1. Character truncation:
If the character truncation is a generic path, you can use both the basename and dirname tools:
BaseName can truncate a filename from a file path
For example:
Copy Code code as follows:
$ basename/home/file.tar
File.tar
DirName can be truncated to a directory path from a file path
For example:
Copy Code code as follows:
$ dirname/home/file.tar
/home
Do not use external tools for character truncation
Bash has its own features to truncate variables, generally using the "# #", "#", "%", "%", "*" combination to achieve. For example:
Copy Code code as follows:
$ string=hellowbashshell
$ echo ${string##*sh}
Ell
$ echo ${string#*sh}
Shell
$ echo ${string%%sh*}
Hellowba
$ echo ${string%sh*}
Hellowbash
"#" is dropped from the beginning of the character and immediately after the match
"# #" means to remove from the beginning of the character, searching for the longest match of the entire string to remove
"%" is removed from the end of the character and immediately removed when the match is made into a public
Percent% means that the end of the character is removed and the longest match is searched for the entire character to remove
The "*" wildcard character, commonly associated with "# #" or "#", is placed on the left side of the search string, for example: ${string#*sh} (on the left of SH), and "%" or "%" will be placed on the right side of the matching string, for example: ${string%%sh*}
Common skills:
Take filename in path: ${path##*/} (same function as basename)
Directory path in Path: ${path%/*} (same function as dirname)
Fetch file extension: ${path##*.}
2. The receipt of the independent variable
Receives arguments passed in from the command line, the first parameter is represented by $, and the second argument is $ ... Analogy Note: $ represents the script filename. Another common use in shell programming is "$@", which represents all the parameters. You can use a loop to traverse this argument. If you use Java analogy, you can think of $@ as the array defined in the man function.
3.if statement:
Format:
Copy Code code as follows:
if [condition]
Then
Action
Fi
Note: Spaces are required between "if" and "[", and if you do not have spaces, the shell will report syntax errors. That's what I've been wasting for so long.
Conditon Test Type comparison table
| Operator |
Describe |
Example |
| File comparison Operators |
| -e filename |
True if filename exists |
[-e/var/log/syslog] |
| -D FileName |
True if filename is a directory |
[-d/tmp/mydir] |
| -F filename |
True if filename is a regular file |
[-f/usr/bin/grep] |
| -L filename |
True if filename is a symbolic link |
[-l/usr/bin/grep] |
| -R FileName |
True if filename is readable |
[-r/var/log/syslog] |
| -W filename |
True if filename is writable |
[-w/var/mytmp.txt] |
| -X filename |
True if filename is executable |
[-l/usr/bin/grep] |
| filename1-nt filename2 |
True if filename1 is newer than filename2 |
[/tmp/install/etc/services-nt/etc/services] |
| filename1-ot filename2 |
True if the filename1 is older than filename2 |
[/boot/bzimage-ot Arch/i386/boot/bzimage] |
| string comparison operators (note the use of quotes, which is a good way to prevent spaces from disrupting code) |
| -Z string |
True if the string length is zero |
[-Z "$myvar"] |
| -N String |
True if the string length is Non-zero |
[-N ' $myvar] |
| string1= string2 |
True if string1 is the same as string2 |
["$myvar" = "One Two Three"] |
| string1!= string2 |
True if string1 is different from string2 |
["$myvar"!= "one Two three"] |
| Arithmetic comparison operators |
| num1-eq num2 |
Equals |
[3-eq $mynum] |
| num1-ne num2 |
Not equal to |
[3-ne $mynum] |
| num1-lt num2 |
Less than |
[3-lt $mynum] |
| num1-le num2 |
Less than or equal to |
[3-le $mynum] |
| num1-gt num2 |
Greater than |
[3-gt $mynum] |
| num1-ge num2 |
Greater than or equal to |
[3-ge $mynum] |
If you feel that the IF in bash is more intelligent than some other languages, in bash, testing the existence of a file is no different than comparing the size of two numbers;)
4.for Statement
The statements in bash are always so human, so close to the natural language that you can almost iterate over any data type similar to the collection in a For statement (perhaps that's not true, but I really can't think of a better word to replace it).
Look at an example:
Copy Code code as follows:
#!/bin/bash
For args in $@
Todo
Echo $args
Done
Save the above code entry as Showargs.sh set to executable (chmod +x showargs.sh) Execution:
Copy Code code as follows:
$./showargs.sh arg1 arg2 Arg3 arg4
Arg1
Arg2
Arg3
Arg4
In this example, we use the "$@", which represents all command-line arguments. In this case, the system iterates through the $@, takes the command-line arguments out of the args, and then outputs them using the Echo $args.
For more frequent traversal of directories, the following example is used to list the names of all files and folders under the current directory
Copy Code code as follows:
$ for file in *
> Do
> Echo $file
> Done
Here the * represents the current directory, listing all the names of files and folders, where folders and files are not separate, and if you need to, you should use if [D-${file}] to make a judgment.
What's even more interesting about file traversal is that you can connect multiple expressions after "in". In other words, you can traverse multiple directories at once.
The following code copies the files in the Go folder and do folder under the current directory to the FO folder
Copy Code code as follows:
#!/bin/bash
For args in./go/*./do/*
Todo
CP ${args}./FO
echo "Copying ${args} to./fo/${args}"
Done
