使用命令列擴充套件

Bash shell有多種擴充套件命令列的方式，包括模式匹配、主目錄擴充套件、字串擴充套件和變數替換。

元字元和匹配項表

示例：

簡單模式匹配：

通過星號和問號字元以及一類字元來匹配其中一些檔名

[jumpserver@192 bin]$ ls a*
abrt-action-analyze-backtrace             abrt-dump-xorg                   ar
abrt-action-analyze-c                     abrt-handle-upload               arch
abrt
 
-action-analyze-ccpp-local            abrt-merge-pstoreoops            arecord
[jumpserver@192 bin]$ ls *a*
abrt-action-analyze-backtrace             nl-cls-add
abrt-action-analyze-c                     nl-link-enslave
abrt-action-analyze-ccpp-local            nl-link-ifindex2name
abrt-action-analyze-core                  nl
 
-link-name2ifindex
jumpserver@192 bin]$ ls [ac]*
abrt-action-analyze-backtrace             analog             catman              clear
abrt-action-analyze-c                     aplay              cc                  clevis
abrt-action-analyze-ccpp-local            aplaymidi          cd                  clevis-decrypt
abrt-action-analyze-core                  appstream-compose  cd-convert          clevis-decrypt-sss
[jumpserver@192 bin]$ ls ???
atq  c99  col  dig  env  ftp  gjs  jjs  lua  mdu  ptx  red  scp  sum  tee  top  who  zip
[jumpserver@192 bin]$ ls ?????
alias  chacl  crash  fgrep  guild  lscpu  mmcli  nproc  pchrt  pslog  sg_dd  tload  vlock  xmlwf
amidi  chage  dconf  flock  guile  lsiio  mmove  nroff  pgrep  quota  shred  totem  watch  xzcat

波形符擴充套件：

波形符（~）可匹配當前使用者的主目錄

[jumpserver@192 /]$ ls ~jumpserver
Desktop  Documents  Downloads  Music  Pictures  Public  Templates  Videos

大括號擴充套件：

1）大括號擴充套件用於生成任意字串

2）大括號包含字串的逗號分隔列表或順序表示式

3）雙句點語法（..）可擴充套件成一個序列，使得{m..p}擴充套件成m n o p

[jumpserver@192 test]$ echo {Sunday,Monday,Tuesday,Wednesday}.log
Sunday.log Monday.log Tuesday.log Wednesday.log
[jumpserver@192 test]$ echo file{1..3}.txt
file1.txt file2.txt file3.txt
[jumpserver@192 test]$ echo file{a..e}.txt
filea.txt fileb.txt filec.txt filed.txt filee.txt
[jumpserver@192 test]$ echo file{a,b}{1,2}.txt
filea1.txt filea2.txt fileb1.txt fileb2.txt
[jumpserver@192 test]$ echo file{a{1,2},b,c}.txt
filea1.txt filea2.txt fileb.txt filec.txt
[jumpserver@192 test]$ mkdir rhel{6,7,8}
[jumpserver@192 test]$ ls R*
ls: cannot access 'R*': No such file or directory
[jumpserver@192 test]$ ls r*
rhel6:
rhel7:
rhel8:

變數擴充套件：

變數的作用類似於可以在記憶體中儲存值的命名容器

通過變數，可以從命令列或在shell指令碼內輕鬆訪問和修改儲存的資料

[jumpserver@192 test]$ USERNAME=operator
[jumpserver@192 test]$ echo $USERNAME
operator
為了避免因其他shell擴充套件而引起的錯誤，可將變數的名稱放在大括號中
[jumpserver@192 test]$ echo ${USERNAME}
operator

命令替換：

1）命令替換允許命令的輸出替換命令列上的命令本身

2）當命令括在括號中並且前面有美元符號（$）時，會發生命令替換

[jumpserver@192 test]$ echo Today is $(date)
Today is Sun Nov 15 23:24:08 PST 2020

防止引數被擴充套件：

反斜槓（\）是Bash shell中的轉義字元。它可以防止緊隨其後的字元被擴充套件

[jumpserver@192 /]$ echo the value is $HOME is your home directory
the value is /home/jumpserver is your home directory
[jumpserver@192 /]$ echo the value is \$HOME is your home directory
the value is $HOME is your home directory

如果需要保護較長的字串，則使用單引號（''）或雙引號（""）來括起字串

單引號：阻止通配和shell擴充套件

雙引號：阻止大部分shell擴充套件

使用雙引號可以阻止通配和shell擴充套件，但依然允許命令和變數替換
[jumpserver@192 /]$ 
echo "---------hostname is ${myhost}------------"
---------hostname is 192------------
[jumpserver@192 /]$ echo "will varialbe $myhost evaluate to $(hostname -s)"
will varialbe 192 evaluate to 192
[jumpserver@192 /]$ echo 'will varialbe $myhost evaluate to $(hostname -s)'
will varialbe $myhost evaluate to $(hostname -s)