[Shell] Bash Programming Summary

As a result of the work, several bash scripts were written in the previous months, primarily for automated testing, packaging, installation packages, and so on. Although compared to C + + programming, to be simple, stupid, but it in the Unix-like system can greatly improve the efficiency of the work. So here's a brief summary of some of the considerations in scripting.

1. Shell Overview

The shell is an interface between the user and the Unix-like operating system kernel (kernel) to protect the kernel from misuse by the user and to create a shell (Shell)around the kernel.

By making a request to the shell, the shell interprets and passes the request to the kernel. While multiple shell requests can be written in a single file, they form a shell script that is itself an interpreted programming language (a relatively compiled programming language), and shell scripts support most of the program elements seen in high-level languages, such as functions, variables, arrays, and program control structures, At the same time, any command that can be typed at the Unix terminal prompt can be placed in a shell script file.

The more widely used shells are: Bourne Again shell (bash), Bourne Shell (sh), C Shell (csh), Korn Shell (ksh ) and so on, the default Shell in most Unix - like systems today is bash.

View the SHELL used by the current environment: "Echo $SHELL".

View the shell that the current release can use: "Cat/etc/shells".

2. Bash Programming

Shell script file format,

1#!/bin/Bash2 3 function1 ()4 {5 }6 7 function2 ()8 {9 }Ten  One # Main program A Echo"Hello World" # Print Hello world - ... - ... the...

A variable that starts with $ in bash,

$#	The number of arguments passed to the script
$*	Values passed to all parameters of the script
[Email protected]	With $*
$$	The process number of the script
$!	Process number of the last process running in the background
$-	Displays the current options used by the shell
$?	Displays the exit status of the command (or function), 0 is correct, and 1 is the error
$	The program name of the shell script (file name)
$n	The nth parameter option for a script (or function) (starting with 1)

String manipulation,

${#str}	Returns the length of the string variable str
${STR:N}	Returns STR from nth character to last string (n starts from 0)
${str:n:len}	Returns STR starting with the nth character, Len length (n from 0)
${str#pattern}	Remove the first part of STR that matches the pattern at the beginning
${str# #pattern}	Remove most of the first part of STR that matches pattern
${str%pattern}	Remove the minimal portion of the end of STR that matches the pattern
${str%%pattern}	Remove the most part of the end of STR that matches the pattern
${str/old/new}	Replace the first occurrence of the old in Str with new
${str//old/new}	Replace all old in Str with new
${str/#old/new}	Replace the STR prefix with new with the old
${str/%old/new}	Replace the str suffix with new with the old
=	Determine if two strings are the same
!=	Determine if two strings are not the same
-N	Determines whether the string length is greater than 0
-X	Determines whether the string length equals zero
Note: The shell's built-in string operations perform better than awk and SED for various operations (at the very least, there is no need to start processes such as awk, SED, and so on).

Test the status of the file,

-D	is a directory
-F	is a file
-L	Whether it is a symbolic link
-R	Whether it is readable
-W	Whether it can be written
-X	is executable
-A	is not empty

Numerical tests,

-eq	equal
-ge	greater than or equal to
-le	less than equals
-ne	Not equal to
-gt	< P> greater than
-lt	less than

String test,

-eq	equal
-ge	greater than or equal to
-le	less than equals
-ne	Not equal to
-gt	< P> greater than
-lt	less than

logical operators,

!	Logical inversion
-A	Logic and
-O	Logical OR

3. script Debugging

Sh-x a.sh	Trace the execution of the script, perform a replacement for each command in the script, then display, then execute
Sh-v a.sh	Print each line in the script as entered, and print one line
Sh-n a.sh	The script is checked for syntax, but does not perform

4. some tips

Masking command Printing	cmd >/dev/null # Mask normal output CMD 2>/dev/null # Mask Error Output cmd >/dev/null 2>/dev/null # Mask normal output and error output cmd >/dev/null 2>&1 # Mask normal output and error output CMD &>/dev/null # Mask normal output and error output
Whether the command executed successfully	CMD && echo "Failed" | | echo "OK"
Connection file 1 All Lines	Paste–sd "," 1 # Connect with commas Paste–sd "" 1 # Connect with a space
Filter Files 1 Duplicate rows	Sort-u 1
Setting temporary environment variables	In the script file, export acts on the current script, VAR=ENV_STR CMD environment variable only for this cmd (no need to export first, then unset)
String Default value	${str:-0} # If STR is not declared or is empty, set to 0, can be used to determine the command line or function parameters
Special Parameters--	echo "Abc-a" | grep---I #--Subsequent arguments are not resolved as options
Get a soft link to a file	Readlink file
Emptying a file (or creating a file)	echo "" > File Cat/dev/null > File > File
Multi-process in shell	Use (cmd) to fork a child shell to execute the command, and wait for all child processes to end with the wait command
......	......
To be supplemented in the future

[Shell] Bash Programming Summary