概述

? 學習自定義?Linux?bash?shell?環境來滿足用戶需求。學習：

·?修改全局和用戶配置文件

·?在登錄或生成新?shell?時設置環境變量

·?為常用命令序列創建?bash?函數

·?為新用戶帳戶維護框架目錄

·?設置命令搜索路徑

Linux?shell

? ? ? 在終端上使用一個?Linux?shell?程序，通過鍵入命令（輸入流）來與系統交互，并在同一個終端上查看輸出（輸出流）和錯誤消息（錯誤流）。有時，您需要在系統引導以前運行命令，以便允許終端建立連接，有時您需要定期運行命令，無論您是否登錄。shell?也可以為您完成這些任務。標準輸入和輸出流不需要來自或定向到終端上的真實用戶。在本教程中，將進一步了解?shell?和自定義用戶的環境。具體地講，您將學習?bash?(Bourne?again)?shell（原始?original?Bourne?shell?的增強版），并了解使?bash?更符合可移植操作系統接口?(Portable?Operating?System?Interface,?POSIX)?標準的更改。我會在此過程中介紹其他一些?shell?特性。

Shell?和環境

? ? shell 與操作系統提供了一個層。借助?Linux（和?UNIX）shell，組合基本函數來構建復雜的操作。然后可以使用編程結構，構建能在?shell?中直接執行的函數或將函數保存為?shell?腳本。shell?腳本是一個命令序列，它存儲在?shell 運行的文件中。

表?1.?常見?bash?環境變量

名稱	用途
USER	登錄用戶的名稱
UID	登錄用戶的用戶?ID?數字
HOME	用戶的主目錄
PWD	當前工作目錄
SHELL	shell?的名稱
PPID	父進程的?PID?—啟動此進程的進程的進程?ID

shell?還會設置一些特殊參數，但這些參數無法修改。

表?2.?常見?bash?參數

名稱	用途
$	（運行的?bash?shell?[?或其他?]?進程）的進程?ID（或?PID）
?	上一個命令的退出代碼
0	shell?或?shell?腳本的名稱

使用變量

在名稱前面加上?$作為前綴來使用變量的值，如1所示。

1.?使用變量值

?[ian@atticf22?~]$?echo?$UID?1000?

?[ian@atticf22?~]$?echo?$HOME?/home/ian

設置變量值并讓它們可用

? ? 通過鍵入一個名稱后不留空格，立即鍵入等號?(=)，在?bash?shell?中創建或設置?shell?變量。變量名是僅由字母數字字符和下劃線組成的單詞。名稱以字母字符或下劃線開頭。變量名是區分大小寫的，所以?var1和?VAR1是不同的變量。變量名（特別是導出變量）通常采用大寫形式，就像?表?1中的示例一樣，但這是一種約定，而不是一種要求。一些變量（比如?$$和?$?）實際上是?shell?參數而不是變量?—只能引用它們，不能向它們賦值。

Shell?變量僅對您創建它們時所在的進程可見，除非?導出它們，以便子進程可以看到和使用它們。子進程不能將變量導出到父進程。可以使用export命令導出變量。在?bash?shell?中，可以一步完成分配和導出，但不是所有?shell?都支持這項功能。

? ? 探索這些概念的一種好方法是使用另一個?shell?來創建子進程?？梢允褂?ps命令幫助跟蹤您所在的位置和正在運行的進程。

2.?設置?shell?變量并將它們導出到環境

ian@atticf22?~]$?#?Use?the?ps?command?to?list?current?PID,?parent?PID?and?running?command?name

?[ian@atticf22?~]$?ps?-p?$$?-o?"pid?ppid?cmd"

??PID??PPID?CMD?

?12761??9169?bash?

?[ian@atticf22?~]$?#?Start?a?child?bash?process

?[ian@atticf22?~]$?bash

?[ian@atticf22?~]$?#?Assign?two?variables

?[ian@atticf22?~]$?VAR1=v1?

?[ian@atticf22?~]$?VAR2="Variable?2"

?[ian@atticf22?~]$?#?Export?examples

?[ian@atticf22?~]$?export?VAR2?

?[ian@atticf22?~]$?export?VAR3="Assigned?and?exported?in?one?step"

?[ian@atticf22?~]$?#?Use?the?$?character?to?reference?the?variables?$?export?VAR4=var4

?[ian@atticf22?~]$?echo?$VAR1?'/'?$VAR2?'/'?$VAR3

?v1?/?Variable?2?/?Assigned?and?exported?in?one?step?

?[ian@atticf22?~]$?#?What?is?the?value?of?the?SHELL?variable?

?[ian@atticf22?~]$?echo?$SHELL

?/bin/bash?

?[ian@atticf22?~]$?#?Now?start?ksh?child?and?export?VAR4

?[ian@atticf22?~]$?ksh

?$?ps?-p?$$?-o?"pid?ppid?cmd"

??PID??PPID?CMD?

