"Pit" of automatic implicit type conversion in Shell, AWK

1. Questions:

in a variety of programming languages, the weak type of language is quite a lot, on the one hand this "dynamic type" is very convenient to use, while on the other hand, "pit" you do not discuss ~ Common SQL, Shell, AWK will encounter various hidden "implicit type conversion", the following list some Shell, Awk Automatic implicit type conversion case to prevent the pit from falling.

• Note the shell, awk variable is an empty string , the variable is null undefined , and the initial value of the implicit conversion problem:

# dictionary sort comparison under shell [email protected] 10:59:23/opt/script >[[((a > 0)]] && Echo 1 1[email prote  CTED] 10:59:34/opt/script >[[((> 0)] && Echo 1 # Double brackets do not do arithmetic comparisons, but rather a dictionary comparison of strings, equivalent to [[]][[((0 = = 0)]] && Echo 1 1[email protected] 13:04:27/opt/script >[[((>))] && echo 1 [email&n Bsp;protected] 13:04:34/opt/script ># Shell string comparison does not convert, strictly according to the literal dictionary comparison [email protected] 11:51:41/opt/script > [[A = = "]] && echo 1[email protected] 14:00:53/opt/script >[[a >" "] && Echo 1 1[EMAIL&N Bsp;protected] 14:00:59/opt/script >[[a > 0]] && Echo 1 # Shell numeric type implicit conversions: variable undefined will be converted to 0[email prote CTED] 11:53:56/opt/script >[[a-eq 0]] && Echo 1 1[email protected] 11:54:11/opt/script >[[A1- EQ 1]] && echo 1 [email protected] 11:55:11/opt/script >[[1a-eq 1]] && echo 1-bash: [[: 1a: Value too great for base (erRor token is "1a") # awk implicit type only converts 1a, does not convert a1[email protected] 11:55:15/opt/script >echo|awk ' {print 1a = 1} ' # awk An external variable undefined is set to an empty string, and an empty string is not implicitly converted to a number [email protected] 14:12:57/opt/script >echo|awk-va= ' {print a = = 0} ' 0[EMAIL&NB Sp;protected] 14:18:08/opt/script >echo|awk ' {print "" = = 0} ' 0[email protected] 14:13:01/opt/script >echo|a Wk-va= ' {print a = = '} ' # awk built-in variable undefined default is implicitly converted to the number 0 or an empty string [email protected] 14:13:12/opt/script >echo|awk ' {prin t a = = 0} ' 1[email protected] 14:13:48/opt/script >echo|awk ' {print a = = ""} ' # awk built-in variable when comparing, a side with double quotes, the end result As a string comparison (undefined variable or auto-implicit conversion) [email protected] 14:23:43/opt/script >echo|awk ' {print 0b = = ' xx '} ' 0[email  Protected] 14:23:47/opt/script >echo|awk ' {print 0b = = xx} ' 1[email protected] 14:36:57/opt/script >echo|a WK ' {print ' 00 "= = 0} ' # explicitly identifies a string that is no longer implicitly converted to the implicit conversion in the 0# awk: undefined variables are automatically converted implicitly regardless of whether the final result is a numeric comparison [email protected] 14:27:49/opt /script >echo|awk ' {print 0b= = "0"} ' 1echo|awk ' {y=0b; print y = = "0"} ' 1echo|awk ' {print 0b = = 0} ' 1echo|awk ' {y= "0b"; print y = = "0"} ' 0ech O|awk ' {y= "0b"; print +y = = "0"} ' 1[email protected] 23:49:55 ~ >echo|awk ' {y= "0b"; Print +y = "xx"} ' 0[EMAIL&NB Sp;protected] 23:49:59 ~ >echo|awk ' {y= "0b"; print (+y > "xx")} ' 0[email protected] 23:50:14 ~ >echo|awk ' {y = "0b"; Print +y < "xx"} ' 1[email protected] 14:41:16/opt/script >echo|awk ' {print "xx" a = = "xx"} ' # awk external variables participate in the comparison, Unlike the implicit conversion characteristics of built-in variables, a number that contains a non-numeric string is directly treated as a string. # that is to say awk external variables do not have the properties of built-in variables for non-digital implicitly conversions [email protected] 16:16:27/opt/script >a=0b; echo|awk-vm= "$a" ' {print m; print m = = 0; print int (m) = = 0} ' 0b01[email protected] 16:16:39/opt/script >a=00;  echo|awk-vm= "$a" ' {print m; print m = = 0; print int (m) = = 0} ' 0011[email protected] 16:16:59/opt/script >a=00; echo|awk-vm= "$a" ' {print m; print m = = "0"; print int (m) = = "0"} ' 0001[email protected] 16:20:11/opt/script >a= 00; Echo|awk-vm= "$a" ' {print m; print m = = "XX"; print int (m) = = 0} ' 0011[email protected] 00:33:24 ~ >a=0b; echo|awk-vm= "$a" ' {b=0;print 0b; print 0b = = 0; print int (m) = = 0} ' 0001# to avoid the ambiguity and uncertainty of the case, or to cast it honestly with +, int ~[ema Il protected] 00:43:55 ~ >echo|awk ' {print int ("1/*")} ' 1[email protected] 00:47:31 ~ >echo|awk ' {print Int ("*/1")} ' 0[email protected] 00:47:38 ~ >echo|awk ' {print + "*/1"} ' 0[email protected] 00:47:47 ~ > Echo|awk ' {print + ' 1/* '} ' 1[email protected] 00:47:53 ~ >

2. Conclusion: (1) The Shell's automatic implicit type conversion is rather weak, and awk is relatively fault-tolerant .
(2) in case, if the automatic implicit type conversion by Shell or awk is quite unreliable, it is extremely error-prone .
(3) to get a definite result, or an honest cast, such as awk using +, int and so on.

"Pit" of automatic implicit type conversion in Shell, AWK