$# expands to the number of positional parameters in decimal.

If PROMPT_COMMAND is set, the value is interpreted as a command to execute before the printing of each primary prompt ($PS1).

Yes, nameref variables can reference array variables and subscripted array variables.

BASHPID expands to the process id of the current Bash process. This differs from $$ under certain circumstances, such as subshells that do not require Bash to be re-initialized.

A zero-length (null) directory name in the value of PATH indicates the current directory. A null directory name may appear as two adjacent colons, or as an initial or trailing colon.

If set to a number greater than zero, PROMPT_DIRTRIM is used as the number of trailing directory components to retain when expanding the \w and \W prompt string escapes. Characters removed are replaced with an ellipsis.

When used in the text of the message, $_ expands to the name of the current mail file.

The default value of HISTFILESIZE is 500. This is the maximum number of lines contained in the history file.

GROUPS is an array variable containing the list of groups of which the current user is a member.

Yes, BASHOPTS is readonly.

TIMEFORMAT='%R' displays the elapsed time in seconds.

Unsetting BASH_XTRACEFD or assigning it the empty string causes the trace output to be sent to the standard error.

EUID contains the numeric effective user id of the current user. This variable is readonly.

PWD is the current working directory as set by the cd builtin.

The optional 'l' specifies a longer format, including minutes, of the form MMmSS.FFs. The value of p determines whether or not the fraction is included.

BASH_SOURCE is an array variable whose members are the source filenames corresponding to the elements in the FUNCNAME array variable.

Yes, SHELLOPTS is readonly.

SHLVL is incremented by one each time a new instance of Bash is started. This is intended to be a count of how deeply your Bash shells are nested.

A variable is a parameter denoted by a name. A variable has a value and zero or more attributes.

HISTCONTROL='ignoreboth' is shorthand for both 'ignorespace' and 'ignoredups'.

BASH_REMATCH is an array variable whose members are assigned by the '=~' binary operator to the [[ conditional command. The element with index 0 is the portion of the string matching the entire regular expression. The element with index n is the portion of the string matching the nth parenthesized subexpression.

The default value of HISTFILE is ~/.bash_history. This is the file to which the command history is saved.

FCEDIT is the editor used as a default by the -e option to the fc builtin command.

When $@ expansion occurs within double quotes and word splitting is performed, each parameter expands to a separate word. That is, '$@' is equivalent to '$1' '$2' …

IGNOREEOF controls the action of the shell on receipt of an EOF character as the sole input. If set, the value denotes the number of consecutive EOF characters that can be read as the first character on an input line before the shell will exit.

HISTCMD is the history number, or index in the history list, of the current command.

COLUMNS is automatically set upon receipt of a SIGWINCH signal.

A HISTIGNORE pattern of '[ ]*' is identical to ignorespace.

LINES is used by the select builtin command to determine the column length for printing selection lists. It is automatically set upon receipt of a SIGWINCH.

BASH_COMMAND is the command currently being executed or about to be executed, unless the shell is executing a command as the result of a trap, in which case it is the command executing at the time of the trap.

No, assignment to DIRSTACK will not change the current directory.

If PS3 is not set, the select command prompts with '#? '.

MACHTYPE is a string that fully describes the system type on which Bash is executing, in the standard gnu cpu-company-system format.

When HISTCONTROL includes 'ignorespace', lines which begin with a space character are not saved in the history list.

BASH_VERSINFO[0] contains the major version number (the release).

If set to a value greater than zero, TMOUT is treated as the default timeout for the read builtin.

DIRSTACK is an array variable containing the current contents of the directory stack. Directories appear in the stack in the order they are displayed by the dirs builtin.

FUNCNAME is an array variable containing the names of all shell functions currently in the execution call stack. The element with index 0 is the name of any currently-executing shell function. The bottom-most element is 'main'.

INPUTRC is the name of the Readline initialization file, overriding the default of ~/.inputrc.

HISTIGNORE is a colon-separated list of patterns used to decide which command lines should be saved on the history list.

When += is applied to a variable with the integer attribute set, the variable's current value and the new value are each evaluated as arithmetic expressions, and the sum is assigned as the variable's value.

$? expands to the exit status of the most recently executed command.

HOSTFILE contains the name of a file in the same format as /etc/hosts that should be read when the shell needs to complete a hostname.

histchars contains up to three characters which control history expansion, quick substitution, and tokenization. The first character is the history expansion character (normally '!'), the second is quick substitution (normally '^'), and the optional third is the history comment character (usually '#').