?26212?22923?ksh?

?$?export?VAR4=var4

?$?#?See?what?is?visible

?$?echo?$VAR1?'/'?$VAR2?'/'?$VAR3?'/'?$VAR4?'/'?$SHELL

?/?Variable?2?/?Assigned?and?exported?in?one?step?/?var4?/?/bin/bash?

?$#?No?VAR1?and?shell?is?/bin/bash?-?is?that?right?

?$?exit?

?[ian@atticf22?~]$?ps?-p?$$?-o?"pid?ppid?cmd"

??PID??PPID?CMD?

?22923?12761?bash?

?[ian@atticf22?~]$?#?See?what?is?visible

?[ian@atticf22?~]$?echo?$VAR1?'/'?$VAR2?'/'?$VAR3?'/'?$VAR4?'/'?$SHELL

?v1?/?Variable?2?/?Assigned?and?exported?in?one?step?/?/?/bin/bash?

?[ian@atticf22?~]$?#?No?VAR4?-?our?child?cannot?export?back?to?us

?[ian@atticf22?~]$?exit

?exit?

?[ian@atticf22?~]$?ps?-p?$$?-o?"pid?ppid?cmd"

??PID??PPID?CMD?

?12761??9169?bash?

?[ian@atticf22?~]$?#?See?what?is?visible

?[ian@atticf22?~]$?echo?$VAR1?'/'?$VAR2?'/'?$VAR3?'/'?$VAR4?'/'?$SHELL

?/?/?/?/?/bin/bash?

?[ian@atticf22 ~]$ # None of VAR1 through VAR4 is exported back to parent

? ? 2中否注意到，ksh?沒有設置?SHELL變量？但在您登錄時，或者使用?su命令和創建?登錄?shell的選項來切換到另一個用戶時，通常會設置此變量。

可使用?echo命令查看?表?1和?表?2中的一些常見?bash?變量

3.?常見環境變量和?shell?變量

[ian@atticf22?~]$?echo?$USER?$UID

?ian?1000?

?[ian@atticf22?~]$?echo?$SHELL?$HOME?$PWD

?/bin/bash?/home/ian?/home/ian?

?[ian@atticf22?~]$?(exit?0);echo?$?;(exit?4);echo?$?

?0?

?4?

?[ian@atticf22?~]$?echo?$0

?bash?

?[ian@atticf22?~]$?echo?$$?$PPID

?12761?9169?

?[ian@atticf22?~]$?#?see?what?my?process?and?its?parent?are?running

?[ian@atticf22?~]$?ps?-p?$$,$PPID?-o?"pid?ppid?cmd"

??PID??PPID?CMD?

?9169?1?/usr/libexec/gnome-terminal-server?

?12761 ?9169 bash

? ? 在?bash?shell?中，還可以通過在命令前加上名稱?=?值對作為前綴，將環境值設置為在單個命令的持續時間內有效

4.?為單個命令設置?bash?環境值?

?[ian@atticf22?~]$?echo?"$VAR5?/?$VAR6"

?/?

?[ian@atticf22?~]$?VAR5=5?VAR6="some?value"?bash

?[ian@atticf22?~]$?echo?"$VAR5?/?$VAR6"

?5?/?some?value?

?[ian@atticf22?~]$?exit

?[ian@atticf22?~]$?echo?"$VAR5?/?$VAR6"

readonly和其他變量屬性 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ??上一頁? ? ? ?回頁首??

一些?shell?參數無法修改。還可以將變量限制為?readonly、integer或?string。可以使用?declare命令設置變量屬性。使用?-p選項顯示變量和各種屬性。了解?declare命令：

?info?bash?"Shell?Builtin?Commands"?"Bash?Builtins"??--index-search?declare?

或

?help?declare

5.?變量屬性

?[ian@atticf22?~]$?declare?-r?rov1="this?is?readonly"

?[ian@atticf22?~]$?rov="Who?says?it's?read?only?"

?[ian@atticf22?~]$?readonly?rov2="another?constant?value"

?[ian@atticf22?~]$?rov2=3

?bash:?rov2:?readonly?variable?

?[ian@atticf22?~]$?UID=99

?bash:?UID:?readonly?variable?

?[ian@atticf22?~]$?declare?-pr

?declare?-r?BASHOPTS="checkwinsize:cmdhist:complete_fullquote:expand_aliases:extglob:extquote:?

?force_fignore:histappend:interactive_comments:progcomp:promptvars:sourcepath"

?declare?-ir?BASHPID?

?declare?-r?BASH_COMPLETION_COMPAT_DIR="/etc/bash_completion.d"

?declare?-ar?BASH_VERSINFO='([0]="4"?[1]="3"?[2]="42"?[3]="1"?[4]="release"?[5]=?

