Some keywords in shell programming-to be sorted

1 shell $! , $ ?, $, $ @

 

• $ N $1 the first parameter, $2 the second...
• $ # The number of command-line parameters.
• $0 the name of current program.
• $? Last Command or function's return value.
• $ The program's PID.
• $! Last program's PID.
• $ @ Save all the parameters.

Almost any shell book will talk about them, from which you can get their detail usages.

2 Linux Shell if command parameter description

•-B returns true if the file exists and is a block File
•-C returns true if the file exists and is a character file
•-D returns true if pathname exists and is a directory
•-E returns true if the file or directory specified by pathname exists
•-F returns true if the file exists and is a regular file
•-G returns true if a file or directory specified by pathname exists and the sgid bit is set.
•-H returns true if the file exists and is a symbolic link file. This option is invalid in some old systems.
•-K returns true if the file or directory specified by pathname exists and a "Sticky" bit is set
•-P returns true if the file exists and is a command Pipeline
•-R returns true if a file or directory specified by pathname exists and is readable
•-S returns true if the file size is greater than 0
•-U returns true if a file or directory specified by pathname exists and SUID is set
•-W returns true if the file or directory specified by pathname exists and is executable. A directory must be executable for its content access.
•-O returns true if a file or directory specified by pathname exists and is owned by the user specified by the valid user ID of the current process.
Comparison character writing in Unix shell:

•-EQ equals
•-Ne is not equal
•-GT greater
•-Lt is less
•-Le is less than or equal
•-Ge is greater than or equal
•-Z empty string
• = Two equal characters
•! = Two characters
•-N non-empty strings
Summary:

Document comparison operator
-E filename if filename exists, it is true [-E/var/log/syslog]
-D filename if filename is a directory, it is true [-D/tmp/mydir]
-F filename if filename is a regular document, it is true [-F/usr/bin/grep]
-L filename if filename is a symbolic link, it is true [-L/usr/bin/grep]
-R filename is true if filename is readable [-r/var/log/syslog]
-W filename if filename is writable, it is true [-W/var/mytmp.txt]
-X filename if filename is executable, it is true [-L/usr/bin/grep]
Filename1-nt filename2 if filename1 is newer than filename2, true [/tmp/install/etc/services-nt/etc/services]
Filename1-ot filename2 if filename1 is older than filename2, true [/boot/bzimage-ot ARCH/i386/boot/bzimage]
String comparison operator (please note the use of quotation marks, which is a good way to prevent space from disturbing the code)
-Z string if the string length is zero, it is true [-z "$ myvar"]
-N string if the string length is not zero, it is true [-n "$ myvar"]
String1 = string2 if string1 and string2 are the same, it is true ["$ myvar" = "One Two Three"]
String1! = String2 if string1 is different from string2, it is true ["$ myvar "! = "One Two Three"]
Arithmetic comparison operator
Num1-eq num2 equals to [3-EQ $ mynum]
Num1-ne num2 is not equal to [3-ne $ mynum]
Num1-lt num2 less than [3-lt $ mynum]
The num1-le num2 is less than or equal to [3-Le $ mynum]
Num1-gt num2 greater than [3-GT $ mynum]
Num1-ge num2 is greater than or equal to [3-ge $ mynum]

This document is excerpted from the network

++ ++

Shell checks whether a file or directory exists or has permissions.
Source: http://hi.baidu.com/haigang/blog/item/e5f582262d639c118b82a167.html
#! /Bin/sh
Mypath = "/var/log/httpd /"
Myfile = "/var/log/httpd/access. log"
# Here, the-x parameter determines whether $ mypath exists and has executable permissions.
If [! -X "$ mypath"]; then
Mkdir "$ mypath"
Fi
# Here, the-D parameter determines whether $ mypath exists.
If [! -D "$ mypath"]; then
Mkdir "$ mypath"
Fi
# Here, the-F parameter determines whether $ myfile exists.
If [! -F "$ myfile"]; then
Touch "$ myfile"
Fi
# Other parameters include-N, which determines whether a variable has a value.
If [! -N "$ myvar"]; then
Echo "$ myvar is empty"
Exit 0
Fi
# Determine whether two variables are equal
If ["$ var1" = "$ var2"]; then
Echo '$ var1 EQ $ var2'
Else
Echo '$ var1 not EQ $ var2'
Fi

Note: The if format cannot contain fewer spaces.
==========================================================
Shell judgment statement

If the "if" expression of process control is true, the part after then is executed: If...; then.
....
Elif...; then
....
Else
....
Fi
In most cases, you can use test commands to test the conditions. For example, you can compare strings, determine whether a file exists, and whether the file is readable... "[]" Is usually used to represent a conditional test. Note that spaces are important. Make sure that the square brackets have spaces.
[-F "somefile"]: determines whether it is a file.
[-X "/bin/ls"]: determines whether/bin/ls exists and has the executable permission.
[-N "$ Var"]: determines whether the $ var variable has a value.
["$ A" = "$ B"]: determines whether $ A and $ B are equal.-R file users can be read as true.
-W file users can write to true
-X file user executable is true
-F file: the regular file is true.
-D file: the directory is true.
-C file: the special character file is true.
-B file: The Block special file is true.
-S file is true if the file size is not 0
-T file: this parameter is true when the specified device is a terminal in the file descriptor (1 by default ).
######################################## #################
Shell script with conditional Selection
Simple shell scripts for tasks without variables are generally competent. However, when executing some decision-making tasks, it is necessary to include the IF/then condition judgment. Shell script programming supports such operations, including comparison operations and determining whether a file exists. The basic if condition Command Options include-eq-to compare whether two parameters are equal (for example, if [2-EQ 5]).
-Ne-compare whether two parameters are not equal
-Lt-parameter 1: whether it is smaller than parameter 2
-Le-parameter 1: whether it is less than or equal to parameter 2
-GT-whether parameter 1 is greater than parameter 2
-Whether Ge-parameter 1 is greater than or equal to parameter 2
-F-check whether a file exists (for example, if [-F "FILENAME"])
-D-check whether the directory exists
Almost all judgments can be implemented using these comparison operators. In the script, the common-F Command Option checks whether a file exists before executing it. ######################################## ######################### Determine whether a file exists
#! /Bin/sh
Today = 'date-d Yesterday + % Y % m % d'
File = "apache_$today.tar.gz"
CD/home/chenshuo/Shell
If [-F "$ file"]; then
Echo "OK"
Else
Echo "error $ file"> error. Log
Mail-s "fail backup from test" test@test.com <error. Log
Fi

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