Shell script--Symbol Chapter

Shell Execution Script

650) this.width=650; "Width=" 799 "height=" 321 "title=" QQ picture 20160809205710.png "Style=" width:607px;height:229px;float : none; "alt=" Wkiom1ep0pkiz2v4aac1d9tdasu417.png "src=" http://s4.51cto.com/wyfs02/M02/85/9B/ Wkiom1ep0pkiz2v4aac1d9tdasu417.png "/> 650) this.width=650; "title=" QQ picture 20160809212237.png "alt=" wkiom1ep2igxpi3maabpvobgyic455.png "src="/http S2.51cto.com/wyfs02/m01/85/9c/wkiom1ep2igxpi3maabpvobgyic455.png "/> 650) this.width=650; "title=" QQ picture 20160809211211.png "style=" Float:none; "alt=" Wkiol1ep1hzwwsidaaaq03yhbls384.png " src= "Http://s5.51cto.com/wyfs02/M00/85/9B/wKioL1ep1hzwwSIdAAAQ03yhBLs384.png"/> 650) this.width=650; "Width=" 813 "height=" 283 "title=" qq picture 20160809213711.png "style=" WIDTH:669PX;HEIGHT:236PX; "alt= "Wkiol1ep2_ghtyowaadin6ew7ci265.png" src= "Http://s3.51cto.com/wyfs02/M02/85/9B/wKioL1ep2_GhtYOWAADIn6eW7CI265.png"/> Shell is a batch language (batch tasks are processed one time) The shell writes the text file, cannot let the operating system run it, but fork creates the child process, the subroutine replaces the shell interpreter, then passes the text file as the command line argument to the Bash interpreter, the child process one line reads the command, encounters a row to create a child process, So executing the above script is equivalent to executing the program.

Shell variables

650) this.width=650; "Width=" 694 "height=" 223 "title=" QQ picture 20160809214920.png "style=" WIDTH:658PX;HEIGHT:216PX; "alt= "Wkiom1ep3t-xawboaacvdp7nwkc792.png" src= "Http://s1.51cto.com/wyfs02/M02/85/9C/wKiom1ep3t-xAwBOAACVdp7NWkc792.png"/> env Display Environment variables Export imports local variables into environment variables Set display environment variables and local variables unset delete a defined local variable and environment variable

Variable reference

650) this.width=650; "title=" QQ picture 20160809220508.png "alt=" wkiol1ep4nmwm5saaabx6cy3bhw814.png "src="/http S2.51cto.com/wyfs02/m01/85/9b/wkiol1ep4nmwm5saaabx6cy3bhw814.png "/> The shell variable does not need to be explicitly defined, and the value of the shell variable is a string ; For example, define VALUE=32, which means Value is a string of 32, not an integer The shell variable does not need to be defined before it is used, and if a variable is not defined, the value is an empty string

File name substitution

Wildcard *: matches 0 or more arbitrary characters ？ : Matches an arbitrary character [Several characters]: matches one occurrence of any character in square brackets

Transfer characters \ & single quotes & double quotes

Transfer character \ : Used to remove the special meaning of a single character immediately after it For example: Create a $ $ file 650) this.width=650; "title=" QQ picture 20160809222129.png "alt=" wkiol1ep5k6almdlaaaz2ppqz4g299.png "src="/http S2.51cto.com/wyfs02/m02/85/9c/wkiol1ep5k6almdlaaaz2ppqz4g299.png "/> Example: Create a-file and delete-file 650) this.width=650; "title=" QQ picture 20160809222413.png "style=" Float:none; "alt=" Wkiom1ep5yht0p7uaaafwlzon74490.png " src= "Http://s1.51cto.com/wyfs02/M00/85/9C/wKiom1ep5yHT0p7uAAAfwLzOn74490.png"/> 650) this.width=650; "title=" QQ picture 20160809222442.png "style=" Float:none; "alt=" Wkiol1ep5yky0xrdaaahjbnoncm898.png " src= "Http://s4.51cto.com/wyfs02/M00/85/9C/wKioL1ep5yKy0xRdAAAhJbNONcM898.png"/> Single & double quotes: Both the delimiter of the string and not the delimiter of the character Single quotation marks are used to keep the literal value of all characters within the quotation marks, even if the \ and carriage returns in parentheses are no exception, but no single quotation marks can appear in the string, and if the quotation marks are not paired, the shell will give a continuation prompt asking the user to pair the quotes 650) this.width=650; "title=" QQ picture 20160809223937.png "style=" Float:none; "alt=" Wkiom1ep6sgsb-tzaaaqp-adezi703.png " src= "Http://s3.51cto.com/wyfs02/M01/85/9C/wKiom1ep6sGSb-TzAAAQP-adEZI703.png"/> Double quotation marks are used to keep the literal value of all characters within the quotation marks, even if carriage returns are no exception 650) this.width=650; "title=" QQ picture 20160809224021.png "style=" Float:none; "alt=" Wkiol1ep6slxjtotaaamzcsra08886.png " src= "Http://s5.51cto.com/wyfs02/M01/85/9C/wKioL1ep6sLxjTotAAAMZCSRa08886.png"/> 650) this.width=650; "title=" QQ picture 20160809225750.png "alt=" wkiol1ep7uiqjo-vaaatwqj6bbm455.png "src="/http S1.51cto.com/wyfs02/m00/85/9c/wkiol1ep7uiqjo-vaaatwqj6bbm455.png "/>

Command substitution "or $ ()

"is also a command, the shell executes the command first, and then the output is immediately substituted to the current command line For example, define a variable to hold the output of the date command: ` ` 650) this.width=650; "title=" QQ picture 20160809233503.png "alt=" wkiol1ep94qz0hdtaaan3i9i7j4037.png "src="/http S4.51cto.com/wyfs02/m01/85/9c/wkiol1ep94qz0hdtaaan3i9i7j4037.png "/> $() 650) this.width=650; "title=" QQ picture 20160809233003.png "alt=" wkiom1ep9mhso6_6aaapsgrybp0502.png "src="/http S2.51cto.com/wyfs02/m02/85/9c/wkiom1ep9mhso6_6aaapsgrybp0502.png "/> Arithmetic substitution $ () 650) this.width=650; "title=" QQ picture 20160809233835.png "alt=" wkiom1ep-h6ikykuaaam1dcxbog900.png "src="/http S2.51cto.com/wyfs02/m02/85/9c/wkiom1ep-h6ikykuaaam1dcxbog900.png "/>

Shell script--Symbol Chapter