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scripting:basics [2015/08/01 20:19]
bill_thomson
scripting:basics [2015/08/02 04:31] (current)
bill_thomson
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 This is interpreted by the kernel ((under specific circumstances,​ also by the shell itself)) of your system. In general, if a file is executable, but not an executable (binary) program, and such a line is present, the program specified after ''#​!''​ is started with the scriptname and all its arguments. These two characters ''#''​ and ''​!''​ must be **the first two bytes** in the file! This is interpreted by the kernel ((under specific circumstances,​ also by the shell itself)) of your system. In general, if a file is executable, but not an executable (binary) program, and such a line is present, the program specified after ''#​!''​ is started with the scriptname and all its arguments. These two characters ''#''​ and ''​!''​ must be **the first two bytes** in the file!
  
-You can follow ​it by using the ''​echo'' ​program ​as fake-interpreter:​+You can follow ​the process ​by using ''​echo''​ as fake interpreter:​
 <​code>​ <​code>​
 #!/bin/echo #!/bin/echo
 </​code>​ </​code>​
-We don't need a script-body here, as the file will never be interpreted and executed by "''​echo''"​, but you can see what the system ​does, it calls "''/​bin/​echo''"​ with the name of the executable file and all that follows.+We don't need a script body here, as the file will never be interpreted and executed by "''​echo''"​. You can see what the Operating System ​does, it calls "''/​bin/​echo''"​ with the name of the executable file and following arguments.
 <​code>​ <​code>​
 $ /​home/​bash/​bin/​test testword hello $ /​home/​bash/​bin/​test testword hello
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 </​code>​ </​code>​
  
-The same way, with ''#​!/​bin/​bash''​ the shell "''/​bin/​bash''"​ is called with the script-file as an argument. It'​s ​exactly ​the same as executing "''/​bin/​bash /​home/​bash/​bin/​test testword hello''"​+The same way, with ''#​!/​bin/​bash''​ the shell "''/​bin/​bash''"​ is called with the script ​filename ​as an argument. It's the same as executing "''/​bin/​bash /​home/​bash/​bin/​test testword hello''"​
  
 If the interpreter can be specified with arguments and how long it can be is system-specific (see [[http://​www.in-ulm.de/​~mascheck/​various/​shebang/​|#​!-magic]]). If the interpreter can be specified with arguments and how long it can be is system-specific (see [[http://​www.in-ulm.de/​~mascheck/​various/​shebang/​|#​!-magic]]).
-When Bash executes a file with a #!/bin/bash-shebang, the shebang itself is ignored, since first character is a hashmark "#",​ which indicates a comment. The shebang is for the operating system, not for the shell. Programs that don't ignore such lines, may not work as shebang-driven interpreters.+When Bash executes a file with a #!/bin/bash shebang, the shebang itself is ignored, since the first character is a hashmark "#",​ which indicates a comment. The shebang is for the operating system, not for the shell. Programs that don't ignore such lines, may not work as shebang driven interpreters.
  
 <WRAP center round important 60%> <WRAP center round important 60%>
-__**Attention:​**__When the specified interpreter is unavailable or not executable (permissions),​ you usually get a "''​bad interpreter''"​ error message., If you get nothing and it fails, check the shebang. Older Bash versions will respond with a "''​no such file or directory''" ​erro for a nonexistant interpreter specified by the shebang.+__**Attention:​**__When the specified interpreter is unavailable or not executable (permissions),​ you usually get a "''​bad interpreter''"​ error message., If you get nothing and it fails, check the shebang. Older Bash versions will respond with a "''​no such file or directory''" ​error for a nonexistant interpreter specified by the shebang.
 </​WRAP>​ </​WRAP>​
  
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   * the needed ''​bash''​ binary must be located in ''​PATH''​   * the needed ''​bash''​ binary must be located in ''​PATH''​
  
-Which one you need, or whether you think which is good, or bad, is up to you. There is no bulletproof portable way to specify an interpreter. **It's a common misconception that it solves all problems. Period.**+Which one you need, or whether you think which one is good, or bad, is up to you. There is no bulletproof portable way to specify an interpreter. **It's a common misconception that it solves all problems. Period.**
  
 ===== The standard filedescriptors ===== ===== The standard filedescriptors =====
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 Usually, they'​re all connected to your terminal, stdin as input file (keyboard), stdout and stderr as output files (screen). When calling such a program, the invoking shell can change these filedescriptor connections away from the terminal to any other file (see redirection). Usually, they'​re all connected to your terminal, stdin as input file (keyboard), stdout and stderr as output files (screen). When calling such a program, the invoking shell can change these filedescriptor connections away from the terminal to any other file (see redirection).
-Why two different output filedescriptors?​ It's convention to send error messages and warnings to stderr and only program output to stdout. This enables the user (you!) ​to decide if they want to see nothing, only the data, only the errors, or both - and where you want to see them.+Why two different output filedescriptors?​ It's convention to send error messages and warnings to stderr and only program output to stdout. This enables the user to decide if they want to see nothing, only the data, only the errors, or both - and where they want to see them.
  
