Logic

#!/bin/bash

# help test for list of options

ExampleGroup 'file tests'

 Example '[[ ... ]] if ... then '
    file_test() {
      if [[ -a /etc/passwd ]]
      then
        echo "file exists"
      else
        echo "no file"
      fi
    } 
    When call file_test
    The output should eq "file exists"
  End

 Example '-a same as -e file exists'
    file_test() {
      [[ -a /etc/passwd ]] && echo "file exists" || echo "no file"
    } 
    When call file_test
    The output should eq "file exists"
  End

 Example 'file is a block device (eg hard-drive or storage0)'
    file_test() {
      [[ -b /dev/nvme0n1p3 ]] && echo "block device" || echo "no file"
    } 
    When call file_test
    The output should eq "block device"
  End

 Example 'file is a character device (eg mouse, null)'
    file_test() {
      [[ -c /dev/null ]] && echo "character device" || echo "no file"
    } 
    When call file_test
    The output should eq "character device"
  End

 Example 'is a directory and exists'
    directory_test() {
      if [[ -d /etc ]]
      then
        echo "directory exists"
      else
        echo "no directory"
      fi
    } 
    When call directory_test
    The output should eq "directory exists"
  End

 Example '-e file exists'
    file_test() {
      if [[ -e /etc/passwd ]]
      then
        echo "file exists"
      else
        echo "no file"
      fi
    } 
    When call file_test
    The output should eq "file exists"
  End

 Example 'regular file exists'
    file_test() {
      if [[ -f /etc/passwd ]]
      then
        echo "regular file exists"
      else
        echo "no file"
      fi
    } 
    When call file_test
    The output should eq "regular file exists"
  End

 Example '-h same as -L symlink exists'
    file_test() {
      if [[ -h /dev/stdout ]]
      then
        echo "symlink exists"
      else
        echo "no file"
      fi
    } 
    When call file_test
    The output should eq "symlink exists"
  End

 Example 'directory has sticky bit set'
    file_test() {
      if [[ -k /tmp ]]
      then
        echo "sticky bit set"
      else
        echo "no sticky bit"
      fi
    } 
    When call file_test
    The output should eq "sticky bit set"
  End

 Example '-L symlink exists'
    file_test() {
      if [[ -L /dev/stdout ]]
      then
        echo "symlink exists"
      else
        echo "no file"
      fi
    } 
    When call file_test
    The output should eq "symlink exists"
  End

 Example 'file is a named pipe'
    file_test() {
      if [[ -p /var/run/dmeventd-client ]]
      then
        echo "named pipe"
      else
        echo "no file"
      fi
    } 
    When call file_test
    The output should eq "named pipe"
  End

 Example 'file exists and is not empty'
    file_test() {
      if [[ -s /etc/passwd ]]
      then
        echo "file exists"
      else
        echo "no file"
      fi
    } 
    When call file_test
    The output should eq "file exists"
  End

 Example '-S socket exists'
    file_test() {
      if [[ -S /run/dbus/system_bus_socket ]]
      then
        echo "socket exists"
      else
        echo "no file"
      fi
    } 
    When call file_test
    The output should eq "socket exists"
  End

End

Whenever you’re making a Bash script, you should always use [[ rather than [. — BashGuide

Predicates (aka boolean functions, conditionals, tests…) are generally within double square brackets [[...]] for text or double round brackets ((...)) for arithmetic. Historically they were within single brackets, and the double bracket notation comes from the Korn shell.

A key advantage of double square brackets is they support regex patterns using string =~ regex, and confusingly the syntax isn’t the same as Bash globbing. The manual has the details in the conditiional constructs section.

Whereas double quoting left-hand-side variables was required by [ and isn’t assuming there are no spaces in [[, it’s a good habit to double quote both sides.

Both the single and double bracketed tests support numerous flags to check if files exist, are empty and the same for strings and variables. The manual contains the full list in its conditional expressions section.

Pitfalls with `[[...]]`

Arrays implicitly concatenate in [[ ]]. Use a loop (or explicit * instead of @). SC2199
Brace expansion doesn’t happen in [[ ]]. Use a loop. SC2201
Globs are ignored in [[ ]] except right of =/!=. Use a loop. SC2203

An if can test any command, not just conditions enclosed within brackets. — Test Constructs of Advanced Bash-Scripting Guide.

if…then is just and

Bash is prologish in that the return status of functions can often be used navigate without if predicate; then ... fi:

if [[ -z $var ]]; then continue; fi

can be simplified to:

[[ -z $var ]] && continue

Logic

Pitfalls with [[...]]

if…then is just and

Pitfalls with `[[...]]`