BASH (shopt) Shell Options Explained

bash_shopt.md

Bash Shel Options ( shopt ) Explained

I wasted time WTFM so RTFM

Enable Option	Disable Option	List Enabled Options	Enabled Options Sample Output
shopt -s <opt>	shopt -u <opt>	echo $BASHOPTS	checkwinsize:cmdhist:complete_fullquote

This table illustrates the behavior of various shopt options in bash. By toggling these options, you can influence how the shell handles things like filename completion, history management, and error handling.

Option	Description	Disabled	Enabled	Example
assoc_expand_once	Suppress multiple evaluation of associative arrays during arithmetic expressions, variable assignments, and array dereferencing.	Normal evaluation	Only evaluates once	Consider an associative array data with data[key1]=value1 and data[key2]=value2. With this option disabled, result=$((data[key1] + data[key2])) would evaluate data[key1] and data[key2] twice. Enabling assoc_expand_once ensures the array elements are evaluated only once.
autocd	Execute directory names as cd commands (interactive shells only).	Manual cd required	Automatic cd	If autocd is on, typing documents in the terminal will change the working directory to "documents" as if you typed cd documents.
cdible_vars	Use variable values as directory for cd	Error for non-directories	Uses variable value	Suppose you have a variable project_dir set to /home/user/projects. With cdible_vars enabled, cd $project_dir would change the directory. If cdible_vars is off, this command would result in an error since $project_dir is not a valid directory by itself.
cdspell	Correct minor errors in directory spelling	Manual correction required	Suggests corrections	With cdspell on, a typo like teh_wrong_dir might be corrected to the_wrong_dir if that directory exists.
checkhash	Verify hashed commands exist before execution	Re-attempts execution	Skips execution	If a command is not found in the hash table (think command cache), bash normally searches the PATH directories. With checkhash enabled, bash would skip this search and throw an error instead.
checkjobs	List stopped/running jobs before exiting interactive shell	Silent exit	Lists jobs	When exiting a shell with exit, checkjobs (if enabled) will show a list of background jobs before prompting you to confirm exit.
checkwinsize	Update terminal dimensions after each command	Potentially outdated dimensions	Accurate dimensions	This option ensures the shell maintains an accurate representation of your terminal window size for features like cursor positioning.
cmdhist	Save multi-line commands as a single history entry	Separate entries for each line	Single entry for entire command	With cmdhist on, a multi-line command like git add .\n git commit -m "Fixed the bug" would be saved as one entry in the history.
complete_fullquote	Quote all shell metacharacters in filename completion	Metacharacters might be interpreted	Protects metacharacters	This option helps prevent unintended behavior when completing filenames with special characters like * or $.
direxpand	Replace directory names with their expanded forms during completion	Shows original directory name	Shows expanded path	Imagine a directory named proj*ect. With direxpand enabled, completing proj* might show the full path /home/user/projects.
dotglob	Include hidden files (starting with .) in filename expansion	Hidden files excluded	Hidden files included	If dotglob is on, ls would list files like .bashrc along with regular files.
expand_aliases	Expand aliases when referenced	Aliases not expanded	Aliases expanded	With expand_aliases enabled, using an alias like lg (defined as alias lg="grep") would trigger the grep command.
extglob	Enable extended pattern matching features	Basic pattern matching	More powerful matching	Extended patterns allow for complex matching criteria. For instance, shopt -s extglob enables patterns like *.{txt,jpg} to match files with .txt or .jpg extensions.
failglob	Errors on failed filename expansion patterns	Patterns might silently expand to empty	Errors shown for invalid patterns	This helps avoid unintended consequences when using patterns that might not match any files.
force_fignore	Consider ignored words during filename completion (even if only option)	Might complete ignored words	Ignores even only completion	This option ensures that even if a partially typed word is the only possible completion according to the FIGNORE variable, bash won't suggest it.
globstar	Allow '**' pattern to match directories recursively	Limited directory matching	Recursive matching	shopt -s globstar enables ** to match all files and subdirectories within a directory. For example, rm -rf dir/** would recursively delete everything inside the dir directory.
histappend	Append history to history file on exit	Overwrites history file	Maintains history across sessions	This ensures your command history is preserved for future use.
histverify	Ask for confirmation before using history substitutions	Automatic history substitution	User verification for substitutions	This can prevent accidental execution of unintended commands based on history substitutions.
inherit_errexit	Inherit errexit option in command substitutions	errexit unset in subshells	Maintains errexit behavior	This ensures subshells also exit on encountering errors if the main shell has errexit enabled.
interactive_comments	Allow comments starting with '#' in interactive shells	Comments require manual execution	Comments directly disable lines	This lets you temporarily disable lines in your shell script by adding # at the beginning.
lithist	Save multi-line commands with newlines (if cmdhist is enabled)	Uses semicolons to separate lines	Preserves newlines in multi-line history entries	This ensures better readability of multi-line commands stored in the shell history.
nocaseglob	Perform case-insensitive filename expansion	Case-sensitive matching	Case-insensitive matching	With nocaseglob enabled, FILE and File would match the same file.
nocasematch	Perform case-insensitive pattern matching	Case-sensitive matching	Case-insensitive matching	Similar to nocaseglob, but applies to broader pattern matching contexts.
nullglob	Expand unmatched filename patterns to empty string	Patterns might expand to themselves	Empty string for no matches	This avoids unintended behavior when working with patterns that might not match any files.
promptvars	Allow variable expansion and other operations in prompt strings	Limited prompt customization	Richer prompt customization	This enables you to create dynamic prompts that display things like the current directory or username.
shift_verbose	Print error message for excessive shift usage	Silent errors	Informs about invalid shifts	This helps identify potential errors in scripts that rely on shifting positional parameters.
sourcepath	Use PATH to locate files for the . (source) builtin	Might not find files not in PATH	Uses PATH for locating source files	This ensures the shell can find scripts to source even if they are not in the current directory.
varredir_close	Automatically close file descriptors opened using {varname} redirection	File descriptors stay open	Closes file descriptors	This helps prevent resource leaks by ensuring file descriptors opened using syntax like > {output_file} are closed after the command finishes.
xpg_echo	Enable backslash-escape sequence interpretation in echo by default	Manual escaping required	Automatic backslash interpretation	With xpg_echo enabled, echo \n would print a newline character because \n is interpreted as a newline escape sequence.