How to use bash file testing flags?

This entry is reference copy of this great StackOverflow answer with some additional examples and description.

Bash File Testing

-b filename - Block special file
-c filename - Special character file
-d directoryname - Check for directory Existence
-e filename - Check for file existence, regardless of type (node, directory, socket, etc.)
-f filename - Check for regular file existence not a directory
-G filename - Check if file exists and is owned by effective group ID
-G filename set-group-id - True if file exists and is set-group-id
-k filename - Sticky bit
-L filename - Symbolic link
-O filename - True if file exists and is owned by the effective user id
-r filename - Check if file is a readable
-S filename - Check if file is socket
-s filename - Check if file is nonzero size
-u filename - Check if file set-user-id bit is set
-w filename - Check if file is writable
-x filename - Check if file is executable

How to use:

#!/bin/bash
file=./file
if [ -e "$file" ]; then
    echo "File exists"
else 
    echo "File does not exist"
fi

A test expression can be negated by using the ! operator

#!/bin/bash
file=./file
if [ ! -e "$file" ]; then
    echo "File does not exist"
else 
    echo "File exists"
fi

The flags can be combined using && expression. Each test need to be in it's own square brackets. Example below shows how to check if file is readable and writable:

#!/bin/bash
file=./file
if [ -r "$file" ] && [ -w "$file" ]; then
    echo "File is readable and writable"
else
    echo "File is not readable or writable"
fi

Check if it's real directory, not symlinked one:

#!/bin/bash
file=./file
if [ -d "$file" ] && [ ! -L "$file" ]; then
    echo "Real directory"
else
    echo "Not real directory, possibly symbolic link"
fi

Further Explanation of Each Flag

• -b filename: Checks for block special files like hard drives and other block devices. These are files that provide buffered I/O access.
• -c filename: Checks for character special files such as terminals or keyboards. These files provide unbuffered, direct access to hardware devices.
• -d directoryname: Verifies if the specified directory exists. This will also evaluate to true for symlinks to directories.
• -e filename: Checks for the existence of a file regardless of its type.
• -f filename: Determines if the specified file is a regular file (not a directory, device, etc.).
• -G filename: Confirms if the file exists and is owned by the effective group ID of the user running the script.
• -g filename: Checks if the set-group-ID (SGID) bit is set on the file, which means it will be executed with the permissions of the group that owns the file.
• -k filename: Tests if the sticky bit is set, which affects directory deletion permissions.
• -L filename: Checks if the file is a symbolic link.
• -O filename: Verifies if the file is owned by the effective user ID.
• -r filename: Tests if the file is readable by the user running the test.
• -S filename: Checks if the file is a socket, which is used for inter-process communication.
• -s filename: Determines if the file size is greater than zero.
• -u filename: Checks if the set-user-ID (SUID) bit is set, which means the file will be executed with the permissions of the owner of the file.
• -w filename: Tests if the file is writable by the user running the test.
• -x filename: Determines if the file is executable by the user running the test.