bash assign default value

Question

parameter  word                  Assign Default Values   If parameter is unset or null  the   expansion of word is assigned to   parameter   The value of parameter is   then  substituted    Positional    parameters  and  special                 parameters may not be assigned to in this way    I thought I could use this feature to write   LONG VARIABLE NAME  hello  instead of the longer LONG VARIABLE NAME   LONG VARIABLE NAME -hello   but now bash also tries to execute  hello  and that gives a command not found  Any way to avoid that  Or will I have to stick to the latter  Can someone give an example where the assign default is actually useful

User · Answer

The default value parameter expansion is often useful in build scripts like the example one below. If the user just calls the script as-is, perl will not be built in. The user has to explicitly set WITH_PERL to a value other than "no" to have it built in.

$ cat defvar.sh
#!/bin/bash

WITH_PERL=${WITH_PERL:-no}

if [[ "$WITH_PERL" != no ]]; then
    echo "building with perl"
    # ./configure --enable=perl
else
    echo "not building with perl"
    # ./configure
fi

Build without Perl

$ ./defvar.sh
not building with perl

Build with Perl

$ WITH_PERL=yes ./defvar.sh
building with perl

User · Answer

Please look at http   www tldp org LDP abs html parameter-substitution html for examples    parameter-default     parameter -default    If parameter not set  use default  After the call  parameter is still not set  Both forms are almost equivalent  The extra   makes a difference only when parameter has been declared  but is null   unset EGGS echo 1   EGGS-spam      1 spam echo 2   EGGS -spam     2 spam  EGGS  echo 3   EGGS-spam      3 echo 4   EGGS -spam     4 spam  EGGS cheese echo 5   EGGS-spam      5 cheese echo 6   EGGS -spam     6 cheese       parameter default     parameter  default    If parameter not set  set parameter value to default  Both forms nearly equivalent  The   makes a difference only when parameter has been declared and is null    sets variable without needing to reassign   colons suppress attempting to run the string unset EGGS     EGGS spam  echo 1  EGGS       1 spam unset EGGS     EGGS  spam  echo 2  EGGS       2 spam  EGGS      EGGS spam  echo 3  EGGS       3         set  but blank - gt  leaves alone  EGGS      EGGS  spam  echo 4  EGGS       4 spam  EGGS cheese     EGGS  spam  echo 5  EGGS       5 cheese EGGS cheese     EGGS spam  echo 6  EGGS       6 cheese       parameter alt value     parameter  alt value    If parameter set  use alt value  else use null string  After the call  parameter value not changed  Both forms nearly equivalent  The   makes a difference only when parameter has been declared and is null  unset EGGS echo 1   EGGS spam     1 echo 2   EGGS  spam    2  EGGS  echo 3   EGGS spam     3 spam echo 4   EGGS  spam    4  EGGS cheese echo 5   EGGS spam     5 spam echo 6   EGGS  spam    6 spam

User · Answer

You can also use    construct to assign and decide on action in one step  Consider following example    Example of setting default server and reporting it s status  server  1 if      server  localhost      a-z             localhost  assigned here to  server then    echo  quot server is localhost quot             echo is triggered since letters were found in  server else         echo  quot server was set quot    numbers were passed fi  If  1 is not empty  localhost will be assigned to server in the if condition field  trigger match and report match result  In this way you can assign on the fly and trigger appropriate action

User · Answer

Use a colon       A  hello    The colon is a null command that does nothing and ignores its arguments  It is built into bash so a new process is not created

[bash] bash assign default value

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