# My How and Why: Aliases, Functions, Symlinks and Shortcuts

I spend a decent amount of time at the commandline, for various reasons. At work, it’s usually at Windows cmd; at home, it’s either cmd, or bash on Debian. In both places, a major activity is working with Python, virtual environments, git, etc.; on the Linux box, I’m also doing a variety of other stuff as well. On both platforms I have a bunch of shell invocations that I don’t want to have to type out in full every time. This post lays out some ways that I’ve found to implement these sorts of shortcuts, and which of these approaches I currently prefer to use.

## bash (Debian Linux)

Prior to writing this post, I had only been using symlinks and functions; now, I’m using some custom-defined aliases as well.

### Aliases

bash aliases let you define static abbreviations for shell commands (both builtins and executables). I’ve most often seen them used to avoid needing to supply flags/arguments that are used on ~every invocation:

$du -s 16720220 .$ alias du="du -h"
$du -s 16G .$ alias pyc="python3 -c"
$pyc 'print("Hello!")' Hello!  I actually hadn’t used any of these before researching them for this post; but, now that I’ve looked closely, I’ve added a few to a new ~/.bash_aliases (they could also be added directly into ~/.bashrc or ~/.profile): # ls, full listing & show dotfiles alias lla="ls -la" # ls, minimal display alias l1="ls -1" # Always human-readable du alias du="du -h" # Always human-readable df alias df="df -h" # Grepped history and ps alias hg="history | grep " alias psag="ps aux | grep " # Quick access to apt update and upgrade alias aptupd="sudo apt-get update" alias aptupg="sudo apt-get upgrade"  Note that it does work fine to ‘redefine’ an existing command to include default options/flags, as was done above for du and dfalias doesn’t recurse its substitutions. In situations like this, the original command can be accessed by enclosing it in quotes: $ du -s
16G     .
$"du" -s 16695720 .  However, as I did for lla and l1, in most cases I expect I’ll define aliases as new names, so that I can get at the original commands without quoting them. Note also that aliases cannot contain explicit positional arguments. Any arguments passed to the alias always get transferred directly to the tail of the expanded command. ### Functions bash functions are a considerably more flexible means for defining custom commands. They can enclose multi-line commands, and can use arguments in arbitrary ways. The syntax is a bit unusual, as can be seen in this function I wrote for activating a python virtual environment in a sub-directory of the current working directory: vact () { source "env$1/bin/activate"
}


The parentheses will always be empty; they’re purely a syntactic marker to tell bash that you’re defining a function. Note that even though this function is only a single command, it could not be implemented as an alias, since the argument indicating which environment to activate has to be substituted into the middle of the command. (Most of the time, I use this function without an argument, which will activate the environment in env; however, if I define multiple environments in a common location, this lets me activate a specific environment: $vact foo would activate an environment residing in envfoo.) Note that, unlike with aliases, arguments passed to a function call are NOT automatically passed through to any particular command inside the function body—you have to specifically indicate where they should be used: $ pygood () { python3 $*; }$ pybad () { python3; }
$pygood -c "print('Hi')" Hi$ pybad -c "print('Hi')"
Python 3.7.3 (default, Apr  3 2019, 05:39:12)
[GCC 8.3.0] on linux
>>>


As with aliases, functions can put directly into ~/.profile or ~/.bashrc, or kept segregated in ~/.bash_aliases.

Aliases and functions are great for abbreviating direct invocations from the command line, but they have some disadvantages as compared to symlinks (created with ln -s [target] [new link]). One significant disadvantage is the fact that aliases and functions are defined per-user, whereas symlinks exist on the filesystem and (given suitable permissions) can thus be used by anyone logged in to the machine. Also, since symlinks provide an effectively invisible pass-through to the target executable, they can be used in complex invocations in ways that aliases and functions might not support. Initially, I thought that piped command sequences were one example of this, but it turns out that both aliases and functions handle pipes and input redirects just fine:

$alias py="python3"$ py3 () { python3 $*; }$ echo "print('Hi')" | python3
Hi
$echo "print('Hi')" | py Hi$ echo "print('Hi')" | py3
Hi
$echo "print('Hi')" > in$ py < in
Hi
$py3 < in Hi  Depending on your needs, it may make sense to put some symlinks in a central location that won’t conflict with the system package manager. It sounds like /usr/local/bin is a pretty standard location for things like this, and looks to be included on PATH by default. If you would want any of these symlinks to be available to users only if they want to opt-in to them, something non-standard like /usr/custom/bin is what I would use. (For more information on the various bin directories, see here.) For my use cases to date, though, I’ve pretty much always created my symlinks in a per-user fashion, placing them in a ~/bin directory, since I haven’t needed to make them accessible to others. I’ve done this even though I’m using multiple logins (to keep various responsibilities separated), because most of the commands are specific to each of the different logins I use to keep concerns segregated. However, for my custom Python builds, as described below, I’m considering moving their installation locations to someplace centralized, such as /usr/custom, and switching to building with the superuser. This would in particular avoid the need to rebuild Python for each user. I would still curate the symlinks per-user, though, in ~/bin. In order to make the symlinks available for execution, I just add a command in ~/.bashrc to prepend ~/bin (in its fully expanded form) to PATH: export PATH="/home/username/bin:$PATH"


Other paths, such as /usr/local/bin, can be added to PATH in the same fashion. On all subsequent logins with username, these symlinks will be available for direct execution in the shell.

As noted above, my main current use for these symlinks is to allow easy access to multiple locally-compiled versions of Python. While there are tools out there that provide for automatic management of Python versions (e.g., pyenv and pyflow ), I would rather have more direct control over what’s installed and how it’s compiled. For per-user installs, I install my custom Pythons into ~/python/x.y.z/, and then create symlinks in ~/bin:

$cd /home/username/bin$ ln -s /home/username/python/3.8.0/bin/python3.8 python3.8


This setup works really well with tox, such that the Python executables can be set just as:

[testenv:linux]
platform=linux
basepython=
py39: python3.9
py38: python3.8
py37: python3.7
py36: python3.6
py35: python3.5


The packages associated with each Python version can be changed with pip per usual:

$python3.8 -m pip ...  And, new virtual environments can be created with a given Python version via one of: $ python3.8 -m virtualenv env --prompt="(envname) "


or

$python3.8 -m venv env --prompt="envname"  Obviously, the first option only works after a python3.x -m pip install virtualenv. ## Windows Prior to doing the research for this post, as far as I knew Windows cmd wasn’t nearly as flexible an environment for defining these sorts of helpers—the only option was to use batch files. One alternative might have been to switch to PowerShell, but cmd was working well enough for me and I had no real desire to take the time to learn a completely new (and from what I could tell crushingly verbose) syntax. So, my approach for this on Windows has always been to add a per-user bin directory and put it on %PATH%, in a fashion directly analogous to the above approach for bash. The Windows 10 equivalent of ~ is usually C:\Users\Username, but whatever the location of the home directory for a user actually is, it’s stored in the environment variables as %USERPROFILE%. Once created, that directory needs to be added to %PATH%; Windows defines both a system-wide and a user-specific PATH, and since I’m usually creating a user-specific %USERPROFILE%\bin, I usually add it to the user-specific %PATH%. There’s a good discussion of %PATH% and how to add entries here on SuperUser. With the bin directory created and on %PATH%, I just create batch files for everything I want to streamline in there. For functionality that’s like a bash alias or symlink, the batch files typically are simple two-liners. For commands that lead to further interaction at the command line, I’ll use: %USERPROFILE%\bin\python3.8.bat =============================== @echo off C:\Python\python38\python.exe %*  For commands that kick off a GUI application (e.g., WordPad), or an application that I always want to run in the background as a new process, I’ll use: %USERPROFILE%\bin\wordpad.bat ============================= @echo off start "C:\Program Files\Windows NT\Accessories\wordpad.exe" %*  In these, @echo off is the first line used in basically every DOS/Windows batch script ever (why is echo on by default?!), to turn off echoing to stdout of the commands executed by the script. The %* argument passes the entire set of arguments provided to the script (if any) through to the command to be run. Another thing that I learned in the course of researching this post is that cmd batch files do handle content either piped in from a preceding command or redirected from a file on disk. Per here, this content stream is implicitly provided to the first command of the batch file that is capable of processing it. Thus, both of these invocation cases work fine with the above python3.8.bat: C:\Temp>type test.py import sys from pathlib import Path print(sys.version_info) print(str(Path().resolve())) C:\Temp>type test.py | python3.8 sys.version_info(major=3, minor=8, micro=1, releaselevel='final', serial=0) C:\Temp C:\Temp>python3.8 < test.py sys.version_info(major=3, minor=8, micro=1, releaselevel='final', serial=0) C:\Temp  For actions that are more complex, analogous to the use-case of bash functions, the script typically ends up structured differently, and sometimes ends up being longer. For example, this is my Windows equivalent of that vact helper for activating virtual environments: %USERPROFILE%\bin\vact.bat ========================== @echo off env%1\scripts\activate  So, while I can’t do everything with batch files in cmd that I can in bash, I can get pretty darn close—certainly enough to handle all of my routine tasks, and even most non-routine ones. In the course of researching for this post, I discovered that apparently Windows does support functionality for symlinks (since Windows Vista!), and provides the DOSKEY macro mechanism that behaves somewhat similar to bash aliases. They both have significant downsides/problems, though, that make me strongly prefer the batch-file approach. Symlinks don’t seem to work correctly out of the box in all situations for executing Python, one of my must-have use cases—it appears that they sometimes don’t set various elements of the execution context correctly, at minimum the path. They are created with the mklink command, which has to be run in a console with administrator privileges (a further annoyance; press Win+R to open the “Run” dialog, type cmd, then press Ctrl+Shift+Enter for elevated execution): C:\Users\Username\bin>mklink python3.8-sl C:\Python\Python38\python.exe symbolic link created for python3.8-sl <<===>> C:\Python\Python38\python.exe  However, execution of Python via this symlink doesn’t work reliably. It functions okay on my work PC, but when I try something like this at home… C:\Users\Username\bin>cd \temp C:\Temp>python3.8-sl test.py  … I get a system error: So, not knowing exactly what’s going on here, and given the friction involved in creating the things, I’ll be continuing to avoid Windows symlinks for the time being. DOSKEY aliases work for a subset of applications, but have a number of key limitations. They’re created and used using a syntax similar to bash’s: C:\Temp>doskey python38=C:\Python\Python38\python.exe$*

C:\Temp>python38
Python 3.8.0 (tags/v3.8.0:fa919fd, Oct 14 2019, 19:37:50) [MSC v.1916 64 bit (AMD64)] on win32
>>> ^Z

C:\Temp>python38 -c "print('HELLO WORLD')"
HELLO WORLD


The \$* argument serves the same role as in bash functions, passing through to the expanded command all arguments provided to the alias/macro.

Unfortunately, unlike with batch scripts, you can’t pipe text into a DOSKEY macro:

C:\Temp>type test.py | python38
'python38' is not recognized as an internal or external command,
operable program or batch file.


Also, per here, DOSKEY macros can’t be used within batch files, and setting them up to be loaded at the start of every console session requires a registry modification. So, batch files again remain my preferred Windows solution.

Written on June 9, 2020