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Updated SC3012 (markdown)

Vidar Holen
2021-03-31 20:46:48 -07:00
parent dbcf6e6139
commit 75d146b08b
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SC3012.md

@@ -1,4 +1,4 @@
## In POSIX sh, lexicographical \< is undefined. ## In POSIX sh, lexicographical `\<` is undefined.
### Problematic code: ### Problematic code:
@@ -16,25 +16,23 @@ fi
First, make sure you wanted a lexicographical comparison (aka dictionary order), and not a numerical comparison. First, make sure you wanted a lexicographical comparison (aka dictionary order), and not a numerical comparison.
The easiest fix is to switch to a shell that *does* support lexicographical comparison with `>`/`\>`, such as `bash`, `dash`, or `ksh`. Otherwise, you can use `awk` to compare lexicographically by making sure the strings contain at least some non-numerical data: Then to compare as string, you can use `expr` and make sure that the strings are not interpreted numerically by adding some non-numerical data to them. Here, an apostrophe is prepended:
``` ```
#!/bin/sh #!/bin/sh
x="aardvark" x="aardvark"
y="zebra" y="zebra"
if awk -v first="'$x'" -v second="'$y'" 'BEGIN { exit (first < second) ? 0 : 1 }' if expr "'$x" \< "'$y" > /dev/null
then then
echo "$x comes before $y in the dictionary" echo "$x comes before $y in the dictionary"
fi fi
``` ```
The extra quotes in the awk `-v` flag are there to add non-numerical data to the string, otherwise `10 < 2` is treated as a numerical comparison (being false) instead of a lexicographical string comparison (being true). Any other non-numerical data can be used instead, such as `-v x="FOO$x"`.
### Rationale: ### Rationale:
The `test` binary operators `>`, `\>`, `<`, and `\<` are not part of POSIX and not guaranteed to be supported in scripts targeting `sh`. The `test` binary operators `>`, `\>`, `<`, and `\<` are not part of POSIX and not guaranteed to be supported in scripts targeting `sh`.
The `awk` functionality is POSIX and can safely be relied on. The `expr` functionality is specified by POSIX.
### Exceptions: ### Exceptions:
@@ -42,4 +40,4 @@ If you know your `sh` will be e.g. `dash`, consider explicitly using `#!/bin/das
### Related resources: ### Related resources:
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html#tag_20_128 * POSIX: [test](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html#tag_20_128), [expr](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/expr.html)