File test operators in Bash

Following are the file test operators in Bash.

-e  -> file exists
-a -> file exists

This is identical in effect to -e. It has been "deprecated," [1] and its use is discouraged.

-f -> file is a regular file (not a directory or device file)
-s -> file is not zero size
-d -> file is a directory
-b -> file is a block device
-c -> file is a character device

device0="/dev/sda2"    # /   (root directory)
if [ -b "$device0" ]
then
  echo "$device0 is a block device."
fi

# /dev/sda2 is a block device.



device1="/dev/ttyS1"   # PCMCIA modem card.
if [ -c "$device1" ]
then
  echo "$device1 is a character device."
fi

# /dev/ttyS1 is a character device.

-p -> file is a pipe

function show_input_type()
{
   [ -p /dev/fd/0 ] && echo PIPE || echo STDIN
}

show_input_type "Input"                           # STDIN
echo "Input" | show_input_type                    # PIPE

# This example courtesy of Carl Anderson.

-h -> file is a symbolic link
-L -> file is a symbolic link
-S -> file is a socket
-t -> file (descriptor) is associated with a terminal device

This test option may be used to check whether the stdin [ -t 0 ] or stdout [ -t 1 ] in a given script is a terminal.

-r -> file has read permission (for the user running the test)
-w -> file has write permission (for the user running the test)
-x -> file has execute permission (for the user running the test)
-g -> set-group-id (sgid) flag set on file or directory

If a directory has the sgid flag set, then a file created within that directory belongs to the group that owns the directory, not necessarily to the group of the user who created the file. This may be useful for a directory shared by a workgroup.

-u -> set-user-id (suid) flag set on file

A binary owned by root with set-user-id flag set runs with root privileges, even when an ordinary user invokes it. [2] This is useful for executables (such as pppd and cdrecord) that need to access system hardware. Lacking the suid flag, these binaries could not be invoked by a non-root user.

          -rwsr-xr-t    1 root       178236 Oct  2  2000 /usr/sbin/pppd

A file with the suid flag set shows an s in its permissions.

-k -> sticky bit set

Commonly known as the sticky bit, the save-text-mode flag is a special type of file permission. If a file has this flag set, that file will be kept in cache memory, for quicker access. [3] If set on a directory, it restricts write permission. Setting the sticky bit adds a t to the permissions on the file or directory listing. This restricts altering or deleting specific files in that directory to the owner of those files.

          drwxrwxrwt    7 root         1024 May 19 21:26 tmp/

If a user does not own a directory that has the sticky bit set, but has write permission in that directory, she can only delete those files that she owns in it. This keeps users from inadvertently overwriting or deleting each other's files in a publicly accessible directory, such as /tmp. (The owner of the directory or root can, of course, delete or rename files there.)
-O -> you are owner of file
-G -> group-id of file same as yours
-N -> file modified since it was last read
f1 -nt f2
file f1 is newer than f2
f1 -ot f2
file f1 is older than f2
f1 -ef f2
files f1 and f2 are hard links to the same file
!
"not" -- reverses the sense of the tests above (returns true if condition absent).