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 ${file} ]]
```
## String expressions
* True if the shell variable varname is set (has been assigned a value).
> Here, `varname` is the name of the variable. The `-v` operator expects a variable name as an argument rather than a value, so if you pass `${varname}` instead of `varname`, the expression will return false.
* 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
