Paths as bash aliases only work in parent directories?

Question

Can bash aliases be paths?

Hi and sorry in advance for the noob question. I noticed a strange behavior when using bash aliases.

My goal

is to use short bash aliases for often used paths, to make navigation in the shell easier. As an esample, to quickly navigate to a university (uni) folder on a data drive mounted at /mnt/data I used the bash alias alias uni='/mnt/data/uni'. Now, if I am in the parent directory /mnt/data and run cd uni, this works as I though it would and I move to /mnt/data/uni.

The issue

However if I am on another drive, say in my root directory and I run the same, I get the error bash: cd: uni: No such file or directory. If I only execute uni, the output is bash: /mnt/data/uni: Is a directory indicating that bash "understands" that this is a directory. However, for some reason, when not in the parent directory, the alias can not be used with the cd function to move to the directory.

If I add the cd command to my alias so that it reads alias uni='cd /mnt/data/uni', the alias works from all directories. However I would like to have only the path in the alias to be able to use other functions with it. Now I could live with including the cd into the alias because 99% of operations I do with these directories is just navigation but what bugs me most is that I obviously don't fully understand how aliases work in this situation.

My question

Could somebody explain why my alias behaves this way and what I am missing here? A solution to only alias the path would also be handy.

Thanks a lot! Cheers

Answer 1

No, aliases can't be paths. Or, more precisely, they can only be paths to something that can be executed. Aliases are run as commands, so alias uni=/bin/ls' will work because this is a path that points to a command. However, alias uni=/mnt/data/uni will not work because uni is a directory and that can't be executed.

The reason it works when you are in /mnt/data is because you have a subdirectory there named uni, so cd uni simply moves into that directory. The alias isn't even being invoked here. It isn't relevant at all. All the shell sees is a command (cd) and a target (the directory uni), the alias functionality isn't being called at all.

If you want to have directories easily accessible like that, you can set them up as variables. Add this to your ~/.bashrc file:

uni="/mnt/data/uni"

Save the file, open a new terminal, and you can now do cd "$uni" to move to /mnt/data/uni from anywhere.

Answer 2

Aliases can't be paths. They can only be command. If you run cat uni/file.txt or even cd uni, bash never tries to expand uni as an alias. If you want this to happen, the closest thing would be to make it a variable.

uni=/mnt/data/uni
cd $uni
cat $uni/file.txt

Note that if the path contain spaces (or other special characters rarely found in file names), you need double quotes when using the variable:

pol="/mnt/data/Pol Sci"
cat "$pol/file.txt"              # works
cat "$pol"/file.txt              # works
cat $pol/file.txt   # doesn't work: tries to print the content of /mnt/data/Pol and of Sci.txt

If you enable the cdable_vars option, you can save typing the $ and double quotes.

shopt -s cdable_vars
pol="/mnt/data/Pol Sci"
cd "$pol"                        # works
cd $pol                          # doesn't work ("cd: too many arguments")
cd pol                           # works thanks to cdable_vars

If you're only interested in a shortcut for the cd command, however, you can simply include cd in the alias.

alias uni='cd /mnt/data/uni'
uni                              # equivalent to cd /mnt/data/uni
alias pol='cd "/mnt/data/Pol Sci"'
pol                              # equivalent to cd "/mnt/data/Pol Sci"

Alternatively, or in addition, you can enable the autocd option. This causes bash to interpret a directory in command position as the instruction to change to a directory.

shopt -s autocd
/some/where                 # equivalent to cd /some/where

Even if you do this, you might as well include cd in your uni alias, because the alias can't do anything else.

Another related feature worth mentioning is the CDPATH variable. It lets you define “quick directory change” locations. For example:

CDPATH=$HOME:/mnt/data
cd /some/where
cd uni

The command cd uni will try first to change to /some/where/uni, then if this doesn't exist it will try $HOME/uni, then if that doesn't exist it will try /mnt/data/uni. You may or may not like this feature because on the one hand it's sometimes convenient, but on the other hand it makes it easy to accidentally change into a directory you didn't intend and end up working on the wrong project.

Answer 3

While the other answerers are correct that bash doesn't allow aliases for paths, you could consider switching to zsh, which has global aliases, which could be a path.

alias -g uni=/mnt/data/uni
cd uni

Works in zsh, but not bash.