docker powershell shell command-prompt line-continuation

What is the significance of backslashes, backticks, and carets in various shells

I am following https://docs.docker.com/get-started/06_bind_mounts/#start-a-dev-mode-container on a Windows PC and am stuck here:

Run the following command. We’ll explain what’s going on afterwards:

 docker run -dp 3000:3000 \
     -w /app -v "$(pwd):/app" \
     node:12-alpine \
     sh -c "yarn install && yarn run dev"

If you are using PowerShell then use this command:

 docker run -dp 3000:3000 `
     -w /app -v "$(pwd):/app" `
     node:12-alpine `
     sh -c "yarn install && yarn run dev"

When using Command Prompt, I get errors (tried multiple variations as shown below), and when using PowerShell, I don't appear to get errors but am not running anything as showed when executing docker ps. Note that I would rather use Command Prompt and not PowerShell as I could use Linux commands with ComandPrompt on my PC.

What is the significance of backslashes when using Dockers with Command Prompt (and tick marks with PowerShell for that matter)?

I have since found that docker run -dp 3000:3000 -w /app -v "%cd%:/app" node:12-alpine sh -c "yarn install && yarn run dev" works without errors (got rid of backslashes, put on one line, and used %cd% instead of $(pwd)), but would still like to know why using the exact script in the example results in errors.

Using Command Prompt

C:\Users\michael\Documents\Docker\app>docker run -dp 3000:3000 \
docker: invalid reference format.
See 'docker run --help'.

C:\Users\michael\Documents\Docker\app>     -w /app -v "$(pwd):/app" \
'-w' is not recognized as an internal or external command,
operable program or batch file.

C:\Users\michael\Documents\Docker\app>     node:12-alpine \
The filename, directory name, or volume label syntax is incorrect.

C:\Users\michael\Documents\Docker\app>     sh -c "yarn install && yarn run dev"
sh: yarn: command not found

C:\Users\michael\Documents\Docker\app>docker run -dp 3000:3000 \ -w /app -v "$(pwd):/app" \ node:12-alpine \ sh -c "yarn install && yarn run dev"
docker: invalid reference format.
See 'docker run --help'.

C:\Users\michael\Documents\Docker\app>docker run -dp 3000:3000 -w /app -v "$(pwd):/app" node:12-alpine sh -c "yarn install && yarn run dev"
docker: Error response from daemon: create $(pwd): "$(pwd)" includes invalid characters for a local volume name, only "[a-zA-Z0-9][a-zA-Z0-9_.-]" are allowed. If you intended to pass a host directory, use absolute path.
See 'docker run --help'.

C:\Users\michael\Documents\Docker\app>

Using PowerShell

PS C:\Users\michael\Documents\Docker>  docker run -dp 3000:3000 `
>>      -w /app -v "$(pwd):/app" `
>>      node:12-alpine `
>>      sh -c "yarn install && yarn run dev"
849af42e78d4ab09242fdd6c3d03bcf1b6b58de984c4485a441a2e2c88603767
PS C:\Users\michael\Documents\Docker> docker ps
CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES
PS C:\Users\michael\Documents\Docker>

Solution

would still like to know why using the exact script in the example results in errors.

Because the command with the line-ending \ characters is meant for POSIX-compatible shells such as bash, not for cmd.exe

POSIX-compatible shells (sh, bash, dash, ksh, zsh):
- use \ for line-continuation (continuing a command on the following line) and escaping in general.
- use $varName to reference both environment and shell-only variables.
- support $(...) for embedding the output from a command (...) in command lines (command substitution).
- support both double-quoted ("...", interpolating) and single-quoted ('...', verbatim) strings; use '\'' to - in effect - include a ' inside '...'.
  (Additionally, in bash, ksh, and zsh, there are the rarely used ANSI C-quoted strings, $'...', and, in bash and ksh, perhaps even more rarely, localizable strings, $"...").
cmd.exe:
- uses ^ for line-continuation and escaping in general (in unquoted arguments only).
- uses %varName% to reference environment variables (the only variable type supported).
- doesn't support command substitutions at all.
- supports only "..." strings (interpolating).
PowerShell:
- uses ` (the backtick) for line-continuation and escaping in general.
- uses $env:varName to reference environment variables, $varName to reference shell-only variables.
- supports $(...), called subexpressions, the equivalent of command substitutions (outside of double-quoted strings, (...) is usually sufficient).
- supports both double-quoted ("...", interpolating) and single-quoted ('...', verbatim) strings; use '' to embed a ' inside '...'.
- Note: A common pitfall is that PowerShell has more metacharacters compared to both POSIX-compatible shells and cmd.exe, notably including @ { } , ;, which therefore require individual `-escaping in unquoted arguments or embedding in quoted strings - see this answer.

Potential line-continuation pitfall: in all of the shells discussed, the escape character must be the very last character on the line - not even trailing (intra-line) whitespace is allowed (because the escape character would then apply to it rather than to the newline).

The information above is summarized in the following table:

Feature	POSIX shells _	cmd.exe _	PowerShell _
Line-continuation / escape character	Backslash (`\`)	Caret (`^`)	Backtick (`)
Double-quoted strings (interpolating)	✅	✅	✅
Single-quoted strings (verbatim)	✅	❌	✅
Get / set environment variables	`$varName` / `export varName=...`	`%varName%` / `set varName=...`	`$env:varName` / `$env:varName = ...`
Get / set shell-only variables	`$varName`/ `varName=...`	❌ (no such variables exist, but you can limit the scope of env. vars. with `setlocal`)	`$varName`/ `$varName = ...`
Command substitutions, subexpressions	`$(...)`	❌	`(...)` / `$(...)`, esp. in strings

Note re setting variables with respect to whitespace on either side of the = symbol:

In POSIX-like shells, there must not be whitespace around =.
In cmd.exe, such whitespace is significant and becomes part of the variable / value name, and is therefore usually to be avoided.
In PowerShell, such whitespace is optional - you may use it to enhance readability; any string value to be assigned requires quoting (e.g., $var = 'hi!')