"x86_64-redhat-linux-gnu")'

?declare?-ir?EUID="1000"

?declare?-ir?PPID="12761"

?declare?-r?SHELLOPTS="braceexpand:emacs:hashall:histexpand:history:interactive-comments:monitor"

?declare?-ir?UID="1000"

?declare?-r?rov1="this?is?readonly"

?declare?-r?rov2="another?constant?value"

?[ian@atticf22?~]$?help?declare

?declare:?declare?[-aAfFgilnrtux]?[-p]?[name[=value]?...]?

????Set?variable?values?and?attributes.?

? ??

????Declare?variables?and?give?them?attributes.??If?no?NAMEs?are?given,?

????display?the?attributes?and?values?of?all?variables.?

? ??

????Options:?

??????-f???restrict?action?or?display?to?function?names?and?definitions?

??????-F???restrict?display?to?function?names?only?(plus?line?number?and?

???????source?file?when?debugging)?

??????-g???create?global?variables?when?used?in?a?shell?function;?otherwise?

???????ignored?

??????-p???display?the?attributes?and?value?of?each?NAME?

? ??

????Options?which?set?attributes:?

??????-a???to?make?NAMEs?indexed?arrays?(if?supported)?

??????-A???to?make?NAMEs?associative?arrays?(if?supported)?

??????-i???to?make?NAMEs?have?the?`integer'?attribute?

??????-l???to?convert?NAMEs?to?lower?case?on?assignment?

??????-n???make?NAME?a?reference?to?the?variable?named?by?its?value?

??????-r???to?make?NAMEs?readonly?

??????-t???to?make?NAMEs?have?the?`trace'?attribute?

??????-u???to?convert?NAMEs?to?upper?case?on?assignment?

??????-x???to?make?NAMEs?export?

? ??

????Using?`+'?instead?of?`-'?turns?off?the?given?attribute.?

? ??

????Variables?with?the?integer?attribute?have?arithmetic?evaluation?(see?

????the?`let'?command)?performed?when?the?variable?is?assigned?a?value.?

? ??

????When?used?in?a?function,?`declare'?makes?NAMEs?local,?as?with?the?`local'

????command.??The?`-g'?option?suppresses?this?behavior.?

? ??

????Exit?Status:?

????Returns?success?unless?an?invalid?option?is?supplied?or?a?variable?

取消設置變量 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ??上一頁? ? ? ?回頁首

使用?unset命令從?bash?shell?刪除一個變量。使用?-v選項確保刪除了變量定義。函數可擁有與變量相同的名稱，刪除一個函數定義，可使用?-f。如果沒有?-f或?-v。

6.?取消設置?bash?變量

?ian@atticf22?~]$?bash

?[ian@atticf22?~]$?VAR1=v1

?[ian@atticf22?~]$?declare?-i?VAR2

?[ian@atticf22?~]$?VAR2=3+4

?[ian@atticf22?~]$?echo?$VAR1?$VAR2

?v1?7?

?[ian@atticf22?~]$?unset?VAR1

?[ian@atticf22?~]$?echo?$VAR1?$VAR2

?7?

?[ian@atticf22?~]$?unset?-v?VAR2

?[ian@atticf22?~]$?echo?$VAR1?$VAR2

?[ian@atticf22?~]$?exit

?exit

注意，變量被定義為?integer，向它賦的值會計算為算術表達式。

默認情況下，bash?將未設置的變量視為擁有空值。為什么取消設置一個變量，而不是向它分配一個空值？在?bash?和其他許多?shell?中，如果應用未定義的變量，可能會產生一個錯誤。可使用?set?-u命令生成未定義變量的錯誤，使用?set?+u禁用該警告。

7.?錯誤和取消設置變量

?[ian@atticf22?~]$?bash

?[ian@atticf22?~]$?set?-u

?[ian@atticf22?~]$?VAR1=var1

?[ian@atticf22?~]$?echo?$VAR1

?var1?

?[ian@atticf22?~]$?unset?VAR1

?[ian@atticf22?~]$?echo?$VAR1

?bash:?VAR1:?unbound?variable?

?[ian@atticf22?~]$?VAR1=

?[ian@atticf22?~]$?echo?$VAR1

?[ian@atticf22?~]$?unset?VAR1

?[ian@atticf22?~]$?echo?$VAR1

?bash:?VAR1:?unbound?variable?

?[ian@atticf22?~]$?unset?-v?VAR1

?[ian@atticf22?~]$?set?+u

?[ian@atticf22?~]$?echo?$VAR1

?[ian@atticf22?~]$?exit

?exit

取消設置一個不存在的變量不會出錯，甚至在指定了?set?-u時也是如此。

上一頁? ? ? ?回頁首

轉載于:https://blog.51cto.com/risingair/1868669

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