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This article will explain in detail how to use Bash programming to achieve logical operators and shell extensions. The editor thinks it is very practical, so I share it with you as a reference. I hope you can get something after reading this article.

Logical operator

There are a large number of logical operators in Bash for different conditional expressions. The most basic is the if control structure, which determines a condition and executes some program statements if the condition is true. There are three types of operators: file, numeric, and non-numeric operators. If the condition is true, all operators return a true value (0) and a false value (1) if the condition is false.

The functional syntax of these comparison operators is that an operator plus one or two parameters is placed in parentheses, followed by a series of program statements. If the condition is true, the program statement executes, and there may be another list of program statements. The list is executed when the condition is false:

If [arg1 operator arg2]; then list or if [arg1 operator arg2]; then list; else list; fi

As in the example, spaces cannot be omitted in comparative expressions. Each part of the brackets, [and], is the same traditional Bash symbol as the test command:

If test arg1 operator arg2; then list

There is also an updated syntax that provides a little convenience, which some system administrators prefer to use. This format is slightly less compatible with different versions of Bash and some shell such as ksh (Korn shell). The format is as follows:

If [[arg1 operator arg2]]; then list file operator

File operators are a series of powerful logical operators in Bash. Figure 1 lists more than 20 different operators for Bash processing files. It is used very frequently in my script.

Operator description-a filename returns the true value if the file exists The file can be empty or have content, but as long as it exists, it returns truth-b filename if the file exists and is a block device, such as / dev/sda or / dev/sda1, return truth-c filename if the file exists and is a character device, such as / dev/TTY1, return truth-d filename if the file exists and is a directory, return truth-e filename if the file exists, return truth Same as-an above-f filename if the file exists and is a general file, not a directory, device file, link, etc., return truth-g filename if the file exists and the SETGID tag is set on it, return truth-h filename if the file exists and is a symbolic link, return truth-k filename if the file exists and the sticky bit is set Return truth-p filename if the file exists and is a named pipe (FIFO), return truth-r filename if the file exists and has readable permissions (its readable bit is set), return truth-s filename if the file exists and the size is greater than 0, return truth Returns a false value-t fd if a file exists but the size is 0-t fd if the file descriptor fd is opened and associated to a terminal device, return true value-u filename if the file exists and its SETUID bit is set, return true value-w filename if the file exists and has writeable permission, return true value-x filename if the file exists and has executable permission Return truth-G filename if the file exists and the group ID of the file is the same as the current user, return truth-L filename if the file exists and is a symbolic link, return truth (same as-h)-N filename if the file exists and has been modified since the file was last read, return truth-O filename if the file exists and you are the owner of the file, return truth-S filename if the file exists and the file is a socket Return truth value file1-ef file2 if file file1 and file file2 point to the same INODE number of the same device, return truth value (that is, hard link) file1-nt file2 if file file1 is newer than file2 (according to modification date), or if file1 exists but file2 does not exist, return truth value file1-ot file2 if file file1 is older than file2 (based on modification date), or file1 does not exist but file2 exists

Chart 1:Bash file operator

For example, to test the existence of a file:

[student@studentvm1 testdir] $File= "TestFile1"; if [- e $File]; then echo "The file $File exists."; else echo "The file $File does not exist."; fiThe file TestFile1 does not exist. [student@studentvm1 testdir] $

Create a file to test and name it TestFile1. Currently, it does not need to contain any data:

[student@studentvm1 testdir] $touch TestFile1

In this short CLI program, it is easier to change the value of the $File variable than to change the value of a string that represents a file name in multiple places:

[student@studentvm1 testdir] $File= "TestFile1"; if [- e $File]; then echo "The file $File exists."; else echo "The file $File does not exist."; fiThe file TestFile1 exists. [student@studentvm1 testdir] $

Now, run a test to determine whether a file exists and is not 0 in length (indicating that it contains data). Suppose you want to judge three situations:

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File does not exist

The file exists and is empty

The file exists and contains data.

Therefore, you need a more complex set of test code-to test all cases, use the elif statement in the if-elif-else structure:

[student@studentvm1 testdir] $File= "TestFile1"; if [- s $File]; then echo "$File exists and contains data."; fi [student@studentvm1 testdir] $

In this case, the file exists but does not contain any data. Add some data to the file and run it again:

[student@studentvm1 testdir] $File= "TestFile1"; echo "This is file $File" > $File; if [- s $File]; then echo "$File exists and contains data."; fiTestFile1 exists and contains data. [student@studentvm1 testdir] $

This set of statements can return normal results, but only test an exact condition in three known cases. Add an else statement so that you can test it more accurately. Delete the file and you can test the new code completely:

[student@studentvm1 testdir] $File= "TestFile1"; rm $File; if [- s $File]; then echo "$File exists and contains data."; else echo "$File does not exist or is empty."; fiTestFile1 does not exist or is empty.

