Move pages under tldr, lint run.sh, update docs

tldr/if

@@ -0,0 +1,42 @@
+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# if
+
+> Performs conditional processing in shell scripts.
+> See also: `test`, `[`.
+> More information: <https://www.gnu.org/software/bash/manual/bash.html#Conditional-Constructs>.
+
+- Execute the specified commands if the condition command's exit status is zero:
+
+`if {{condition_command}}; then {{echo "Condition is true"}}; fi`
+
+- Execute the specified commands if the condition command's exit status is not zero:
+
+`if ! {{condition_command}}; then {{echo "Condition is true"}}; fi`
+
+- Execute the first specified commands if the condition command's exit status is zero otherwise execute the second specified commands:
+
+`if {{condition_command}}; then {{echo "Condition is true"}}; else {{echo "Condition is false"}}; fi`
+
+- Check whether a [f]ile exists:
+
+`if [[ -f {{path/to/file}} ]]; then {{echo "Condition is true"}}; fi`
+
+- Check whether a [d]irectory exists:
+
+`if [[ -d {{path/to/directory}} ]]; then {{echo "Condition is true"}}; fi`
+
+- Check whether a file or directory [e]xists:
+
+`if [[ -e {{path/to/file_or_directory}} ]]; then {{echo "Condition is true"}}; fi`
+
+- Check whether a variable is defined:
+
+`if [[ -n "${{variable}}" ]]; then {{echo "Condition is true"}}; fi`
+
+- List all possible conditions (`test` is an alias to `[`; both are commonly used with `if`):
+
+`man [`