号（**;**）。当一段 CLI shell 程序在命令行中被串起来作为单行指令使用时，必须使用分号来终结每个语句并将其与下一条语句分开。但 CLI shell 程序中的最后一条语句可以使用显式或隐式的分号。
|
||||
|
||||
### 一些基本语法
|
||||
|
||||
下面的例子会阐明这一语法规则。这段程序由单条命令组成，还有一个显式的终止符：
|
||||
|
||||
|
||||
```
|
||||
[student@studentvm1 ~]$ echo "Hello world." ;
|
||||
Hello world.
|
||||
```
|
||||
|
||||
看起来不像一个程序，但它确是我学习每个新编程语言时写下的第一个程序。不同语言可能语法不同，但输出结果是一样的。
|
||||
|
||||
让我们扩展一下这段微不足道却又无所不在的代码。你的结果可能与我的有所不同，因为我的家目录有点乱，而你可能是在 GUI 桌面中第一次登陆账号。
|
||||
|
||||
|
||||
```
|
||||
[student@studentvm1 ~]$ echo "My home directory." ; ls ;
|
||||
My home directory.
|
||||
chapter25 TestFile1.Linux dmesg2.txt Downloads newfile.txt softlink1 testdir6
|
||||
chapter26 TestFile1.mac dmesg3.txt file005 Pictures Templates testdir
|
||||
TestFile1 Desktop dmesg.txt link3 Public testdir Videos
|
||||
TestFile1.dos dmesg1.txt Documents Music random.txt testdir1
|
||||
```
|
||||
|
||||
现在是不是更明显了。结果是相关的，但是两条语句彼此独立。你可能注意到我喜欢在分号前后多输入一个空格，这样会让代码的可读性更好。让我们再运行一遍这段程序，这次不要带结尾的分号：
|
||||
|
||||
|
||||
```
|
||||
`[student@studentvm1 ~]$ echo "My home directory." ; ls`
|
||||
```
|
||||
|
||||
输出结果没有区别。
|
||||
|
||||
### 关于变量
|
||||
|
||||
像所有其他编程语言一样，Bash 支持变量。变量是个象征性的名字，它指向内存中的某个位置，那里存着对应的值。变量的值是可以改变的，所以它叫“变~量”。
|
||||
|
||||
Bash 不像 C 之类的语言，需要强制指定变量类型，比如：整型、浮点型或字符型。在 Bash 中，所有变量都是字符串。整数型的变量可以被用于整数运算，这是 Bash 唯一能够处理的数学类型。更复杂的运算则需要借助 [**bc**][9] 这样的命令，可以被用在命令行编程或者脚本中。
|
||||
|
||||
变量的值是被预先分配好的，这些值可以用在命令行编程或者脚本中。可以通过变量名字给其赋值，但是不能使用 **$** 符开头。比如，**VAR=10** 这样会把 VAR 的值设为 10。要打印变量的值，你可以使用语句 **echo $VAR**。变量名必须以文本（即非数字）开始。
|
||||
|
||||
Bash 会保存已经定义好的变量，直到它们被取消掉。
|
||||
|
||||
下面这个例子，在变量被赋值前，它的值是空（null）。然后给它赋值并打印出来，检验一下。你可以在同一行 CLI 程序里完成它：
|
||||
|
||||
|
||||
```
|
||||
[student@studentvm1 ~]$ echo $MyVar ; MyVar="Hello World" ; echo $MyVar ;
|
||||
|
||||
Hello World
|
||||
[student@studentvm1 ~]$
|
||||
```
|
||||
|
||||
_注意：变量赋值的语法非常严格，等号（**=**）两边不能有空格。_
|
||||
|
||||
那个空行表明了 **MyVar** 的初始值为空。变量的赋值和改值方法都一样，这个例子展示了原始值和新的值。
|
||||
|
||||
正如之前说的，Bash 支持整数运算，当你想计算一个数组中的某个元素的位置，或者做些简单的算术运算，这还是挺有帮助的。然而，这种方法并不适合科学计算，或是某些需要小数运算的场景，比如财务统计。这些场景有其它更好的工具可以应对。
|
||||
|
||||
下面是个简单的算术题：
|
||||
|
||||
|
||||
```
|
||||
[student@studentvm1 ~]$ Var1="7" ; Var2="9" ; echo "Result = $((Var1*Var2))"
|
||||
Result = 63
|
||||
```
|
||||
|
||||
好像没啥问题，但如果运算结果是浮点数会发生什么呢？
|
||||
|
||||
|
||||
```
|
||||
