Linux shell number variables add All In One

Linux shell number variables add All In One

bash / zsh

error ❌

# string number / 字符串数字
$ var1=1
$ var2=2

$ echo $var1
# 1
$ echo $var2
# 2

$ echo $var1$var2
# 12
$ echo $var1 + $var2
# 1 + 2

$ echo ${var1 + var2}
# zsh: bad substitution ❌
$ echo ${var1} + ${var2}
# 1 + 2

solutions ✅

1. declare -i

$ var1=1
$ echo $var1
# 1
$ var1+=1
$ echo $var1
# 11

# 声明为 integer 整数类型 ✅ `declare -i`
$ declare -i num1
$ num1=1
$ echo $num1
# 1
$ num1+=1
$ echo $num1
# 2

2. Use $(()) for bash arithmetic expansion.

使用 $(()) 进行 bash 算术扩展。

$ var1=1 
$ var2=2

$ echo $var1 + $var2
# 1 + 2
$ echo $(($var1 + $var2))
# 3

https://infoheap.com/bash-add-number-variable/

3. ??? 逆波兰表达式

4. let

# let

https://stackoverflow.com/questions/6348902/how-can-i-add-numbers-in-a-bash-script

https://linux.die.net/man/1/dc

https://www.computerhope.com/unix/bash/let.htm

Linux bash ${} vs $()

\((command) is “command substitution”. \)(command) 是“命令替换”。

\({parameter} is “parameter substitution”. \){parameter} 是“参数替换”。

demos

$ animal=cat
$ cat=kitty

$ echo $animal
# cat
$ echo $cat
# kitty

# $ echo ${!animal}
# kitty ❌

#  echo ${!animal} => echo ${animal=cat} ❓
$ echo ${!animal}
$ echo ${animal=cat}
# cat

$ animal=cat

# String length
$ echo ${#animal}
# 3
# Substitution, 字符串替换：c`at`/at/`ow` => cow 
$ echo ${animal/at/ow}
# cow

$ echo ${animal/a/x}  
# cxt
$ echo ${animal/t/y}
# cay

https://superuser.com/questions/935374/difference-between-and-in-a-shell-script

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refs

https://github.com/xgqfrms/linux/issues/25

https://superuser.com/questions/935374/difference-between-and-in-a-shell-script

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