-You should follow some rules when you write a script:+When you write a script:
   * always read user-input from ''​stdin''​   * always read user-input from ''​stdin''​
   * always write diagnostic/​error/​warning messages to ''​stderr''​   * always write diagnostic/​error/​warning messages to ''​stderr''​
  
-To learn more about the standard filedescriptors,​ especially about redirecting ​and piping ​them, see:+To learn more about the standard filedescriptors,​ especially about redirection ​and piping, see:
   * [[howto:​redirection_tutorial | An illustrated redirection tutorial]]   * [[howto:​redirection_tutorial | An illustrated redirection tutorial]]
  
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 |''​PS2''​|''​PS4''​|''​PS3''​|''​PWD''​|''​SHELL''​|''​SHELLOPTS''​| |''​PS2''​|''​PS4''​|''​PS3''​|''​PWD''​|''​SHELL''​|''​SHELLOPTS''​|
 |''​SHLVL''​|''​TERM''​|''​UID''​|''​USER''​|''​USERNAME''​|''​XAUTHORITY''​| |''​SHLVL''​|''​TERM''​|''​UID''​|''​USER''​|''​USERNAME''​|''​XAUTHORITY''​|
-This list is incomplete. **The safest way is to use only lowercase variable names.**+This list is incomplete. **The safest way is to use all-lowercase variable names.**
  
  
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 ===== Exit codes ===== ===== Exit codes =====
-Every program you start terminates with a so-called ​exit code and reports it to the operating system. This exit code can be utilized by Bash. You can show it, you can react on it, you can control ​the script'​s ​flow with it. +Every program you start terminates with an exit code and reports it to the operating system. This exit code can be utilized by Bash. You can show it, you can act on it, you can control script flow with it. 
-The code is a number between 0 and 255, where the part from 126 to 255 is reserved ​to be used by the shell directly or for special purposes, like reporting a termination by a signal: +The code is a number between 0 and 255. Values ​from 126 to 255 are reserved ​for use by the shell directlyor for special purposes, like reporting a termination by a signal: 
-  * **126**: the requested command (file) can't be executed ​(but was found)+  * **126**: the requested command (file) ​was found, but can't be executed
   * **127**: command (file) not found   * **127**: command (file) not found
   * **128**: according to ABS it's used to report an invalid argument to the exit builtin, but I wasn't able to verify that in the source code of Bash (see code 255)   * **128**: according to ABS it's used to report an invalid argument to the exit builtin, but I wasn't able to verify that in the source code of Bash (see code 255)
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 The lower codes 0 to 125 are not reserved and may be used for whatever the program likes to report. The lower codes 0 to 125 are not reserved and may be used for whatever the program likes to report.
-A value of **0 means successful** termination,​ a value **not 0 means unsuccessful** termination. This behavior (== 0, != 0) is also what Bash reacts ​on in some code flow control statements.+A value of 0 means **successful** termination,​ a value not 0 means **unsuccessful** termination. This behavior (== 0, != 0) is also what Bash reacts ​to in some flow control statements.
  
 An example of using the exit code of the program ''​grep''​ to check if a specific user is present in /​etc/​passwd:​ An example of using the exit code of the program ''​grep''​ to check if a specific user is present in /​etc/​passwd:​
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 fi fi
 </​code>​ </​code>​
-A common command ​to use for decisions ​is the command ​"''​test''"​ or its equivalent "''​[''"​. But note that, when using the test-command ​with the command ​name "''​[''", ​**the braces ​are not part of the shell syntax, ​they are the test command!** :!:+ 
 +A common ​decision making ​command is "''​test''"​ or its equivalent "''​[''"​. But note that, when calling ​test with the name "''​[''",​ the square brackets  ​are not part of the shell syntax, the left bracket **is** the test command! 
 <code bash> <code bash>
 if [ "​$mystring"​ = "Hello world" ]; then if [ "​$mystring"​ = "Hello world" ]; then
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 Read more about [[commands:​classictest | the test command]] Read more about [[commands:​classictest | the test command]]
  
-A common ​way of checking the exit code is using the "''​||''"​ or "''&&''"​ operators, when you do something special only when the command ​has either failed ​or succeeded:+A common exit code check method uses the "''​||''"​ or "''&&''"​ operators. This lets you execute a command ​based on whether ​or not the previous command completed successfully:
 <code bash> <code bash>
 grep ^root: /etc/passwd >/​dev/​null || echo "root was not found - check the pub at the corner."​ grep ^root: /etc/passwd >/​dev/​null || echo "root was not found - check the pub at the corner."​
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 </​code>​ </​code>​
  
-Please, when your script exits on errors, provide a "​FALSE"​ exit code, that others can check the script execution.+Please, when your script exits on errors, provide a "​FALSE"​ exit code, so others can check the script execution.
  