If Bash is started with the -c option, $0 is set to the first argument after the string to be executed, if one is present. Otherwise, it is set to the filename used to invoke Bash.

If POSIXLY_CORRECT is in the environment when bash starts, the shell enters posix mode before reading the startup files, as if the --posix invocation option had been supplied.

When there are no positional parameters, '$@' and $@ expand to nothing (i.e., they are removed).

The shell sets BASH_ARGC only when in extended debugging mode (extdebug option to the shopt builtin).

Setting BASH_XTRACEFD to 2 (the standard error file descriptor) and then unsetting it will result in the standard error being closed.

If HOSTFILE is set, but has no value, or does not name a readable file, Bash attempts to read /etc/hosts to obtain the list of possible hostname completions.

When += is applied to an array variable using compound assignment, the variable's value is not unset (as it is when using '='), and new values are appended to the array beginning at one greater than the array's maximum index for indexed arrays, or added as additional key-value pairs in an associative array.

If value is not given, the variable is assigned the null string.

If set to an integer corresponding to a valid file descriptor, Bash will write the trace output generated when 'set -x' is enabled to that file descriptor.

No, FUNCNAME exists only when a shell function is executing.

If IFS is unset, the parameters in $* are separated by spaces.

BASH_CMDS is an associative array variable whose members correspond to the internal hash table of commands as maintained by the hash builtin.

$0 expands to the name of the shell or shell script, set at shell initialization.

Assigning a value to RANDOM seeds the random number generator.

$! expands to the process ID of the job most recently placed into the background, whether executed as an asynchronous command or using the bg builtin.

UID contains the numeric real user id of the current user. This variable is readonly.

OPTIND is initialized to 1 each time the shell or a shell script is invoked.

The shell sets BASH_ARGV only when in extended debugging mode (extdebug option to the shopt builtin).

FIGNORE is a colon-separated list of suffixes to ignore when performing filename completion. A sample value is '.o:~'.

The initial value of BASH_SUBSHELL is 0.

The default value of PS4 is '+ '. This is the prompt printed before the command line is echoed when the -x option is set.

BASH_VERSINFO[2] contains the patch level.

LC_COLLATE determines the collation order used when sorting the results of filename expansion, and determines the behavior of range expressions, equivalence classes, and collating sequences within filename expansion and pattern matching.

Yes, if HISTTIMEFORMAT is set, timestamps are written to the history file so they may be preserved across shell sessions.

COMP_LINE is the current command line. This variable is available only in shell functions and external commands invoked by the programmable completion facilities.

A nameref is a reference to another variable, created using the -n option to declare or local. Whenever the nameref variable is referenced, assigned to, unset, or has its attributes modified, the operation is actually performed on the variable specified by the nameref variable's value.

When HISTCONTROL includes 'erasedups', all previous lines matching the current line are removed from the history list before that line is saved.

TIMEFORMAT='%S' displays the number of CPU seconds spent in system mode.

GLOBIGNORE is a colon-separated list of patterns defining the set of filenames to be ignored by filename expansion. If a filename matched by a filename expansion pattern also matches one of the patterns in GLOBIGNORE, it is removed from the list of matches.

BASH_LINENO is an array variable whose members are the line numbers in source files corresponding to each member of FUNCNAME.

The first character of PS4 is replicated multiple times, as necessary, to indicate multiple levels of indirection when tracing with set -x.

If DIRSTACK is unset, it loses its special properties, even if it is subsequently reset.

LC_CTYPE determines the interpretation of characters and the behavior of character classes within filename expansion and pattern matching.

If COMP_WORDBREAKS is unset, it loses its special properties, even if it is subsequently reset.

If the variable exists but does not have a numeric value (or has no value), the default is 10.

HISTTIMEFORMAT uses the history comment character to distinguish timestamps from other history lines.

OLDPWD is the previous working directory as set by the cd builtin.

In HISTIGNORE patterns, '&' matches the previous history line.

When += is applied to a string-valued variable, the value is expanded and appended to the variable's value.

If FUNCNAME is unset, it loses its special properties, even if it is subsequently reset.

BASH_VERSINFO[3] contains the build version.

COMP_TYPE is set to an integer value corresponding to the type of completion attempted: TAB for normal completion, '?' for listing completions after successive tabs, '!' for listing alternatives on partial word completion, '@' to list completions if the word is not unmodified, or '%' for menu completion.

If POSIXLY_CORRECT is set while the shell is running, bash enables posix mode, as if the command 'set -o posix' had been executed.

