Bash – What does ${1:0:2} mean in this context

bashshell-script

I am looking into the code displayed below and it checks the input if the row/column arguments start with either -r or -c.
What does ${1:0:2} mean in this context? 

rowArgName="-r"
colArgName="-c"
if [ "${1:0:2}" != $rowArgName ] && [ "${1:0:2}" != $colArgName ]
then
   echo $correctCmdMsg >&2
   exit 1
fi

Best Answer

It's a Substring Expansion (subclass of Parameter Expansion) pattern of shell.

The format is:

${parameter:offset:length}

and indexing starts at 0.

Say, you have a variable foo, then ${foo:0:2} yields the first two characters (from position 0 the next 2).

Example:

$ foo=spamegg
$ echo "${foo:0:2}"
sp

In your case, the first number, 1, refers to variable name $1, which is the first argument passed via command line (in the main program) or the first argument passed to the function.

So in your case, "${1:0:2}" will:

  • start extracting the substring starting from index 0 i.e. first character

  • and continue upto next two characters

so after the operation you will get the first two characters (indexed at 0 and 1) of the input string.

The [ "${1:0:2}" != $rowArgName ] and [ "${1:0:2}" != $colArgName ] are checking if the output subtring is equal to some other strings.

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