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action:Execute_a_Shell_Script [2019/05/03 13:13]
JMichaelTX ADD example of Bash quoting
action:Execute_a_Shell_Script [2019/05/03 23:54] (current)
peternlewis [Quoting Strings]
Line 111: Line 111:
 ===== Quoting Strings ===== ===== Quoting Strings =====
  
-Proper quoting of strings in a Shell Script is often a challenge.+Proper quoting of strings in a Shell Script is often a challenge.  In order to understand it, you have to understand what happens at each level of the processing.
  
-For discussion on how to quote a string, ​see this [[https://forum.keyboardmaestro.com/t/having-trouble-with-quotes-and-shell-variables/6236/9 | Keyboard Maestro Forum post by @PeterNLewis]].+Thankfully, in an Execute Script action, Keyboard Maestro itself does not do any processing, so that is one less place for confusion. 
 + 
 +With shell commands, the important thing to understand is that, with only few exceptional programs, it is not the command that does the processing, it is the shell (bash usually, although there are many shells). 
 + 
 +So for example with Keyboard Maestro variable set to `"​Selection Style" -int 0` and en Execute Shell Script command of: 
 + 
 +```bash 
 +defaults write /​Users/​me/​Library/​Application\ Support/​Witch/​Settings $KMVAR_witchPref 
 +``` 
 + 
 +It is the *bash* tool that sees the string and it processes it in to an array of strings which it passes to the *defaults* tool.  This is important - the defaults command receives not single ​string, ​but an array of strings, in this case the array will be: 
 + 
 +* `defaults` 
 +* `write` 
 +*  `/Users/me/Library/Application Support/​Witch/​Settings` 
 +* `"​Selection` 
 +* `Style"​` 
 +* `-int` 
 +* `0` 
 + 
 +It is bash that has processed the variable substitution for $KMVAR_witchPref,​ split the line in to seven parts, processed the backslash in "​Application\ Support"​ and then executed the defaults tool with the seven parts as arguments (the command itself is the 0th argument, normally mostly ignored by tools). 
 + 
 +So the problem is that the word-breaking, variable substitution,​ de-quoting ​and de-backslash is all happening at as the line is processed by bash, after which you are left with quotes from within variable substitution,​ but they no longer have any meaning, they are just characters. 
 + 
 +So variables ​that are meant to be a single parameter should be surrounded by double quotes in the line, but not contain quotes in the variable, and generally you do not want to pass multiple parameters within a single parameter. ​ So instead, perhaps set one variable to the setting name, and one variable to the type and one variable to the value, and then use a command like this: 
 + 
 +```bash 
 +defaults write "/Users/me/​Library/​Application Support/​Witch/​Settings"​ "​$KMVAR_Setting"​ -"​$KMVAR_Type"​ "​$KMVAR_Value"​ 
 +``` 
 + 
 +Now a further complication happens when you want to sent a string containing double quotes and variables to a parameter For example, perhaps you want a command like this: 
 + 
 +```bash 
 +emacsclient -e '​(w3m-browse-url "​$KMVAR_SafariURL"​)'​ 
 +``` 
 + 
 +This will not work because bash only expands variables within double-quoted strings. ​ Within single quoted strings, they remain as the text `$KMVAR_SafariURL` (unchanged). ​ And so emactsclient will receive the array: 
 + 
 +* `emacsclient` 
 +* `-e` 
 +* `(w3m-browse-url "​$KMVAR_SafariURL"​)` 
 + 
 +But it will not expand the variable (that is the shell’s job). 
 + 
 +To work around this you need to use either all double quotes (and backslash any double quotes within them) such as: 
 + 
 +```bash 
 +emacsclient -e "​(w3m-browse-url \"​$KMVAR_SafariURL\"​)"​ 
 +``` 
 + 
 +or use a combination of strings, some double quoted and some single quoted: 
 + 
 +```bash 
 +emacsclient -e '​(w3m-browse-url "'"​$KMVAR_SafariURL"'"​)'​ 
 +``` 
 + 
 +Note that that parameter is now made up of three stings combined into one parameter:  
 + 
 +* `(w3m-browse-url "` - single quoted string 
 +* the value of $KMVAR_SafariURL - double quoted string 
 +* `")` - single quoted string
  
  
action/Execute_a_Shell_Script.txt · Last modified: 2019/05/03 23:54 by peternlewis