那些 Shell、Awk 中自动隐式类型转换的“坑”

# shell 下的字典排序比较
root@localhost 10:59:23 /opt/script >
[[ (( a > 0 )) ]] && echo 1  							
1									
root@localhost 10:59:34 /opt/script >
[[ (( 00 > 0 )) ]] && echo 1  
1
# 双括号不做算术比较，而是字符串的字典比较，等同于 [[]]
[[ (( 0 == 0 )) ]] && echo 1   							
1						
root@localhost 13:04:27 /opt/script >
[[ (( 11 > 110 )) ]] && echo 1   
root@localhost 13:04:34 /opt/script >
# shell 的字符串比较不做转换，严格按照字面量字典比较
root@localhost 11:51:41 /opt/script >
[[ a == "" ]] && echo 1		
root@localhost 14:00:53 /opt/script >
[[ a > "" ]] && echo 1 
1
root@localhost 14:00:59 /opt/script >
[[ a > 0 ]] && echo 1  
1
# shell 的数字类型隐式转换：变量未定义会被转换为 0
root@localhost 11:53:56 /opt/script >
[[ a -eq 0 ]] && echo 1    
1														
root@localhost 11:54:11 /opt/script >
[[ a1 -eq 1 ]] && echo 1 								
root@localhost 11:55:11 /opt/script >
[[ 1a -eq 1 ]] && echo 1 
-bash: [[: 1a: value too great for base (error token is "1a")
# awk 隐式类型只转换 1a，不转换 a1
root@localhost 11:55:15 /opt/script >
echo|awk '{print 1a == 1 }'			
1
# awk 外置变量未定义会被置为空字符串，且空字符串不会隐式转换为数字
root@localhost 14:12:57 /opt/script >
echo|awk -va= '{print a == 0 }' 
0				
root@localhost 14:18:08 /opt/script >
echo|awk '{print "" == 0 }' 
0
root@localhost 14:13:01 /opt/script >
echo|awk -va= '{print a == "" }'
1
# awk 内置变量未定义默认会隐式转换为数字 0 或者空字符串
root@localhost 14:13:12 /opt/script >
echo|awk '{print a == 0 }'      
1
root@localhost 14:13:48 /opt/script >
echo|awk '{print a == "" }'
1
# awk 内置变量在比较时，一方加了双引号，则最终结果当做字符串比较（未定义的变量还是会自动隐式转换）
root@localhost 14:23:43 /opt/script >
echo|awk '{print 0b == "00" }' 	
0		
root@localhost 14:23:47 /opt/script >
echo|awk '{print 0b == 00 }'   
1
root@localhost 14:36:57 /opt/script >
echo|awk '{print "00" == 0 }' 	# 明确标识字符串的不再隐式转换为数字
0
# awk 中的隐式转换：无论最终结果是否以数字比较，未定义的变量都会自动隐式转换
root@localhost 14:27:49 /opt/script >
echo|awk '{print 0b == "0" }' 	
1			
echo|awk '{y=0b; print y == "0" }'     
1
echo|awk '{print 0b == 0 }'  
1
echo|awk '{y="0b"; print y == "0" }' 
0
echo|awk '{y="0b"; print +y == "0" }' 
1
Jun@VAIO 23:49:55 ~ >
echo|awk '{y="0b"; print +y == "00" }'
0
Jun@VAIO 23:49:59 ~ >
echo|awk '{y="0b"; print (+y > "00") }'
0
Jun@VAIO 23:50:14 ~ >
echo|awk '{y="0b"; print +y < "00" }'
1
root@localhost 14:41:16 /opt/script >
echo|awk '{print "00"a == "00" }' 
1
# awk 的外置变量参与比较时，与内置变量的隐式转换特性不同，如果数字中含有非数字字符串则直接被当做字符串。
# 也就是说 awk 外置变量不具有内置变量对 非数字字符隐式转换 的特性
root@localhost 16:16:27 /opt/script >
a=0b; echo|awk -vm="$a" '{print m; print m == 0; print int(m) == 0 }'	
0b
0
1
root@localhost 16:16:39 /opt/script >
a=00; echo|awk -vm="$a" '{print m; print m == 0; print int(m) == 0 }'  
00
1
1
root@localhost 16:16:59 /opt/script >
a=00; echo|awk -vm="$a" '{print m; print m == "0"; print int(m) == "0" }'
00
0
1
root@localhost 16:20:11 /opt/script >
a=00; echo|awk -vm="$a" '{print m; print m == "00"; print int(m) == 0 }'  
00
1
1
Jun@VAIO 00:33:24 ~ >
a=0b; echo|awk -vm="$a" '{b=0;print 0b; print 0b == 0; print int(m) == 0 }'
00
0
1
# 为避免上述 case 的各种歧义和不确定性，还是老老实实的用 +、int 强制转换吧~
Jun@VAIO 00:43:55 ~ >
echo|awk '{print int("1/*")}'
1
Jun@VAIO 00:47:31 ~ >
echo|awk '{print int("*/1")}'
0
Jun@VAIO 00:47:38 ~ >
echo|awk '{print +"*/1"}'
0
Jun@VAIO 00:47:47 ~ >
echo|awk '{print +"1/*"}'
1
Jun@VAIO 00:47:53 ~ >