Autosave: 2023-02-08 13:47:35

Programming_Languages/Shell/Test.md

@@ -9,6 +9,65 @@ tags:
 
 `test` is a built-in command that is used to compare values or determine whether something is the case.
 
-When we run a test the result we get back is a return status of a `0` or a `1`
+We can use the command `test` but it is more common to test a condition implicity by using square brackets. The square brackets are an alias for `test`. We use this alias when we use `IF` logic.
 
-We can use the command `test` but it is more common to test a condition implicity by using square brackets. The square brackets are an alias for `test`. We use this alias when we use `IF` logic
+When we run a test the result we get back is a return status of a `0` or a `1`. `0` indicates that the test was a success and `1` means failure. (Bear in mind this is in contrast to most all other programming languages.)
+
+If we run a test in the command line we won't get a `0` or a `1` or back, there will just be silence from the shell. We can explicitly invoke the return value with variable `$?`, e.g:
+
+```bash
+[ -d ~ ] # is the home directory a directory?
+echo $?
+0   # yes
+
+[ -d /bin/zsh ] # is this binary a directory?
+echo $
+01  # no
+```
+
+## Test structures
+
+Many tests can be run with flags as a shorthand like we saw above:
+
+### File operators
+
+```
+-a FILE        True if file exists.
+-d FILE        True if file is a directory.
+-e FILE        True if file exists.
+-h FILE        True if file is a symbolic link.z
+-s FILE        True if file exists and is not empty.
+-N FILE        True if the file has been modified since it was last read.
+```
+
+### String operators
+
+```
+-z STRING      True if string is empty.
+-n STRING      True if string is not empty.
+```
+
+## Differences between comparing numbers and strings
+
+- `=` is reserved for comparing strings
+- For numbers we use, e.g, `[ 4 -lt 5 ]`
+
+## Negation
+
+We can negate a test condition with `!`:
+
+```bash
+[ ! 4 -lt 3 ]; echo $?
+0
+```
+
+## Extended test: `[[...]]`
+
+When we use **double brackets** we are using _extended_ `test`.
+
+The extended test supports the standard `test` comparisons and adds other features:
+
+- The use of Boolean operators:
+  ```bash
+  [[ -d ~ || -a /bin/mash ]]; echo $?
+  ```