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Updated SC2016 (markdown)

Vidar Holen
2021-08-26 17:24:22 -07:00
parent b4f691bdaa
commit 0c7d198e5a
1 changed files with 20 additions and 9 deletions
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SC2016.md

@@ -4,33 +4,44 @@
```sh ```sh
name=World name=World
echo 'Hello $name' echo 'Hello $name' # Outputs Hello $name
``` ```
### Correct code: ### Correct code:
```sh ```sh
name=World name=World
echo "Hello $name" echo "Hello $name" # Outputs Hello World
``` ```
### Rationale: ### Rationale:
Single quotes prevent expansion of everything, including variables and command substitution. ShellCheck found an expansion like `$var`, `$(cmd)`, or `` `cmd` `` in single quotes.
If you want to use the values of variables and such, use double quotes instead. Single quotes express all such expansions. If you want the expression to expand, use double quotes instead.
Note that if you have other items that needs single quoting, you can use both in a single word: If switching to double quotes would require excessive escaping of other metacharacters, note that you can mix and match quotes in the same shell word:
```sh ```sh
echo '$1 USD is '"$rate GBP" dialog --msgbox "Filename $file may not contain any of: "'`&;"\#%$' 10 70
``` ```
### Exceptions ### Exceptions
If you want `$stuff` to be a literal dollar sign followed by the characters "stuff", you can [[ignore]] this message. If you know that you want the expression literally without expansion, you can [[ignore]] this message:
ShellCheck tries to be smart about it, and won't warn when this is used with awk, perl and similar, but there are some inherent ambiguities like `'I have $1 in my wallet'`, which could be "one dollar" or "whatever's in the first parameter". ```
# We want this to output $PATH without expansion
# shellcheck disable=SC2016
echo 'PATH=$PATH:/usr/local/bin' >> ~/.bashrc
```
In the particular case of `sed`, ShellCheck uses additional heuristics to try to separate cases like `'s/$foo/bar/'` (failing to replace the variable `$foo`) with from the false positives like `'$d'` (delete last line). If you're still triggering these, consider being more generous with your spaces: use `$ { s/foo/bar; }` instead of `${s/foo/bar/;}` ShellCheck also does not warn about escaped expansions in double quotes:
```
echo "PATH=\$PATH:/usr/local/bin" >> ~/.bashrc
```
### Related resources:
* StackOverflow: [How do I use variables in single quoted strings?](https://stackoverflow.com/questions/21192420/how-do-i-use-variables-in-single-quoted-strings)