Managing paths - directories in shell and scripts

Question

Been using unix cygwin for a time and added many tools recently, command line and scripting are much more familiar. Scripting and programming is my number one goal and 100k dollar question? I am not weened from windows yet though.

The post here -> solved a users dilemma! did a lot to make using / switching paths and directories easier. Better than opening notepad++ and doing it! Specifically this command

# cd "$(cygpath -u 'c:\Program Files\')" 

by ben. works on the fly in bash for me so as to allow building scripts efficiently.

Can someone for programming sake break it down piecemeal?

I noticed there's 2 commands and - quotes and the parenthesis are 'separating' the $ , which I assume is a variable place holder???

I always end up thinking it is shell built in doing it, the easy answer, but still need to find out. And the double quotes outside are for what, while inner singe quotes are for space and literal interpretations by shell.

Exactly what is the shell doing start- to finish? Running a debug could be option I believe, how do I do that to show outputs? Thanks for the response.

Last, I did not mention what I used to try to convert paths. Saw stuff for sed strings but never worked due to complexity, this is much simpler.

Answer 1

A common misconception is that the $ sign means variable. It actually signals bash to perform an expansion (There are cases where variables can be referenced without the $ sign, but I won't get too off topic).

When bash sees the $ sign in front of a variable, it will replace the $VAR with the value (of the variable) and then execute the entire line.

In your example, bash will be doing a command substitution/expansion. Between the parenthesis is a command, bash will execute that command first, take the output and replace the $(command) with it, then execute the cd command.

You can observe this behavior by looking at a trace. Execute your script using bash -x or put set -o xtrace below the header of your script. Example:

#!/bin/bash
set -o xtrace

This will output the exact commands that bash is executing during the scripts which means after substitutions/expansions have taken place.

As for the quotes, they are not 100% necessary, but there can be situations in which the command in your example would not work properly. And that is if there are any spaces in the path returned by cygpath -u 'c:\Program Files\'. Bash command arguments are usually delimited by spaces, and without the quotes, bash will interpret those spaces as a delimiter and not a literal space (also the reason why there are single quotes around the path specified for the cygpath command).

For example, if the returned path was c:\user\Documents and Settings, with out quotes bash would read the command as cd c:\user\Documents and Settings, in which it will think that c:\user\Documents, and, and Settings are all arguments to be passed to the cd command. This is a problem because the cd command is only supposed to have one argument.

Double quotes around the $ substitution prevents space interpretation.