Ubuntu – What does “:” (colon) operator in a bash variable expansion: VAR=${TEMP:3}

bash

What is the meaning of the following line in a variable in bash?

VAR=${TEMP:3}

This is variable expansion and works like this (notice this is only bash and ksh specific and will not work in a POSIX shell):

$ x=1234567890
$ echo ${x:3}
4567890
$ echo ${x:7}
890
$ echo ${x:3:5}
45678

${var:pos} means that the variable var is expanded, starting from offset pos.
${var:pos:len} means that the variable var is expanded, starting from offset pos with length len.

There is more than one way to increment a variable in bash, but what you tried is not correct.

You can use for example arithmetic expansion:

var=$((var+1))
((var=var+1))
((var+=1))
((var++))

Or you can use let:

let "var=var+1"
let "var+=1"
let "var++"

$$ is the PID (process id) of the current process.

$$1 is the above PID followed by the literal string 1.

So it is telling you that your bash is the process with PID 1930.

But... free trick:

show() { eval echo \$$1; }
show PATH
/home/romano/bin:/usr/local/bin:/bin:/usr/bin

(quite convoluted, ain't it?)

More info in TLDP.