Now create an empty file to test:

[student@studentvm1 testdir] $File= "TestFile1"; touch $File; if [- s $File]; then echo "$File exists and contains data."; else echo "$File does not exist or is empty."; fiTestFile1 does not exist or is empty.

Add something to the file, and then test it again:

[student@studentvm1 testdir] $File= "TestFile1"; echo "This is file $File" > $File; if [- s $File]; then echo "$File exists and contains data."; else echo "$File does not exist or is empty."; fiTestFile1 exists and contains data.

Now add an elif statement to tell whether the file does not exist or the file is empty:

[student@studentvm1 testdir] $File= "TestFile1"; touch $File; if [- s $File]; then echo "$File exists and contains data."; elif [- e $File]; then echo "$File exists and is empty."; else echo "$File does not exist."; fiTestFile1 exists and is empty. [student@studentvm1 testdir] $File= "TestFile1"; echo "This is $File" > $File; if [- s $File]; then echo "$File exists and contains data."; elif [- e $File] Then echo "$File exists and is empty."; else echo "$File does not exist."; fiTestFile1 exists and contains data. [student@studentvm1 testdir] $

Now you have a Bash CLI program that can test these three situations, but the possibilities are unlimited.

If you can organize program statements like scripts saved in a file, it is easy to see their logical structure even for more complex command combinations. Figure 2 is an example. The indentation of program statements in each part of the if-elif-else structure makes the logic clearer.

File= "TestFile1" echo "This is $File" > $Fileif [- s $File] then echo "$File exists and contains data." elif [- e $File] then echo "$File exists and is empty." else echo "$File does not exist." fi Chart 2: rewrite a command line program as if it were in a script

For most CLI programs, it takes a lot of code to make these complex commands logical. Although CLI may use Linux or Bash built-in commands, it is more efficient to create a script saved in a file when the CLI program is long or complex, and you can run it at any time after saving it to the file.

String comparison operator

The string comparison operator allows us to compare characters in a string alphabetically. Figure 3 lists only a few string comparison operators.

Operator description-z string returns true value-n string if the length of the string is 0, string1 = = string2 or string1 = string2 if the length of the string is not 0, true value is returned if two strings are equal. In compliance with POSIX conformance, an equal sign = should be used in the test command. When used with the command [[, the pattern matching described above (mixed command) occurs. String1! = string2 two strings are not equal, return the true value string1

String2 if string1 and string2 are sorted alphabetically, string1 comes after string2

Figure 3: Bash string logical operator

First, check the length of the string. The double quotation marks around $MyVar in the comparison expression cannot be omitted (you should still be in the directory ~ / testdir).

[student@studentvm1 testdir] $MyVar= ""; if [- z ""]; then echo "MyVar is zero length."; else echo "MyVar contains data"; fiMyVar is zero length. [student@studentvm1 testdir] $MyVar= "Random text"; if [- z ""]; then echo "MyVar is zero length."; else echo "MyVar contains data"; fiMyVar is zero length.

You can also do this:

[student@studentvm1 testdir] $MyVar= "Random text"; if [- n "$MyVar"]; then echo "MyVar contains data."; else echo "MyVar is zero length"; fiMyVar contains data. [student@studentvm1 testdir] $MyVar= ""; if [- n "$MyVar"]; then echo "MyVar contains data."; else echo "MyVar is zero length"; fiMyVar is zero length

Sometimes you need to know the exact length of a string. Although this is not a comparison, it is also related to the comparison. Unfortunately, there is no easy way to calculate the length of a string. There are many ways to calculate, but I think using the expr (evaluation expression) command is the relatively simple one. Read the man page of expr to learn more about it. Note that the quotation marks around the string or variable you detect in the expression should not be omitted.

[student@studentvm1 testdir] $MyVar= "; expr length" $MyVar "0 [student@studentvm1 testdir] $MyVar=" How long is this? "; expr length" $MyVar "17 [student@studentvm1 testdir] $expr length" We can also find the length of a literal string as well as a variable. "70

With regard to the comparison operator, a large number of operations are used in our script to detect whether two strings are equal (for example, whether two strings are actually the same string). I used a non-POSIX version of the comparison expression:

[student@studentvm1 testdir] $Var1= "Hello World"; Var2= "Hello World"; if ["$Var1" = "$Var2"]; then echo "Var1 matches Var2"; else echo "Var1 and Var2 do not match."; fiVar1 matches Var2 [student@studentvm1 testdir] $Var1= "Hello World"; Var2= "Hello world"; if ["$Var1" = = "$Var2"]; then echo "Var1 matches Var2"; else echo "Var1 and Var2 do not match."; fiVar1 and Var2 do not match.

