Bash – Making a function that displays a variable’s name and its value

This is a well known bash pitfall, due to this feature:

Each command in a pipeline is executed as a separate process (i.e., in a subshell).

so that modified variables are local to the subshell, and not visible once back in the parent.

To avoid that, rephrase your code to avoid the pipeline, with a process substitution:

 for arg in "$@"
    do  
        readlink -e "$arg" || (( ++r ))

    done > >(sort -u)

To summarize, you want to retrieve a value out of script1.sh without running all the commands in script1.sh. I will assume that script1.sh is not under your control, possibly because it is vendor-provided, or managed by an obstinate colleague, or similar such issue, which makes it so that script1.sh has to be used as is.

I will present two approaches. The first is good for getting specific variables out of script1.sh, one at time:

var=$(awk -F'"' '/^variable=/ {print $2}' script1.sh )
echo $var

This uses awk to read (not execute, just read) script1.sh. awk looks for the line that begins with variable= and then writes what appears after the double-quote mark on that line. The output from awk is capture into variable var.

In more detail:

• The statement is of the form var=$(...). This means that whatever is in the parentheses is run as a bash command and its standard output is assigned to the variable var.

• Inside the parentheses, we have the awk command: awk -F'"' '/^variable=/ {print $2}' script1.sh. Let us consider it in parts.

• awk breaks lines (records) into fields. -F'"' tells awk to use double-quote as a field separator.

• /^variable=/ tells awk to restrict operation to lines that begin with variable=. Thus, all the miscellaneous commands in script1.sh will be ignored.

• {print $2} tells awk to print the second field. This means whatever is between the first and second occurence of a double-quote character.

• The last argument to awk tells it what file to read: script1.sh.

The above approach is good for handling one variable at a time and allows you to rename the variable if you like.

How to handle many variables

If you want to source all the variables assigned in script1.sh, consider:

source <(grep -E '^\w+=' script1.sh)

This uses grep to extract all lines from script1.sh that look like variable assignments. These lines are then run in the current shell, assigning the variables.

If you use this approach first check to make sure that you want all the variables and that there aren't any that will interfere with what you are doing. If there are such, we can exclude them with a second grep command.

Considering the pieces in turn:

• source file tells the shell to execute the file in the current shell.

• <(...) is called process substitution. It allows us to use the output of a command in place of a file name.

• The command grep -E '^\w+=' script1.sh extracts all lines that seem like variable assignments. If you run this command by itself on the command line, you should see something like:

  variable="Hello"
  var2="Value2"
  

  and so on. You should do this first and inspect the output to make sure that these are the lines that you want to execute.