automated terminal push

docs/.Bash-Scripting/009-bash-conditional-expressions.md

@@ -0,0 +1,186 @@
+# Bash Conditional Expressions
+
+In computer science, conditional statements, conditional expressions, and conditional constructs are features of a programming language, which perform different computations or actions depending on whether a programmer-specified boolean condition evaluates to true or false.
+
+In Bash, conditional expressions are used by the `[[` compound command and the `[`built-in commands to test file attributes and perform string and arithmetic comparisons. 
+
+Here is a list of the most popular Bash conditional expressions. You do not have to memorize them by heart. You can simply refer back to this list whenever you need it!
+
+## File expressions
+
+* True if file exists.
+
+```bash
+[[ -a ${file} ]]
+```
+
+* True if file exists and is a block special file.
+
+```bash
+[[ -b ${file} ]]
+```
+
+* True if file exists and is a character special file.
+
+```bash
+[[ -c ${file} ]]
+```
+
+* True if file exists and is a directory.
+
+```bash
+[[ -d ${file} ]]
+```
+
+* True if file exists.
+
+```bash
+[[ -e ${file} ]]
+```
+
+* True if file exists and is a regular file.
+
+```bash
+[[ -f ${file} ]]
+```
+
+* True if file exists and is a symbolic link.
+
+```bash
+[[ -h ${file} ]]
+```
+
+* True if file exists and is readable.
+
+```bash
+[[ -r ${file} ]]
+```
+
+* True if file exists and has a size greater than zero.
+
+```bash
+[[ -s ${file} ]]
+```
+
+* True if file exists and is writable.
+
+```bash
+[[ -w ${file} ]]
+```
+
+* True if file exists and is executable.
+
+```bash
+[[ -x ${file} ]]
+```
+
+* True if file exists and is a symbolic link.
+
+```bash
+[[ -L ${file} ]]
+```
+
+## String expressions
+
+* True if the shell variable varname is set (has been assigned a value).
+
+```bash
+[[ -v ${varname} ]]
+```
+
+True if the length of the string is zero.
+
+```bash
+[[ -z ${string} ]]
+```
+
+True if the length of the string is non-zero.
+
+```bash
+[[ -n ${string} ]]
+```
+
+* True if the strings are equal. `=` should be used with the test command for POSIX conformance. When used with the `[[` command, this performs pattern matching as described above (Compound Commands).
+
+```bash
+[[ ${string1} == ${string2} ]]
+```
+
+* True if the strings are not equal.
+
+```bash
+[[ ${string1} != ${string2} ]]
+```
+
+* True if string1 sorts before string2 lexicographically.
+
+```bash
+[[ ${string1} < ${string2} ]]
+```
+
+* True if string1 sorts after string2 lexicographically.
+
+```bash
+[[ ${string1} > ${string2} ]]
+```
+
+## Arithmetic operators
+
+* Returns true if the numbers are **equal**
+
+```bash
+[[ ${arg1} -eq ${arg2} ]]
+```
+
+* Returns true if the numbers are **not equal**
+
+```bash
+[[ ${arg1} -ne ${arg2} ]]
+```
+
+* Returns true if arg1 is **less than** arg2
+
+```bash
+[[ ${arg1} -lt ${arg2} ]]
+```
+
+* Returns true if arg1 is **less than or equal** arg2
+
+```bash
+[[ ${arg1} -le ${arg2} ]]
+```
+
+* Returns true if arg1 is **greater than** arg2
+
+```bash
+[[ ${arg1} -gt ${arg2} ]]
+```
+
+* Returns true if arg1 is **greater than or equal** arg2
+
+```bash
+[[ ${arg1} -ge ${arg2} ]]
+```
+
+As a side note, arg1 and arg2 may be positive or negative integers.
+
+As with other programming languages you can use `AND` & `OR` conditions:
+
+```bash
+[[ test_case_1 ]] && [[ test_case_2 ]] # And
+[[ test_case_1 ]] || [[ test_case_2 ]] # Or
+```
+
+## Exit status operators
+
+* returns true if the command was successful without any errors
+
+```bash
+[[ $? -eq 0 ]]
+```
+
+* returns true if the command was not successful or had errors
+
+```bash
+[[ $? -gt 0 ]]
+```