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Update content of section, "Rationale."

wileyhy
2024-10-06 01:44:33 -07:00
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SC1014.md

@@ -16,15 +16,20 @@ if grep -q pattern file
then then
echo "Found a match" echo "Found a match"
fi fi
``` ```
### Rationale: ### Rationale:
`[ .. ]` is not part of shell syntax like `if` statements. It is not equivalent to parentheses in C-like languages, `if (foo) { bar; }`, and should not be wrapped around commands to test. `[ ... ]` as shell syntax is a simple command that tests for whether certain conditions are true or false, such as whether the value assigned to a variable has a non-zero length (`[ -n "${foo}" ]`) or whether a file system object is a directory (`[ -d "${dir}" ]`). `If-then-(elif-then)-else-fi` statements are logical constructs which themselves contain lists of commands which can include simple commands.
`[` is just regular command, like `whoami` or `grep`, but with a funny name (see `ls -l /bin/[`). It's a shorthand for `test`. `[` is just regular command, like `whoami` or `grep`, but with a funny name (see `ls -l /bin/[`). It's a shorthand for `test`. `[[` is similar to both `[` and `test`, but `[[` offers some additional unary operators, such as '=~' the regular expression comparison operator. It allows one to use extglobs such as `@(foo|bar)` (a "bashism"), among some other less commonly used features.
If you want to check the exit status of a certain command, use that command directly as demonstrated in the correct code. `[[`, `[` and `test` are often used within `if...fi` constructs in the conditional commands position: which is between the 'if' and the 'then.'
There are certain shell syntaxes which can be wrapped directly around simple commands, in particular: (1) `{ ...;}` compound command groupings, (2) `$( ... )` command substitutions and (3) `<( ... )` and `>( ... )` process substitutions. Some examples include `{ [ -d "${HOME}/file" ]; echo $?;}`, `var=$(logname)` and `readarray -d "" -t files < <( find ~ -iname '*bash*' -print0 2> /dev/null )`, respectively. However, the various testing commands' syntaxes are not to be included among this list.
If you want to check the exit status of a certain command, use that command directly as demonstrated in the correct code, above.
If you want to check the output of a command, use `"$(..)"` to get its output, and then use `test` or `[`/`[[` to do a string comparison: If you want to check the output of a command, use `"$(..)"` to get its output, and then use `test` or `[`/`[[` to do a string comparison: