Configuring Shell Hooks

Jun 03, 2020 Jerven Clark Chua

Working with multiple python projects, especially with very different library versions is a complete nightmare. That’s why tools like virtualenv¹ is a godsend. My only complain is that, constantly activating, reactivating and deactivating environment is very tiresome or at least for me because I’m extremely lazy. So, I decided to automate activation and deactivationn when switching directories.

ZSH

Luckily for zsh, it comes with a few hook functions² we can use to accomplish this. On my .zshrc, I added a check for the existence of the venv directory and activate it exists.

# Hook to execute whenever the current working directory is changed
chpwd() {
    activate_virtualenv
}

# Activate virtualenv on change directory
activate_virtualenv() {
    if [ -d $PWD/venv ]; then
        . $PWD/venv/bin/activate
    fi
}

Easy enough right? Deactivation, on the other hand, is a bit of a longer process. For this, we need to get the working directory for our venv. I’m gonna store it to the env variable VIRTUAL_ENV_WORKDIR right after we activate venv. Like so:

# truncated code
...

. $PWD/venv/bin/activate
VIRTUAL_ENV_WORKDIR=$PWD

...
# truncated code

Now that we have a reference, we can test if the target directory we are changing to is a directory outside of our venv working directory. If it is indeed outside, we are going to deactivate venv and unset our VIRTUAL_ENV_WORKDIR to things back to what they were before.

# Hook to execute whenever the current working directory is changed
chpwd() {
    activate_virtualenv
    deactivate_virtualenv
}

# truncated code
...

# Deactivate virtualenv on change directory
deactivate_virtualenv() {
    if [[ "$PWD" != "$VIRTUAL_ENV_WORKDIR"* ]]; then
        deactivate
        unset VIRTUAL_ENV_WORKDIR
    fi
}

It works but we can refactor our code further:

# Hook to execute whenever the current working directory is changed
chpwd() {
    configure_virtualenv
}

# Configure virtualenv on change directory
configure_virtualenv() {
    if [ -d $PWD/venv ]; then
        . $PWD/venv/bin/activate
        VIRTUAL_ENV_WORKDIR=$PWD
    elif [[ "$PWD" != "$VIRTUAL_ENV_WORKDIR"* ]] && [ "$VIRTUAL_ENV" ]; then
        deactivate
        unset VIRTUAL_ENV_WORKDIR
    fi
}

Bash

The code above doesn’t translate directly to bash tho. For it to work, we have to make a few minor adjustments. Instead of declaring a chpwd, we need to use PROMPT_COMMAND. This will be run just before bash displays a prompt.

PROMPT_COMMAND=configure_virtualenv

Putting everything together:

#!/bin/bash

# Hook to execute whenever the current working directory is changed
if [ ${ZSH_VERSION} ]; then
    # 'ZSH_VERSION' only defined in Zsh
    chpwd() {
        configure_virtualenv;
    }
elif [ ${BASH_VERSION} ]; then
    # 'BASH_VERSION' only defined in Bash
    PROMPT_COMMAND=configure_virtualenv
fi

# Configure virtualenv on change directory
configure_virtualenv() {
    if [ -d $PWD/venv ]; then
        . $PWD/venv/bin/activate
        VIRTUAL_ENV_WORKDIR=$PWD
    elif [[ "$PWD" != "$VIRTUAL_ENV_WORKDIR"* ]] && [ "$VIRTUAL_ENV" ]; then
        deactivate
        unset VIRTUAL_ENV_WORKDIR
    fi
}

Notes

virtualenv is a tool for managing multiple python environments. ↩
zsh hook functions ↩