 ===== Comments ===== ===== Comments =====
-In a bigger ​script, it's wise to comment the code. Also for debugging ​purposes ​or tests. Comments are introduced ​with # (hashmark) and go from that to the end of the line:+In a larger, or complex ​script, it's wise to comment the code. Comments can help with debugging or tests. Comments are stat with the character ​(hashmark) and continue ​to the end of the line:
 <code bash> <code bash>
 #!/bin/bash #!/bin/bash
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 echo "Be liberal in what you accept, and conservative in what you send" # say something echo "Be liberal in what you accept, and conservative in what you send" # say something
 </​code>​ </​code>​
-The first thing was already explained, it's the so-called shebang, for the shell, **only a comment**. The second one is a comment from the beginning of the line, where the third comment starts after a valid command. All three comments are in valid syntax.+The first thing was already explained, it's the so-called shebang, for the shell, **only a comment**. The second one is a comment from the beginning of the line, the third comment starts after a valid command. All three syntactically correct.
  
  
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-==== Mass commenting ==== +==== Block commenting ==== 
-To temporarily disable complete blocks of code you normally ​would have to preceede ​every line of that block with a # (hashmark) to make it being a comment. ​Now, there's a little trick, using the pseudo command '':''​ (colon) and input redirection. The '':''​ does nothing, it's a pseudo command, so it also does not care about its standard input. In the following code (you don't have to understand the code, just look what I do with the stuff), you want to test only the things that don't harm (mail, logging) ​but actually don't do anything to the system (dump database, shutdown):+To temporarily disable complete blocks of code you would normally ​have to prefix ​every line of that block with a # (hashmark) to make it a comment. ​There's a little trick, using the pseudo command '':''​ (colon) and input redirection. The '':''​ does nothing, it's a pseudo command, so it does not care about standard input. In the following code example, you want to test mail and logging, but not dump the database, ​or execute a shutdown:
 <code bash> <code bash>
 #!/bin/bash #!/bin/bash
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 ##### The ignored codeblock ends here ##### The ignored codeblock ends here
 </​code>​ </​code>​
-What happened, againWell, the '':''​ pseudo command was given some input by redirection (a here-document) - the pseudo command didn't care about it, effectively, ​this complete ​block is ignored ​now. +What happened? ​The '':''​ pseudo command was given some input by redirection (a here-document) - the pseudo command didn't care about it, effectively, ​the entire ​block was ignored.
-One could say, **the whole block is a comment**. For completeness:​ To make the here-document possible, the shell might generate a temporary file unter ''/​tmp''​ or similar.+
  
 The here-document-tag was quoted here **to avoid substitutions** in the "​commented"​ text! Check [[syntax:​redirection#​tag_heredoc | redirection with here-documents]] for more The here-document-tag was quoted here **to avoid substitutions** in the "​commented"​ text! Check [[syntax:​redirection#​tag_heredoc | redirection with here-documents]] for more
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-In Bash, the scope of user variables is generally //global//. That means, whether a variable is set in the "main program"​ or in a "​function"​, it does **not** matter, the variable is defined everywhere.+In Bash, the scope of user variables is generally //global//. That means, ​it does **not** matter ​whether a variable is set in the "main program"​ or in a "​function",​ the variable is defined everywhere.
  
-Compare the following //​equivalent//​ code snips:+Compare the following //​equivalent//​ code snippets:
 <code bash> <code bash>
 myvariable=test myvariable=test
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 ==== Environment variables ==== ==== Environment variables ====
  
-The environment space is not directly related to the topic about scope, but it's worth to mention.+The environment space is not directly related to the topic about scope, but it's worth mentioning.
  
-Every UNIX(r) process has a so-called //​environment//​. ​Beside other stuff (unimportant for us), variables are saved there, so-called //​environment variables//​. ​Now when a child process is created (in Bash e.g. by simply executing another program, say ''​ls''​ to list files), the whole environment //including the environment variables// is copied to the new process. Reading that from the other side means: **Only variables that are part of the environment are available in the child process.**+Every UNIX(r) process has a so-called //​environment//​. ​Other itemsin addition to variablesare saved there, ​the so-called //​environment variables//​. ​When a child process is created (in Bash e.g. by simply executing another program, say ''​ls''​ to list files), the whole environment //including the environment variables// is copied to the new process. Reading that from the other side means: **Only variables that are part of the environment are available in the child process.**
  
 A variable can be tagged to be part of the environment using the ''​export''​ command: A variable can be tagged to be part of the environment using the ''​export''​ command:
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 </​code>​ </​code>​
  
-Remember that the //​exported//​ variable is a **copy** ​when accessed in the child process. There is no chance ​to "copy it back to the parent"​, it's a //​one-way//​! Also see the article about [[scripting:​processtree | Bash in the process tree]]!+Remember that the //​exported//​ variable is a **copy**. There is no provision ​to "copy it back to the parent." ​See the article about [[scripting:​processtree | Bash in the process tree]]!
  • scripting/basics.1438460357.txt
  • Last modified: 2015/08/01 20:19
  • by bill_thomson