There is a function provided by bash called test which returns a true or false value depending on the result of the tested expression. Its syntax is:
test expression
It can also be implied as follows:
[ expression ]
The tests below are test conditions provided by the shell:
-b file = True if the file exists and is block special file. -c file = True if the file exists and is character special file. -d file = True if the file exists and is a directory. -e file = True if the file exists. -f file = True if the file exists and is a regular file -g file = True if the file exists and the set-group-id bit is set. -k file = True if the files' "sticky" bit is set. -L file = True if the file exists and is a symbolic link. -p file = True if the file exists and is a named pipe. -r file = True if the file exists and is readable. -s file = True if the file exists and its size is greater than zero. -s file = True if the file exists and is a socket. -t fd = True if the file descriptor is opened on a terminal. -u file = True if the file exists and its set-user-id bit is set. -w file = True if the file exists and is writable. -x file = True if the file exists and is executable. -O file = True if the file exists and is owned by the effective user id. -G file = True if the file exists and is owned by the effective group id. file1 –nt file2 = True if file1 is newer, by modification date, than file2. file1 ot file2 = True if file1 is older than file2. file1 ef file2 = True if file1 and file2 have the same device and inode numbers. -z string = True if the length of the string is 0. -n string = True if the length of the string is non-zero. string1 = string2 = True if the strings are equal. string1 != string2 = True if the strings are not equal. !expr = True if the expr evaluates to false. expr1 –a expr2 = True if both expr1 and expr2 are true. expr1 –o expr2 = True is either expr1 or expr2 is true.
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