Tilde Expansion ~
~ : /home or /users or /Users.
# user 'lnh'.
# /home/lnh/.bash_profile or /users/lnh/.bash_profile
echo ~lnh/.bash_profile
# current user home directory
echo ~
Brace Expansion {}
# a b c
echo {a..c}
# 01 02 03 04 05 06 07 08 09 10
echo {01..10}
# 1 3 5
echo {1..5..2}
# a c e
echo {a..f..2}
# file1.txt file2.txt file3.txt file4.txt
echo file{1..4}.txt
Shell Parameter Expansion ${}
${var:-返回默认值不设置}
# 未初始化或者为null
var=
# 输出: 返回默认值不设置
echo ${var:-返回默认值不设置}
${var:=返回默认值并设置}
# 未初始化或者为null
var=
# 输出: 返回默认值并设置
echo ${var:=返回默认值并设置}
${var:?错误提示}
# 未初始化或者为null
var=
# 终止运行并推出
# 输出: -bash: var1: 错误提示
echo ${var:?错误提示} && echo 'ok'
${var:起始索引[:可选长度]}
支持负数索引，表示从后往前（但是注意索引前的空格不要少）。
# 字符串变量
str='01234567890abcdefgh'
# 输出: 7890abcdefgh
echo ${str:7}
# 输出: 7
echo ${str:7:1}
# 输出: 78
echo ${str:7:2}
# 输出: 7890abcdef
echo ${str:7:-2}
# 输出: bcdefgh
echo ${str: -7}
# 输出: bc
echo ${str: -7:2}
# 输出: bcdef
echo ${str: -7:-2}
当变量是数组时，长度不能为负数。
# 数组变量
arr=(0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h)
# 输出: 7 8 9 0 a b c d e f g h
echo ${arr[@]:7}
# 输出: 7
echo ${arr[@]:7:1}
# 输出: 7 8
echo ${arr[@]:7:2}
# 输出: -bash: -2: substring expression < 0
echo ${arr[@]:7:-2}
# 输出: b c d e f g h
echo ${arr[@]: -7}
# 输出: b c
echo ${arr[@]: -7:2}
${#var}
获取变量的长度
# 字符串变量
str='01234567890abcdefghk'
# 输出: 20
echo ${#str}
# 数组变量
arr=(0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h)
# 输出: 19
echo ${#arr[@]}
${var^^}
转换为大写
# 字符串变量
str='01234567890abcdefghk'
# 输出: 01234567890ABCDEFGHK
echo ${str^^}
${var,,}
转换为小
# 字符串变量
str='01234567890ABCDEFGHK'
# 输出: 01234567890abcdefghk
echo ${str,,}