linux:bash

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linux:bash [2022/02/10 09:44] – [Bash Scripting] mhlinux:bash [2026/01/11 11:57] (current) – [Control Operators] mh
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 Bash has **synchronous command execution** meaning it will execute commands one at a time and users cannot interact with bash while it is executing a command. Bash has **synchronous command execution** meaning it will execute commands one at a time and users cannot interact with bash while it is executing a command.
  
-Bash is **not a strict interpreter**. The discipline of writing clear, safe and precise code lies heavily upon the user.  
  
 <WRAP round important> <WRAP round important>
-**Don't write bad bash code.**+Bash is **not a strict interpreter**.  
 + 
 +**Beware of the ambiguity** we are used to while expressing ourselves with human languages. 
 + 
 +The **discipline of writing clear, safe and precise code** lies heavily upon the user.  
 + 
 +__**Don't write bad bash code.**__
 </WRAP> </WRAP>
  
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 Many command types are syntax sugar: their effect can be achieved differently, but they exist to make the job easier.  Many command types are syntax sugar: their effect can be achieved differently, but they exist to make the job easier. 
 </WRAP> </WRAP>
 +
 +==== Variables ====
 +
 +Before a command's name you can insert one or many ''var=value'' assignments. These variables will apply only to that one command.
  
 ==== Basic commands ==== ==== Basic commands ====
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 Bash performs a search using this name and looks for :  Bash performs a search using this name and looks for : 
-  *alias (first)+  *alias (before anything else)
   *functions   *functions
   *built-in commands   *built-in commands
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 $ echo $name $ echo $name
 john john
 +</code>
 +----
 +
 +=== type ===
 +
 +Displays information on a command, and where it is stored.
 +
 +//Note that the **type** command and the **which** program are different and give different outputs//
 +
 +**Example**
 +<code bash>
 +$ type ls
 +ls is aliased to 'ls -al'
 </code> </code>
 ---- ----
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 ---- ----
  
 +=== 2>/dev/null || true ===
 +
 +<code bash>
 +$ something 2>/dev/null || true
 +</code>
 +
 +This means:
 +
 +Try something, but if it errors (exit code ≠ 0): 
 +  * silence the error message (send to ''/dev/null'')
 +  * immediately run ''true'' ( || operator passes along to second part)
 +  * Overall ''exit'' status becomes success (exit code = 0)
 +
 +In plain English:
 +
 +“Try to do the cleanup.
 +If there’s nothing to clean, that’s fine. Move on.”
 +
 +This pattern is **not appropriate** when:
 +  * Failure is meaningful
 +  * You want to be alerted
 +  * Data loss is possible
 +  * You are debugging
 +
 +In those cases, we want ''stderr'' and non-zero exits.
 +
 +So this is not about hiding problems.
 +It’s about acknowledging expected non-problems.
 +
 +----
 +
 +=== && AND Operator ===
 +
 +This control operator tells bash to run a second command **only if the first command before it succeeded**.
 +
 +If the first command fails, the second command is entirely skipped.
 +
 +**Example**
 +<code bash>
 +$ rm hello.txt && echo "Deleted file."
 +rm: cannot remove 'hello.txt': No such file or directory
 +</code>
 +
 +Because the file did not exist, the ''echo'' command was not run.
 ===== Arguments ===== ===== Arguments =====
  
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 They can be a filename, a variable name, the name of a program or just a litteral. They can be a filename, a variable name, the name of a program or just a litteral.
 +
 +Multiple arguments can be used, separated by a blank space
 </WRAP> </WRAP>
  
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 There can be **no whitespace** around the = operator !  There can be **no whitespace** around the = operator ! 
  
-If a whitespace is required in the variable, use '' quotes to create a literal+If a whitespace is required in the variable, use 'single quotes' to create a literal
 </WRAP> </WRAP>
  
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 As a result, both file descriptors are connected to the same stream. As a result, both file descriptors are connected to the same stream.
  
 +Be careful not to write something like ''$ ls -l a b >myfiles.ls 2>myfiles.ls'' as due to the way streams are handled they will most likely end up mixing up and garbling the file 'myfiles.ls'
 </WRAP> </WRAP>
  
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 </code> </code>
  
-This rather more precise alternative invokes the 'env' program giving bash as an argument explicitly asking 'env' to find the path to bash and return it. It is safer and more inclusive for covering more exotic distributions and other operating systems.+This rather more precise alternative invokes the 'env' program giving bash as an argument explicitly asking 'env' to find the path to bash and return it. It is safer and more inclusive for covering more exotic distributions and other operating systems :
 <code bash> <code bash>
 #!/usr/bin/env bash #!/usr/bin/env bash
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