Try these operators in your own script.

Numeric comparison operator

The numeric operator is used to compare two numeric parameters. Like other class operators, most of them are easy to understand.

Operator describes arg1-eq arg2 if arg1 equals arg2, return truth value arg1-ne arg2 if arg1 is not equal to arg2, return truth value arg1-lt arg2 if arg1 is less than arg2, return truth value arg1-le arg2 if arg1 is less than or equal to arg2, return truth value arg1-gt arg2 if arg1 is greater than arg2, return truth value arg1-ge arg2 if arg1 is greater than or equal to arg2, return truth value

Figure 4: Bash numeric comparison logic operator

Let's look at a few simple examples. The first example sets the value of the variable $X to 1, and then detects whether $X is equal to 1. In the second example, $X is set to 0, so the result returned by the comparison expression is not true.

[student@studentvm1 testdir] $X-eq 1; if [$X-eq 1]; then echo "X equals 1"; else echo "X does not equal 1"; fiX equals 1 [student@studentvm1 testdir] $X-eq 0; if [$X-eq 1]; then echo "X equals 1"; else echo "X does not equal 1"; fiX does not equal 1 [student@studentvm1 testdir] $

Try something else by yourself.

Miscellaneous operator

These miscellaneous operators show whether a shell option is set, or whether a shell variable has a value, but it does not display the value of the variable, only whether it has a value.

Operator description-o optname if a shell option optname is enabled (see the list of options below described by the set-o option built into the Bash man page), return true value-v varname if the shell variable varname is set (given a value), return true value-R varname if a shell variable varname is set to a value and is a name reference, it returns true value

Figure 5: miscellaneous Bash logical operators

Practice using these operators yourself.

Expansion

Bash supports several very useful types of extensions and command replacements. According to the Bash man pages, Bash has seven extended formats. This article only introduces five of them: ~ extension, arithmetic extension, path name extension, curly braces extension, and command substitution.

Curly braces extension

Curly braces extension is a way to generate arbitrary strings. (the following example is to create a large number of files with characters in a specific pattern. Brace extensions can be used to generate a list of arbitrary strings and insert them into a specific location surrounded by a static string or at both ends of a static string. This may not be easy to imagine, so let's put it into practice.

First, take a look at the role of curly braces extensions:

[student@studentvm1 testdir] $echo {string1,string2,string3} string1 string2 string3

It doesn't look very useful, does it? But use it in other ways, let's take a look:

[student@studentvm1 testdir] $echo "Hello" {David,Jen,Rikki,Jason} .Hello David. Hello Jen. Hello Rikki. Hello Jason.

This seems to be of some use-we can type a lot less words. Now try this:

[student@studentvm1 testdir] $echo b {ed,olt,ar} sbeds bolts bars

I can continue to give examples, but you should have understood its usefulness.

~ expansion

The data show that the most frequently used extension is the tilde (~) extension. When you use it in a command (such as cd ~ / Documents), Bash shell expands the shortcut into the user's complete home directory.

Use this Bash program to observe the role of ~ extension:

[student@studentvm1 testdir] $echo ~ / home/student [student@studentvm1 testdir] $echo ~ / Documents/home/student/Documents [student@studentvm1 testdir] $Var1=~/Documents; echo $Var1; cd $Var1/home/student/Documents [student@studentvm1 Documents] $pathname extension

Path name extension is another way to expand the file wildcard pattern to match the full path name of the pattern, using matching characters? And *. File wildcard refers to the great flexibility of using specific pattern characters to match file names, paths, and other strings in a large number of operations. These specific pattern characters allow you to match one, more, or specific characters in a string.

?-matches an arbitrary character at a specific position in the string

*-matches 0 or more arbitrary characters at a specific position in the string

This extension is used to match path names. To figure out how to use it, make sure that testdir is the current working directory (PWD), and first execute the basic listing command ls (the content in my home directory is different from yours).

