Scripting case studies

Table of content

This page explains the scripting examples on the homepage, illustrating the advantages of Elvish scripting, especially when compared to traditional shells.

For more examples of Elvish features compared to traditional shells, see the quick tour. For a complete description of the Elvish language, see the language reference.

1. jpg-to-png.elv

This example on the homepage uses GraphicsMagick to convert all the .jpg files into .png files in the current directory:


for x [*.jpg] { gm convert $x (str:trim-suffix $x .jpg).png }

(If you have ImageMagick installed instead, just replace gm with magick.)

It’s equivalent to the following script in traditional shells:

for x in *.jpg do gm convert "$x" "${x%.jpg}".png done

Let’s see how the Elvish version compares to the traditional shell version:

  • You don’t need to double-quote every variable to prevent unwanted effects. A variable in Elvish always evaluates to one value.

  • Instead of ${x%.jpg}, you write (str:trim-suffix $x .jpg). The latter a bit longer, but is easier to remember and understand.

    Moreover, since str:trim-suffix is a normal command rather than a special operator, it’s easy to find its documentation - in the reference doc, in the terminal with doc:show, or by hovering over it in VS Code (support for more editors will come).

  • When there is no file that matches *.jpg, bash will assign $x to the pattern *.jpg itself, which is most likely not what you want.

    Elvish will throw an exception by default, and you can optionally tell Elvish to expand to zero elements with *[nomatch-ok].jpg.

  • Perhaps subjectively, Elvish’s syntax is more readable: instead of keywords like in, do and done, Elvish’s for command doesn’t have in, and uses familiar punctuation to delimit different parts of the command: the list of elements with [ and ], and the loop body with { and }.

This example doesn’t go into the advanced capabilities of Elvish, so differences are small and may seem superficial. However, these small details can quickly add up, and in general it’s much easier to develop and maintain scripts in Elvish.

2. update-servers-in-parallel.elv

This example on the homepage shows how you would perform update commands on multiple servers in parallel:


var hosts = [[&name=a &cmd='apt update'] [&name=b &cmd='pacman -Syu']] # peach = "parallel each" peach {|h| ssh root@$h[name] $h[cmd] } $hosts

Let’s break down the script:

  • The value of hosts is a nested data structure:

    • The outer [] denotes a list.

    • The inner [] denotes maps - &k=v are key-value pairs.

    So the variable $hosts contains a list of maps, each map containing the key name and cmd, describing a host.

  • The peach command takes:

    • A function, in this case an anonymous one. The signature |h| denotes that it takes one argument, and the body is an ssh command using fields from $h.

    • A list, in this case $hosts.

As hinted by the comment, peach calls the function for every element of the list in parallel, running these ssh commands in parallel. It will also wait for all the functions to finish before it returns.

In a real-world script, you’ll likely want to redirect the output of the ssh command, otherwise the output from the ssh commands running in parallel will interleave each other. This can be done with output redirection, not unlike traditional shells:


var hosts = [[&name=a &cmd='apt update'] [&name=b &cmd='pacman -Syu']] peach {|h| ssh root@$h[name] $h[cmd] > ssh-$h[name].log } $hosts

With a traditional shell, you can achieve a similar effect with background jobs:

ssh root@a 'apt update' > ssh-a.log & job_a=$! ssh root@b 'pacman -Syu' > ssh-b.log & job_b=$! wait $job_a $job_b

However, you will have to manage the lifecycle of the background jobs explicitly, whereas the structured nature of peach makes that unnecessary. Alternatively, you can use external commands such as GNU Parallel to achieve parallel execution, but this requires you to learn another tool and structure your script in a particular way.

Like in most programming languages, data structures in Elvish can be arbitrarily nested. This allows you to express more complex workflows in a natural way:


var hosts = [[&name=a &cmd='apt update' &dotfiles=[.tmux.conf .gitconfig]] [&name=b &cmd='pacman -Syu' &dotfiles=[.vimrc]]] peach {|h| ssh root@$h[name] $h[cmd] > ssh-$h[name].log scp ~/(all $h[dotfiles]) root@$h[name]: } $hosts

The expression (all $h[dotfiles]) evaluates to all the elements of $h[dotfiles] (see documentation for all), each of which is then combined with ~/, which evaluates to the home directory.

Traditional shells tend to have limited support for complex data structures, so it can get quite tricky to express the same workflow, especially when coupled with parallel execution.

What’s more, you can easily move the definition of hosts into a JSON file - this can be useful if you’d like to share the script with others without requiring everyone to customize the script:


[ {"name": "a", "cmd": "apt update", "dotfiles": [".tmux.conf", ".gitconfig"]}, {"name": "b", "cmd": "pacman -Syu", "dotfiles": [".vimrc"]} ]


var hosts = (from-json < hosts.json) peach {|h| ssh root@$h[name] $h[cmd] > ssh-$h[name].log scp ~/(all $h[dotfiles]) root@$h[name]: } $hosts

Elvish brings the power of data structures and functional programming to your shell scripting scenarios.

3. Catching errors early

The following interaction in the terminal showcases how Elvish is able to catch errors early:

Terminal: elvish

~> var project = ~/project ~> rm -rf $projetc/bin compilation error: variable $projetc not found

The example on the homepage is slightly simplified for brevity. In fact, Elvish will highlight the exact place of the error, before you even press Enter to execute the code:

Terminal: elvish

~> var project = ~/project ~> rm -rf $projetc/bin elf@host compilation error: [interactive]:1:8-15: variable $projetc not found

In this case, Elvish identifies that the variable name is misspelt and won’t execute the code. Compare this to an interaction in a more traditional shell:

Terminal: sh

$ project=~/project $ rm -rf $projetc/bin [ ...]

Traditional shells by default don’t treat the misspelt variable as an error and evaluates it to an empty string instead. As the result, this will start executing rm -rf /bin, possibly with catastrophic consequences.

Elvish’s early error checking extends beyond terminal interactions, for example, suppose you have the following script:


var project = ~/project # A function... fn cleanup-bin { rm $projetc/bin } # More code...

If you try to run this script with elvish script-with-error.elv, Elvish will find the misspelt variable name within the function cleanup-bin, and refuses to execute any code from the script.

Elvish’s early error checking can help you prevent a lot of bugs from simple typos. There are more places Elvish checks for errors, and more checks are being added.