[student@studentvm1 ~]$ Var1="7" ; Var2="9" ; echo "Result = $((Var1/Var2))"
|
||||
Result = 0
|
||||
[student@studentvm1 ~]$ Var1="7" ; Var2="9" ; echo "Result = $((Var2/Var1))"
|
||||
Result = 1
|
||||
[student@studentvm1 ~]$
|
||||
```
|
||||
|
||||
结果会被取整。请注意运算被包含在 **echo** 语句之中，其实计算在 echo 命令结束前就已经完成了，原因是 Bash 的内部优先级。想要了解详情的话，可以在 Bash 的 man 页面中搜索 "precedence"。
|
||||
|
||||
### 控制运算符
|
||||
|
||||
Shell 的控制运算符是一种语法运算符，可以轻松地创建一些有趣的命令行程序。在命令行上按顺序将几个命令串在一起，就变成了最简单的 CLI 程序：
|
||||
|
||||
|
||||
```
|
||||
`command1 ; command2 ; command3 ; command4 ; . . . ; etc. ;`
|
||||
```
|
||||
|
||||
只要不出错，这些命令都能顺利执行。但假如出错了怎么办？你可以预设好应对出错的办法，这就要用到 Bash 内置的控制运算符， **&&** 和 **||**。这两种运算符提供了流程控制功能，使你能改变代码执行的顺序。分号也可以被看做是一种 Bash 运算符，预示着新一行的开始。
|
||||
|
||||
|
||||
**&&** 运算符提供了如下简单逻辑，“如果 command1 执行成功，那么接着执行 command2。如果 command1 失败，就跳过 command2。”语法如下：
|
||||
|
||||
|
||||
```
|
||||
`command1 && command2`
|
||||
```
|
||||
|
||||
现在，让我们用命令来创建一个新的目录，如果成功的话，就把它切换为当前目录。确保你的家目录（**~**）是当前目录，先尝试在 **/root** 目录下创建，你应该没有权限：
|
||||
|
||||
|
||||
```
|
||||
[student@studentvm1 ~]$ Dir=/root/testdir ; mkdir $Dir/ &&; cd $Dir
|
||||
mkdir: cannot create directory '/root/testdir/': Permission denied
|
||||
[student@studentvm1 ~]$
|
||||
```
|
||||
|
||||
上面的报错信息是由 **mkdir** 命令抛出的，因为创建目录失败了。**&&** 运算符收到了非零的返回码，所以 **cd** 命令就被跳过，前者阻止后者继续运行，因为创建目录失败了。这种控制流程可以阻止后面的错误累积，避免引发更严重的问题。是时候讲点更复杂的逻辑了。
|
||||
|
||||
当一段程序的返回码大于零时，使用 **||** 运算符可以让你在后面接着执行另一段程序。简单语法如下：
|
||||
|
||||
|
||||
```
|
||||
`command1 || command2`
|
||||
```
|
||||
|
||||
解读一下，“假如 command1 失败，执行 command2”。隐藏的逻辑是，如果 command1 成功，跳过 command2。下面实践一下，仍然是创建新目录：
|
||||
|
||||
|
||||
```
|
||||
[student@studentvm1 ~]$ Dir=/root/testdir ; mkdir $Dir || echo "$Dir was not created."
|
||||
mkdir: cannot create directory '/root/testdir': Permission denied
|
||||
/root/testdir was not created.
|
||||
[student@studentvm1 ~]$
|
||||
```
|
||||
|
||||
正如预期，因为目录无法创建，第一条命令失败了，于是第二条命令被执行。
|
||||
|
||||
把 **&&** 和 **||** 两种运算符结合起来才能发挥它们的最大功效。请看下面例子中的流程控制方法：
|
||||
|
||||
|
||||
```
|
||||
`preceding commands ; command1 && command2 || command3 ; following commands`
|
||||
```
|
||||
|
||||
语法解释：“假如 command1 退出时返回码为零，就执行 command2，否则执行 command3。”用具体代码试试：
|
||||
|
||||
|
||||
```
|
||||
[student@studentvm1 ~]$ Dir=/root/testdir ; mkdir $Dir && cd $Dir || echo "$Dir was not created."
