Will Richardson

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Conditionals in SH

January 11, 2017Will Richardson

I’ve been spending more time than I would like writing shell scripts recently, as I spend more time configuring my setup than I do on ‘real’ projects. What I’ve found interesting is how simple the core of a shell is, and the tricks some commands do to build on this.

Most *nix users have probably had a moment were they were writing a shell script and forgotten the syntax for an if statement. I write shell scripts so infrequently I often have to look it up. However all the if statement does is run the condition command and check the exit status, if it is 0 it will run the main block, anything else and it runs the else block.

“But what about the square brackets?” I would think to myself. Well that’s just a command. You know, the [ command. man [ reveals that this is just a standard command with some flags to tell it what kind of thing to check.

Let’s take a simple conditional that checks that two numbers are equal:

if [ $num1 -eq $num2 ]; then
  echo "Equal!"
fi

If num1 is 4 and num2 is 5, the [ command will receive "4", "-eq", "5", and "]" (remember everything is a string in the shell). The command takes the arguments up to the closing square bracket and does the comparison, in this case -eq means integer comparison. As far as I can tell the closing bracket is just for readability - if you have a condition with with logical operators (|| or &&) then each part of the expression can be in separate brackets (or you can use the -o and -a options to keep them in the same set of brackets).

So this means that we can do things like this:

[ -f some/file/path ] && cool_function_on_file some/file/path

Making use of the && builtin, rather than writing a whole if;then;fi block. Or when we remember that the condition can be any command, we can be a bit smarter in scripts:

if git clone "$full_remote$user/$project.git" "$_path"; then
  echo_cd $_path
fi

This will only change to the cloned repos directory if it cloned successfully (indicated by the result of the git command).

for loops work in a similar way, except instead of the condition we have a command that produces an output with each element separated by the $IFS variable. The $IFS is basically just whitespace/ newlines so we can capture the output of ls and iterate through each filename:

for filename in $(ls); do
  echo "It's a thing: $filename"
done

So in short, I have learnt a bit about shell scripting and now think it’s kind of neat rather than getting frustrated at the seemingly nonsensicle syntax.

For reference [[ and == are builtins to BASH and other newer shells. == is no different to = (but it can’t be used to accidentally assign something). [[ works the same as [ apart form the fact that it can be used with < and > for comparison, as it can process them before they are interpreted as IO redirection as part of a normal command.


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