Variables as commands in Bash scripts

eval is not an acceptable practice if your directory names can be generated by untrusted sources. See BashFAQ #48 for more on why eval should not be used, and BashFAQ #50 for more on the root cause of this problem and its proper solutions, some of which are touched on below:

If you need to build up your commands over time, use arrays:

tar_cmd=( tar cv "$directory" )
split_cmd=( split -b 1024m - "$backup_file" )
encrypt_cmd=( openssl des3 -salt )
"${tar_cmd[@]}" | "${encrypt_cmd[@]}" | "${split_cmd[@]}"

Alternately, if this is just about defining your commands in one central place, use functions:

tar_cmd() { tar cv "$directory"; }
split_cmd() { split -b 1024m - "$backup_file"; }
encrypt_cmd() { openssl des3 -salt; }
tar_cmd | split_cmd | encrypt_cmd

I am not sure, but it might be worth running an eval (near "The args are read and concatenated together") on the commands first.

This will let Bash expand the variables $TAR_CMD and such to their full breadth (just as the echo command does to the console, which you say works).

Bash will then read the line a second time with the variables expanded.

eval $TAR_CMD | $ENCRYPT_CMD | $SPLIT_CMD

Page Bash: Why use eval with variable expansion? looks like it might do a decent job at explaining why that is needed.

There is a point to only put commands and options in variables.

#! /bin/bash

if [ $# -ne 2 ]
then
    echo "Usage: `basename $0` DIRECTORY BACKUP_DIRECTORY"
    exit 1
fi

. standard_tools    

directory=$1
backup_directory=$2
current_date=$(date +%Y-%m-%dT%H-%M-%S)
backup_file="${backup_directory}/${current_date}.backup"

${tar_create} "${directory}" | ${openssl} | ${split_1024} "$backup_file"

You can relocate the commands to another file you source, so you can reuse the same commands and options across many scripts. This is very handy when you have a lot of scripts and you want to control how they all use tools. So standard_tools would contain:

export tar_create="tar cv"
export openssl="openssl des3 -salt"
export split_1024="split -b 1024m -"

Quoting spaces inside variables such that the shell will reinterpret things properly is hard. It's this type of thing that prompts me to reach for a stronger language. Whether that's Perl, Python, Ruby, or whatever (I choose Perl, but that's not always for everyone), it's just something that will allow you to bypass the shell for quoting.

It's not that I've never managed to get it right with liberal doses of eval, but just that eval gives me the heebie-jeebies (becomes a whole new headache when you want to take user input and eval it, though in this case you'd be taking stuff that you wrote and evaling that instead), and that I've gotten headaches in debugging.

With Perl, as my example, I'd be able to do something like:

@tar_cmd = ( qw(tar cv), $directory );
@encrypt_cmd = ( qw(openssl des3 -salt) );
@split_cmd = ( qw(split -b 1024m -), $backup_file );

The hard part here is doing the pipes - but a bit of IO::Pipe, fork, and reopening standard output and standard error, and it's not bad. Some would say that's worse than quoting the shell properly, and I understand where they're coming from, but, for me, this is easier to read, maintain, and write. Heck, someone could take the hard work out of this and create a IO::Pipeline module and make the whole thing trivial ;-)