A parameter is an entity that stores values. It can be a name, a number, or one of the special characters listed in the Bash documentation.

At most three places after the decimal point may be specified for the precision p; values of p greater than 3 are changed to 3.

If Bash is invoked with a file of commands, $0 is set to the name of that file.

A HISTIGNORE pattern of '&' is identical to ignoredups.

When HISTCONTROL includes 'ignoredups', lines which match the previous history entry are not saved.

TIMEFORMAT='%P' displays the CPU percentage, computed as (%U + %S) / %R.

BASH_VERSINFO[4] contains the release status (e.g., beta1).

BASH_VERSINFO[1] contains the minor version number (the version).

Each list entry in MAILPATH can specify the message printed when new mail arrives by separating the filename from the message with a '?'.

Attributes are assigned using the declare builtin command. The export and readonly builtins assign specific attributes.

If HISTTIMEFORMAT is set and not null, its value is used as a format string for strftime to print the timestamp associated with each history entry displayed by the history builtin.

Assignment to SECONDS resets the count to the value assigned, and the expanded value becomes the value assigned plus the number of seconds since the assignment.

SHELL contains the full pathname to the shell. If it is not set when the shell starts, Bash assigns to it the full pathname of the current user's login shell.

If IFS is null, the parameters in $* are joined without intervening separators.

MAILCHECK specifies how often (in seconds) the shell should check for mail. The default is 60 seconds.

If Bash finds EMACS in the environment when the shell starts with value 't', it assumes the shell is running in an emacs shell buffer and disables line editing.

The default value of PS2 is '> '. This is the secondary prompt string.

LANG is used to determine the locale category for any category not specifically selected with a variable starting with LC_.

The default value of HISTSIZE is 500. This is the maximum number of commands to remember on the history list.

If set, Bash uses TMPDIR's value as the name of a directory in which Bash creates temporary files for the shell's use.

$$ expands to the process ID of the shell. In a subshell, it expands to the process ID of the invoking shell, not the subshell.

If HISTCMD is unset, it loses its special properties, even if it is subsequently reset.

Each time RANDOM is referenced, a random integer between 0 and 32767 is generated.

If BASH_ENV is set when Bash is invoked to execute a shell script, its value is expanded and used as the name of a startup file to read before executing the script.

COMPREPLY is an array variable from which Bash reads the possible completions generated by a shell function invoked by the programmable completion facility.

COMP_CWORD is an index into ${COMP_WORDS} of the word containing the current cursor position. This variable is available only in shell functions invoked by the programmable completion facilities.

If GROUPS is unset, it loses its special properties, even if it is subsequently reset.

BASH_SUBSHELL is incremented by one each time a subshell or subshell environment is spawned.

BASHOPTS is a colon-separated list of enabled shell options. Each word in the list is a valid argument for the -s option to the shopt builtin command.

When $* expansion occurs within double quotes, it expands to a single word with the value of each parameter separated by the first character of the IFS variable.

Namerefs can be unset using the -n option to the unset builtin. Otherwise, if unset is executed with the name of a nameref variable as an argument, the variable referenced by the nameref variable is unset.

COLUMNS is used by the select builtin command to determine the terminal width when printing selection lists. It is automatically set upon receipt of a SIGWINCH.

No, array variables cannot be given the nameref attribute.

If p is not specified, the value 3 is used for the precision.

TIMEFORMAT='%U' displays the number of CPU seconds spent in user mode.

COMP_WORDS is an array variable consisting of the individual words in the current command line, split as Readline would split it using COMP_WORDBREAKS.

The default value of PS1 is '\s-\v$ '. This is the primary prompt string.

If set to the value 1, Bash displays error messages generated by the getopts builtin command.

No, assignments to FUNCNAME have no effect and return an error status.

If TIMEFORMAT is not set, Bash acts as if it had the value $'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'.

COMP_KEY is the key (or final key of a key sequence) used to invoke the current completion function.

PIPESTATUS is an array variable containing a list of exit status values from the processes in the most-recently-executed foreground pipeline.

LINENO is the line number in the script or shell function currently executing.

The second and subsequent lines of a multi-line compound command are not tested by HISTIGNORE and are added to the history regardless of the value of HISTIGNORE.

HOSTNAME is the name of the current host.

If MAILCHECK is unset, or set to a value that is not a number greater than or equal to zero, the shell disables mail checking.

PPID contains the process id of the shell's parent process. This variable is readonly.

BASH_ARGV is an array variable containing all of the parameters in the current bash execution call stack.