[student@studentvm1 testdir] $lschapter6 cpuHog.dos dmesg1.txt Documents Music softlink1 testdir6 Videoschapter7 cpuHog.Linux dmesg2.txt Downloads Pictures Templates testdirtestdir cpuHog.mac dmesg3.txt file005 Public testdir tmpcpuHog Desktop dmesg.txt link3 random.txt testdir1 umask.test [student@studentvm1 testdir] $

Now list the directories that start with Do, testdir/Documents, and testdir/Downloads:

Documents:Directory01 file07 file15 test02 test10 test20 testfile13 TextFilesDirectory02 file08 file16 test03 test11 testfile01 testfile14file01 file09 file17 test04 test12 testfile04 testfile15file02 file10 file18 test05 test13 testfile05 testfile16file03 file11 file19 test06 test14 testfile09 testfile17file04 file12 file20 test07 test15 testfile10 testfile18file05 file13 Student1.txt test08 test16 testfile11 testfile19file06 file14 test01 test09 test18 testfile12 testfile20 Downloads: [student@studentvm1 testdir] $

However, you didn't get the results you expected. It lists the contents of directories that start with Do. Use the-d option to list only the directories and not their contents.

[student@studentvm1 testdir] $ls-d Do*Documents Downloads [student@studentvm1 testdir] $

In both cases, Bash shell expands the Do* schema to a directory name that matches the pattern. But what happens if there is a file that matches this pattern?

[student@studentvm1 testdir] $touch Downtown; ls-d Do*Documents Downloads Downtown [student@studentvm1 testdir] $

So all files that match the pattern are expanded to the full name.

Command replacement

Command substitution is an extension that allows the standard output data stream of one command to be passed to another command as a parameter, for example, as a series of items processed in a loop. The Bash man page says: "Command replacement allows you to replace the output of a command with the name of the command." This may not be easy to understand.

There are two formats for command substitution: `command` and $(command). Use the backslash (\) in earlier formats and the backslash (\) in the command to maintain the meaning of the text before it is escaped. However, when used in the new version of the parenthesis format, the backslash is treated as a special character. Also note that opening a close command statement in parenthesized format uses a parenthesis.

I often use this capability in command-line programs and scripts, where the result of one command can be used as an argument to another command.

Let's look at a very simple example that uses two formats of this extension (again, make sure testdir is the current working directory):

[student@studentvm1 testdir] $echo "Todays date is `date`" Todays date is Sun Apr 7 14:42:46 EDT 2019 [student@studentvm1 testdir] $echo "Todays date is $(date)" Todays date is Sun Apr 7 14:42:59 EDT 2019 [student@studentvm1 testdir] $

-the seq tool is used for a sequence of numbers:

[student@studentvm1 testdir] $seq 512345 [student@studentvm1 testdir] $echo `seq 5`1 2345 [student@studentvm1 testdir] $

Now you can do something more useful, such as creating a large number of empty files for testing.

[student@studentvm1 testdir] $for I in $(seq-w 5000); do touch file-$I; done

When the seq tool adds the-w option, it completes the generated numbers with 0, so that all results are equal in width, for example, ignore the values of numbers, and their digits are the same. This makes it easy to arrange them in numerical order.

The seq-w 5000 statement produces a sequence of numbers from 1 to 5000. By using the command substitution for the for statement, the for statement can use this sequence of numbers to generate the numeric part of the file name.

Arithmetic extension

Bash can do mathematical calculations of integers, but it's more tedious (as you'll see in a moment). The syntax of the numeric extension is $((arithmetic-expression)), which opens and closes the expression with two parentheses. Arithmetic extensions are similar to command substitution in shell programs or scripts; the result of the expression settlement replaces the expression for subsequent evaluation of shell.

Let's start with a simple usage:

[student@studentvm1 testdir] $echo $((1x 1)) 2 [student@studentvm1 testdir] $Var1=5; Var2=7; Var3=$ ((Var1*Var2)); echo "Var3=$ Var3" Var3= 35

The following division result is 0, because the result of the expression is an integer number less than 1:

[student@studentvm1 testdir] $Var1=5; Var2=7; Var3=$ ((Var1/Var2)); echo "Var3=$ Var3" Var3= 0

This is a simple calculation that I often use in scripts or CLI programs to see how much virtual memory is used in the Linux host. Free does not provide the data I need:

[student@studentvm1 testdir] $RAM= `free | grep ^ Mem | awk'{print $2}'`; Swap= `free | grep ^ Swap | awk'{print $2}'`; echo "RAM= $RAM and Swap= $Swap"; echo "Total Virtual memory is $((RAM+Swap))"; RAM= 4037080 and Swap= 6291452Total Virtual memory is 10328532

I use the `character to demarcate the boundaries used as command replacements.

My scenario of arithmetic extension with Bash is to use a script to check the system resource usage and choose a path for a program to run based on the returned results.

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