${IFS}
Stands for Internal Field Separator in shell scripting. Itโs a special environment variable used in Bash (and similar shells) to define which characters are used to split a string into fields. By default, the characters used as separators are space, tab, and newline.
${PATH}
Defines the list of directories the system searches for executables when a command is entered.
${HOME}
Contains the path to the current userโs home directory.
${PWD}
Represents the current working directory.
${USER}
Holds the name of the current user.
${SHELL}
Displays the path of the command interpreter being used by the user.
${MAIL}
Indicates the location of the userโs mail.
${PS1}
Defines the primary command prompt (the symbol you see in the terminal).
${RANDOM}
Generates a random number between 0 and 32767 each time it is accessed.
${#variable}
Returns the length of the variable variable.
${variable:-default}
Returns the value of variable, or default if variable is not set or is empty.
${variable//pattern/replacement}
Replaces all occurrences of pattern in variable with replacement.
${!variable}
Expands to the value of the variable whose name is the value of variable.
${variable:offset:length}
Extracts a substring from variable, starting at offset and up to length characters.
${variable^}
Converts the first character of variable to uppercase.
${variable,,}
Converts all characters in variable to lowercase.
${variable^pattern}
Capitalizes the first character of each word that matches the specified pattern.
${variable,,pattern}
Converts all characters to lowercase for words matching the specified pattern.
${variable:-default}
Returns default if variable is unset or null; otherwise, it returns variable.
${variable+value}
Returns value if variable is set; otherwise, it returns nothing.
${variable:?error}
Returns variable if it is set and not null; otherwise, it prints error and exits.
${BASH_VERSION}
Displays the version of the Bash shell you are currently using.
${0}
Represents the name of the script or shell itself. This is useful for scripts to know how they were called.
${#}
Returns the number of positional parameters passed to the script or function.
$@
Represents all the positional parameters passed to the script or function, as separate words.
$*
Similar to $@, but treats all positional parameters as a single word.
$?
Contains the exit status of the last command executed. A value of 0 indicates success, while any non-zero value indicates an error.
$$
Represents the process ID (PID) of the current shell or script.
$!
Holds the PID of the last background command executed.
${FUNCNAME}
An array variable that holds the names of the current function and all functions in the call stack.
${LINENO}
Indicates the current line number in the script where it is referenced.
${HISTFILE}
Specifies the file in which command history is saved.
${HISTSIZE}
Determines the number of commands to remember in the command history.
${BASH_ENV}
A file that is sourced whenever a new non-interactive shell is started.
${PROMPT_COMMAND}
A command that is executed before the primary prompt is displayed.