REPLY is the default variable for the read builtin.

BASH_ALIASES is an associative array variable whose members correspond to the internal list of aliases as maintained by the alias builtin.

LC_ALL overrides the value of LANG and any other LC_ variable specifying a locale category.

The bottom-most element of FUNCNAME is 'main'.

BASH_VERSINFO[5] contains the value of MACHTYPE.

No, assignments to GROUPS have no effect and return an error status.

LC_NUMERIC determines the locale category used for number formatting.

Yes, BASH_VERSINFO is a readonly array variable.

COMP_POINT is the index of the current cursor position relative to the beginning of the current command. If the current cursor position is at the end of the current command, the value equals ${#COMP_LINE}.

COMP_WORDBREAKS is the set of characters that the Readline library treats as word separators when performing word completion.

OSTYPE is a string describing the operating system Bash is running on.

LC_MESSAGES determines the locale used to translate double-quoted strings preceded by a '$' (see Locale Translation).

The default path is system-dependent and is set by the administrator who installs bash. A common value is '/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:/sbin'.

In an interactive shell, TMOUT is interpreted as the number of seconds to wait for input after issuing the primary prompt. Bash terminates after that number of seconds if input does not arrive.

SHELLOPTS is a colon-separated list of enabled shell options. Each word in the list is a valid argument for the -o option to the set builtin command.

HOSTTYPE is a string describing the machine Bash is running on.

bash -O extdebug enables extended debugging mode. This provides additional debugging features: the BASH_ARGC, BASH_ARGV, BASH_SOURCE, and BASH_LINENO variables are set, trap handlers receive additional info, and the DEBUG trap is inherited by shell functions.

bash -O cdable_vars allows cd to treat a variable as a directory name if the directory doesn't exist. For example, if mydir='/home/user/docs', then cd mydir would cd to /home/user/docs. The variable value must be an absolute path.

No, -c and -s options are mutually exclusive. The -c option reads commands from a string argument, while -s reads from standard input. You can only use one or the other, not both.

set -e (errexit) does NOT cause exit on: commands in if/while/until conditions, commands whose exit status is being tested with !, part of && or || chains (except the last), piped commands (unless pipefail is set), or any command in a compound command except the last. The ERR trap also overrides this behavior.

The -x option causes bash to print a trace of simple commands, for commands, case commands, select commands, and arithmetic for commands and their arguments or associated word lists after they are expanded and before they are executed. Equivalent to 'set -x' or 'set -o xtrace'.

An interactive login shell reads /etc/profile first, then looks for ~/.bash_profile, ~/.bash_login, and ~/.profile in order, executing the first one found and ignoring the others. When a login shell exits, it reads ~/.bash_logout if it exists.

bash -O huponexit causes bash to send SIGHUP to all jobs when an interactive login shell exits. This terminates background jobs. Disabled by default.

set -o keyword (or -k) causes all argument assignments to be placed in the environment for a command, not just those before the command name. This allows 'VAR=value command' anywhere in the command line.

bash -O shift_verbose causes the shift builtin to print an error message when the shift count exceeds the number of positional parameters. Without this, silent failure occurs.

set -o allexport (or -a) causes all variable definitions to be automatically exported to the environment of child commands. Any variable assignment or modification will be exported automatically without needing 'export' command.

When bash is invoked as 'sh' or with --posix, and running interactively or with a shell script, it uses the value of ENV variable to name the startup file. Bash expands $ENV and executes commands from that file if it exists and is readable.

Bash starts as a login shell if: invoked with -l or --login option, invoked with argument zero (program name) starting with -, or invoked as a login shell by the system (e.g., from /bin/login). Login shells read /etc/profile and ~/.bash_profile.

The -h option causes bash to remember the location of commands as they are looked up for execution. This is enabled by default and is equivalent to 'set -h' or 'set -o hashall'. It affects command hash table behavior.

The --noprofile option prevents bash from reading the system-wide startup file /etc/profile and any personal startup files (~/.bash_profile, ~/.bash_login, or ~/.profile) for a login shell. This skips all profile initialization.

set -o xtrace (or -x) enables debugging mode where commands are printed after expansion but before execution. Each line is prefixed with PS4 (default: +).

