Shell script learning Summary-string processing and branch statements

BASH Shell script learning Summary. It is easy to determine whether a log file is larger than 2 GB. If it is larger than 2 GB, delete it. I haven't written bash for a long time, so I can't even think of it. So I reviewed it and made a summary of today's learning:
1. Character truncation:
You can use BasenameAnd DirnameThese two tools:
BasenameYou can capture a file name from a file path.
For example:

$ Basename/home/file.tar
File.tar

DirnameYou can capture a directory path from a file path.
For example:

$ Dirname/home/file.tar
/Home

Do not use external tools for character Truncation
Bash has its own function to truncation variables. It is generally implemented by combining "#", "#", "%", "%. For example:

$ String = hellowbashshell
$ Echo $ {string # * Sh}
Ell
$ Echo $ {string # * Sh}
Shell
$ Echo $ {string % sh *}
Hellowba
$ Echo $ {string % sh *}
Hellowbash

"#"Indicates that the part is removed from the beginning of the character. It is removed immediately after matching.
"##"Indicates that the string is removed from the starting part of the character, and the longest matching of the entire string is searched.
"%"Indicates to remove the part from the end of the character. It is immediately removed once it is matched to the public.
"%"Indicates that it is removed from the end of the character and will be searched for the longest match in the entire character.
"*"Unified character, usually associated with" # "or" # ", is placed on the left of the search string, for example: $ {string # * Sh} (on the left of SH ), when used together with "%" or "%", it is placed on the right of the matching string, for example: $ {string % sh *}

Common tips:

Get the file name in the path: $ {path ##*/} (same as basename)
Path: $ {PATH %/*} (same as dirname)
File Extension: $ {path ##*.}

2. Receipt of Independent Variables
Receives parameters passed in from the command line. The first parameter is represented by $1, and the second parameter is represented by $2 ,... And so on. Note: $0 indicates the script file name. In shell programming, "$ @" is often used to represent all parameters ,. You can use a loop to traverse this parameter. If you use Java for analogy, You can regard $ @ as the array defined in the man function.

3. If statement:
Format: If [condition]
Then
Action
Fi

Note: a space is required between "if" and "[". If you do not have a space, shell reports a syntax error. I was wasted on this for a while.

Conditon test type table

Operator	Description	Example
File comparison operator
-EFilename	IfFilenameYes, true	[-E/var/log/syslog]
-DFilename	IfFilenameIs a directory, it is true	[-D/tmp/mydir]
-FFilename	IfFilenameIs a regular file, it is true	[-F/usr/bin/grep]
-LFilename	IfFilenameIs a symbolic link, it is true	[-L/usr/bin/grep]
-RFilename	IfFilenameReadable, true	[-R/var/log/syslog]
-WFilename	IfFilenameWritable, true	[-W/var/mytmp.txt]
-XFilename	IfFilenameExecutable, true	[-L/usr/bin/grep]
Filename1-NTFilename2	IfFilename1RatioFilename2New, true	[/Tmp/install/etc/services-nt/etc/services]
Filename1-OtFilename2	IfFilename1RatioFilename2Old, true	[/Boot/bzimage-ot ARCH/i386/boot/bzimage]
String comparison operator(Please pay attention to the use of quotation marks. This is a good way to prevent space from disturbing the code)
-ZString	IfStringIf the length is zero, it is true.	[-Z "$ myvar"]
-NString	IfStringNon-zero length, true	[-N "$ myvar"]
String1=String2	IfString1AndString2Same, true	["$ Myvar" = "One Two Three"]
String1! =String2	IfString1AndString2Otherwise, true	["$ Myvar "! = "One Two Three"]
Arithmetic comparison operator
Num1-EQNum2	Equal	[3-EQ $ mynum]
Num1-NeNum2	Not equal	[3-ne $ mynum]
Num1-LtNum2	Less	[3-lt $ mynum]
Num1-LeNum2	Less than or equal	[3-Le $ mynum]
Num1-GTNum2	Greater	[3-GT $ mynum]
Num1-GeNum2	Greater than or equal	[3-ge $ mynum]

It seems that if in Bash is more intelligent than other languages. In bash, the existence of a file is similar to comparing the size of two numbers.

4. For statement
Bash statements are always so user-friendly and very similar to natural languages. In a for statement, almost any data type similar to a set can be iterated (maybe this is not true, but I really cannot think of better words to replace ).
Let's look at an example:

#! /Bin/bash
For ARGs in $ @
Do
Echo $ ARGs
Done

Save the above code entry as showargs. SH and set it to execute (chmod + x showargs. Sh:

$./Showargs. Sh arg1 arg2 arg3 arg4
Arg1
Arg2
Arg3
Arg4

In this example, we use "$ @", which represents all command line parameters. Here, we use for to traverse it. The system iteratively extracts the command line parameter from $ @ and puts it into args. Finally, we use echo $ ARGs for output.
For is more often used to traverse directories. The following example is used to list the names of all files and folders in the current directory $ for file in *
> Do
> Echo $ File
> Done

Here, "*" represents the current directory and lists the names of all files and folders. Here, folders and files cannot be separated. If you need, you should use if [-d $ {file}] for judgment.
For file traversal, you can add multiple expressions after "in. That is to say, you can traverse multiple directories at a time.
The following code copies the files in the go folder and do folder in the current directory to the fo folder #! /Bin/bash
For ARGs in./go/*./do /*
Do
CP $ {ARGs}./fo
Echo "copying $ {ARGs} To./FO/$ {ARGs }"
Done