My bash script bash.sh only contains one line
echo "${abc:=123}"
I learned that := is used to assign default values. So when I run bash.sh abc=abc, I expect the output to be abc.
However, the output is still 123.
Why is that? Am I call the script in the wrong way? Thanks.
Bash positional arguments are set to $1, $2, etc. Change your script to:
abc=$1
echo "${abc:=123}"
this will make it so if the variable abc is unset the default value is echoed but if another value is passed on the command line abc will be set to that value.
You are passing a parameter and expecting to see it in an environment variable.
If you want to set an environment variable, you can do that before the script name:
$ cat foo
#!/bin/bash
echo "${abc:=123}"
$ ./foo
123
$ abc=hello ./foo
hello
abc=abc ./bash.shto setabcas an environment variable instead of a positional argument (which nothing in your script currently reads).#!/bin/bash, it isn't a bash script at all. (#!/bin/shscripts also aren't bash scripts; they're POSIX sh scripts, which is a more restricted language).