See [man test](https://linux.die.net/man/1/test).
```
[ "$A" = "Mr Flibble" ]     # is the same as
test "$A" = "Mr Flibble"
```
```
( EXPRESSION )              #  EXPRESSION is true
! EXPRESSION                #  EXPRESSION is false
EXPRESSION1 -a EXPRESSION2  #  both EXPRESSION1 and EXPRESSION2 are true
EXPRESSION1 -o EXPRESSION2  #  either EXPRESSION1 or EXPRESSION2 is true
-n STRING                   #  the length of STRING is nonzero
STRING                      #  equivalent to -n STRING
-z STRING                   #  the length of STRING is zero
STRING1 = STRING2           #  the strings are equal
STRING1 != STRING2          #  the strings are not equal
INTEGER1 -eq INTEGER2       #  INTEGER1 is equal to INTEGER2
INTEGER1 -ge INTEGER2       #  INTEGER1 is greater than or equal to INTEGER2
INTEGER1 -gt INTEGER2       #  INTEGER1 is greater than INTEGER2
INTEGER1 -le INTEGER2       #  INTEGER1 is less than or equal to INTEGER2
INTEGER1 -lt INTEGER2       #  INTEGER1 is less than INTEGER2
INTEGER1 -ne INTEGER2       #  INTEGER1 is not equal to INTEGER2
FILE1 -ef FILE2             #  FILE1 and FILE2 have the same device and inode numbers
FILE1 -nt FILE2             #  FILE1 is newer (modification date) than FILE2
FILE1 -ot FILE2             #  FILE1 is older than FILE2
-b FILE                     #  FILE exists and is block special
-c FILE                     #  FILE exists and is character special
-d FILE                     #  FILE exists and is a directory
-e FILE                     #  FILE exists
-f FILE                     #  FILE exists and is a regular file
-g FILE                     #  FILE exists and is set-group-ID
-G FILE                     #  FILE exists and is owned by the effective group ID
-h FILE                     #  FILE exists and is a symbolic link (same as -L)
-k FILE                     #  FILE exists and has its sticky bit set
-L FILE                     #  FILE exists and is a symbolic link (same as -h)
-O FILE                     #  FILE exists and is owned by the effective user ID
-p FILE                     #  FILE exists and is a named pipe
-r FILE                     #  FILE exists and read permission is granted
-s FILE                     #  FILE exists and has a size greater than zero
-S FILE                     #  FILE exists and is a socket
-t FD                       #  file descriptor FD is opened on a terminal
-u FILE                     #  FILE exists and its set-user-ID bit is set
-w FILE                     #  FILE exists and write permission is granted
-x FILE                     #  FILE exists and execute (or search) permission is granted
```