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commands:builtin:let [2012/11/03 08:34]
techlivezheng [Examples]
commands:builtin:let [2015/11/16 04:19] (current)
thebonsai [The let builtin]
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-====== The let builtin ======+====== The let builtin ​command ​======
  
 ===== Synopsis ===== ===== Synopsis =====
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 ===== Portability considerations ===== ===== Portability considerations =====
  
-  * the ''​let''​ command is not specified by POSIX(r). The portable alternative is: <​code>​[ "$(( <​EXPRESSION>​ ))" -ne 0 ]</​code>​ +  * the ''​let''​ command is not specified by POSIX(r). The portable alternative is: <​code>​[ "$(( <​EXPRESSION>​ ))" -ne 0 ]</code>. To make portable scripts simpler and cleaner, ''​let''​ can be defined as: <code> 
 +# POSIX 
 +let() { 
 +    IFS=, command eval test '​$(($*))'​ -ne 0 
 +
 +</​code>​ Aside from differences in supported arithmetic features, this should be identical to the Bash/Ksh ''​let''​. 
 +  * It seems to be a common misunderstanding that ''​let''​ has some legacy purpose. Both ''​let''​ and [[syntax/​ccmd/​arithmetic_eval | ((...))]] were ksh88 features and almost identical in terms of portability as everything that inherited one also tended to get the other. Don't choose ''​let''​ over ''​((''​ expecting it to work in more places.
   * [[http://​pubs.opengroup.org/​onlinepubs/​9699919799/​utilities/​expr.html#​tag_20_42 | expr(1)]] is a command one is likely to come across sooner or later. While it is more "​standard"​ than ''​let'',​ the above should always be preferred. Both [[syntax:​arith_expr | arithmetic expansion]]s and the ''​[''​ test operator are specified by POSIX(r) and satisfy almost all of expr's use-cases. Unlike ''​let'',​ ''​expr''​ cannot assign directly to bash variables but instead returns a result on stdout. ''​expr''​ takes each operator it recognizes as a separate word and then concatenates them into a single expression that's evaluated according to it's own rules (which differ from shell arithmetic). ''​let''​ parses each word it recieves on its own and evaluates it as an expression without generating any output other than a return code.   * [[http://​pubs.opengroup.org/​onlinepubs/​9699919799/​utilities/​expr.html#​tag_20_42 | expr(1)]] is a command one is likely to come across sooner or later. While it is more "​standard"​ than ''​let'',​ the above should always be preferred. Both [[syntax:​arith_expr | arithmetic expansion]]s and the ''​[''​ test operator are specified by POSIX(r) and satisfy almost all of expr's use-cases. Unlike ''​let'',​ ''​expr''​ cannot assign directly to bash variables but instead returns a result on stdout. ''​expr''​ takes each operator it recognizes as a separate word and then concatenates them into a single expression that's evaluated according to it's own rules (which differ from shell arithmetic). ''​let''​ parses each word it recieves on its own and evaluates it as an expression without generating any output other than a return code.
- 
   * For unknown reasons, ''​let''​ is one of the Bash commands with special parsing for arguments formatted like compound array assignments. See: [[commands/​builtin/​eval#​portability_considerations | eval]] for details. There are no known practical uses for this. Parentheses are treated as grouping operators and compound assignment is not possible by arithmetic expressions.   * For unknown reasons, ''​let''​ is one of the Bash commands with special parsing for arguments formatted like compound array assignments. See: [[commands/​builtin/​eval#​portability_considerations | eval]] for details. There are no known practical uses for this. Parentheses are treated as grouping operators and compound assignment is not possible by arithmetic expressions.