Override a variable in a Bash script from the command line

Question

How do you override a variable in your Bash script from the command line?

I know how to pass variables in, but I just want something like ./myscript.sh -Dvar=val.

Answer 1

You need to use parameter expansion for the variable(s) you want to override:

$ cat override.sh #!/bin/bash  : ${var1:=foo} # var1 will take on the value "foo" if not overridden var2=${var2:-foo} # same thing but more typing  echo "var1 is $var1 | var2 is $var2" 

Without Override Values

$ ./override.sh var1 is foo | var2 is foo 

With Override Values

$ var1=bar var2=baz ./override.sh var1 is bar | var2 is baz 

Answer 2

Bash isn't like Make or Ant. Those two programs won't allow you to reset the value of a macro/property once it is set on the command line. Instead in Bash, you'll have to write your scripts in such a way that allows you to set these values from the command line and not override them inside your scripts.

You might want to look at the getopts command which is a Bash builtin. That gives you an easy, flexible way to parse command line arguments and set values from the command line. For example, I have four variables OPT_A, OPT_B, OPT_C, and OPT_D. If I don't pass the parameter, they get their default value. However, I can override that default value on the command line:

 USAGE="$0 [-a <a> -b <b> -c <c> -d <d>]"   OPT_A="Default Value of A"  OPT_B="Default Value of B"  OPT_C="Default Value of C"  OPT_D="Default Value of D"   while getopts ':a:b:c:d:' opt  do      case $opt in          a) OPT_A=$OPTARG;;          b) OPT_B=$OPTARG;;          c) OPT_C=$OPTARG;;          d) OPT_D=$OPTARG;;         \?) echo "ERROR: Invalid option: $USAGE"             exit 1;;      esac done 

---

You can also export your environment variables to allow your Bash scripts access to them. That way, you can set a variable and use that value.

In Bash, the ${parameter:=word} construct says that if $parameter is set, use the value of $parameter. However, if $parameter is null or unset, use word instead.

Now, imagine if you did this:

$ export COMMANDLINE_FOO="FUBAR" 

Now the variable $COMMANDLINE_FOO is set and readable for your shell scripts.

Then, in your shell script, you can do this:

FOO=BARFU  [...]    #Somewhere later on in the program...  echo "I'm using '${COMMANDLINE_FOO:=$FOO}' as the value" 

This will now print

I'm using 'FUBAR' as the value 

instead of

I'm using 'BARFU' as the value 

Answer 3

You should specify the variable with the following syntax:

MYVAR=74 ./myscript.sh

Within the script, check if it is already set before setting it:

if [ ! -z $MYVAR ]; then
    #do something
fi

Answer 4

I would do it like this:

# if JAVA_HOME not set, then set with default value
if [ "x$JAVA_HOME" = "x" ]; then
    JAVA_HOME=/opt/java
fi