|
||||
mkdir: cannot create directory '/root/testdir': Permission denied
|
||||
/root/testdir was not created.
|
||||
[student@studentvm1 ~]$
|
||||
```
|
||||
|
||||
现在我们再试一次，用你的家目录替换 **/root** 目录，你将会有权限创建这个目录了：
|
||||
|
||||
|
||||
```
|
||||
[student@studentvm1 ~]$ Dir=~/testdir ; mkdir $Dir && cd $Dir || echo "$Dir was not created."
|
||||
[student@studentvm1 testdir]$
|
||||
```
|
||||
|
||||
像 **command1 && command2** 这样的控制语句能够运行的原因是，每条命令执行完毕时都会给 shell 发送一个返回码，用来表示它执行成功与否。默认情况下，返回码为 0 表示成功，其他任何正值表示失败。一些系统管理员使用的工具用值为 1 的返回码来表示失败，但其他很多程序使用别的数字来表示失败。
|
||||
|
||||
Bash 的内置变量 **$?** 可以显示上一条命令的返回码，可以在脚本或者命令行中非常方便地检查它。要查看返回码，让我们从运行一条简单的命令开始，返回码的结果总是上一条命令给出的。
|
||||
|
||||
|
||||
```
|
||||
[student@studentvm1 testdir]$ ll ; echo "RC = $?"
|
||||
total 1264
|
||||
drwxrwxr-x 2 student student 4096 Mar 2 08:21 chapter25
|
||||
drwxrwxr-x 2 student student 4096 Mar 21 15:27 chapter26
|
||||
-rwxr-xr-x 1 student student 92 Mar 20 15:53 TestFile1
|
||||
drwxrwxr-x. 2 student student 663552 Feb 21 14:12 testdir
|
||||
drwxr-xr-x. 2 student student 4096 Dec 22 13:15 Videos
|
||||
RC = 0
|
||||
[student@studentvm1 testdir]$
|
||||
```
|
||||
|
||||
在这个例子中，返回码为零，意味着命令执行成功了。现在对 root 的家目录测试一下，你应该没有权限：
|
||||
|
||||
|
||||
```
|
||||
[student@studentvm1 testdir]$ ll /root ; echo "RC = $?"
|
||||
ls: cannot open directory '/root': Permission denied
|
||||
RC = 2
|
||||
[student@studentvm1 testdir]$
|
||||
```
|
||||
|
||||
本例中返回码是 2，表明非 root 用户没有权限进入这个目录。你可以利用这些返回码，用控制运算符来改变程序执行的顺序。
|
||||
|
||||
### 总结
|
||||
|
||||
本文将 Bash 看作一门编程语言，并从这个视角介绍了它的简单语法和基础工具。我们学习了如何将数据输出到 STDOUT，怎样使用变量和控制运算符。在本系列的下一篇文章中，将会重点介绍能够控制指令执行流程的逻辑运算符。
|
||||
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
via: https://opensource.com/article/19/10/programming-bash-part-1
|
||||
|
||||
作者：[David Both][a]
|
||||
选题：[lujun9972][b]
|
||||
译者：[jdh8383](https://github.com/jdh8383)
|
||||
校对：[校对者ID](https://github.com/校对者ID)
|
||||
|
||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
|
||||
|
||||
[a]: https://opensource.com/users/dboth
|
||||
[b]: https://github.com/lujun9972
|
||||
[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/bash_command_line.png?itok=k4z94W2U (bash logo on green background)
|
||||
[2]: https://opensource.com/article/18/10/linux-data-streams
|
||||
[3]: http://www.both.org/?page_id=1183
|
||||
[4]: https://opensource.com/19/9/command-line-heroes-bash
|
||||
[5]: https://en.wikipedia.org/wiki/Comparison_of_command_shells
|
||||
[6]: http://foldoc.org/program
|
||||
[7]: https://wordnet.princeton.edu/
|
||||
[8]: https://en.wikipedia.org/wiki/Computer_program
|
||||
[9]: https://www.gnu.org/software/bc/manual/html_mono/bc.html
|
||||
Loading…
Reference in New Issue
Block a user