The -f option disables pathname expansion (globbing). This prevents special characters like *, ?, and [...] from being expanded to matching filenames. This is equivalent to 'set -f' or 'set -o noglob'.

bash -O cmdhist causes bash to save multi-line commands in a single history entry. This is enabled by default, allowing easy recall of multi-line commands. Without it, each line is a separate history entry.

set -o noexec (or -n) causes commands to be read but not executed. This is useful for syntax checking scripts. Shell syntax is parsed, but commands are not run.

bash -O dotglob includes filenames beginning with . in the results of pathname expansion. By default, * doesn't match dotfiles. With dotglob, ls * would show .bashrc and other hidden files.

set -o errtrace (or -E) causes ERR traps to be inherited by shell functions, command substitutions, and subshells. Without this, ERR traps only trigger in the main shell, not in these contexts.

-e (errexit) causes bash to exit if any command in a script fails (non-zero exit status), but only checks the last command in a pipeline by default. -o pipefail makes the pipeline exit status be the rightmost non-zero exit, so -e will trigger if any command in the pipe fails.

When bash is invoked with the name 'rbash' (e.g., via symlink or rename), it starts as a restricted shell. This is equivalent to invoking bash with the -r or --restricted option.

bash -O expand_aliases enables alias expansion. Aliases are expanded by default in interactive shells but not in non-interactive shell scripts. This option enables alias expansion in scripts if needed.

The -n option causes bash to read commands but not execute them. This is useful for checking shell scripts for syntax errors. Equivalent to 'set -n' or 'set -o noexec'. bash -n script.sh will check syntax without running.

The --login option is equivalent to -l. It makes bash act as if it had been invoked as a login shell, causing it to read and execute login shell startup files like /etc/profile and ~/.bash_profile.

The -T option causes bash to exit if a pipeline command (not just the entire pipeline) returns a non-zero status. This is stronger than set -e which only looks at the last command in a pipeline. Equivalent to 'set -T' or 'set -o pipefail'.

When invoked with -c, bash runs the command string and does NOT read any startup files. It's a non-interactive, non-login shell. However, if BASH_ENV is set and bash is not in POSIX mode, the file specified by BASH_ENV will be sourced before executing the command.

set -o braceexpand enables brace expansion (the default). This allows patterns like {a,b,c} or {1..5} to expand to multiple words. Disabling with +o braceexpand or set +B turns off this feature.

bash -O failglob causes patterns that match no files to cause an error message rather than being left unexpanded. This is useful for catching typos in glob patterns.

Bash processes options in the order they appear on the command line, but certain options like -c affect behavior immediately. Options like -O, -o can set shell options. After option processing, remaining arguments are: if -c was used, the command string and remaining args become $0, $1, etc.; if no -c, the first remaining arg is the script filename, rest become positional parameters.

The --dump-strings option is equivalent to -D. It causes bash to list all double-quoted strings prefixed with $ to standard output without executing the script. These strings are subject to language translation.

set -o emacs enables Emacs-style command-line editing in the interactive shell. This uses key bindings similar to Emacs editor (Ctrl+F for forward, Ctrl+B for backward, etc.). This is typically the default.

bash -O lastpipe causes the last command in a pipeline to be executed in the current shell, not a subshell. This allows variables set in the last pipeline command to persist after the pipeline completes. Only works when job control is disabled.

The --init-file option is a synonym for --rcfile. It takes a filename as an argument and causes bash to execute commands from that file instead of ~/.bashrc for an interactive shell.

set -o noclobber (or -C) prevents shell redirection from overwriting existing regular files. The > operator will fail if the file exists. Use >| to override.

The -l option makes bash act as if it had been invoked as a login shell. A login shell reads certain startup files like /etc/profile, ~/.bash_profile, ~/.bash_login, or ~/.profile (whichever is found first, the rest are ignored).

When bash is started non-interactively (e.g., to run a shell script), if the environment variable BASH_ENV is set and non-empty, bash expands its value and uses that file as startup file, executing commands from it (like sourcing it). This is similar to ENV in POSIX sh.

bash -O xpg_echo causes the echo builtin to expand backslash escape sequences by default, without requiring -e flag. For example, echo 'hello\nworld' would print an actual newline. Without this option, echo doesn't expand escapes unless -e is used.

When bash is invoked with --posix, it follows POSIX startup rules: as a login shell, it reads /etc/profile and then ~/.profile (not ~/.bash_profile). For non-interactive shells, it checks ENV variable instead of BASH_ENV.

bash -O histverify causes the results of history substitution to be shown for review before execution. The substituted command is loaded into readline buffer, requiring Enter to execute. This prevents accidental execution of wrong history substitutions.

bash -O localvar_inherit causes a local variable in a function to inherit the value and attributes of a variable with the same name at the previous scope. Without this, local variables start unset.

When a login shell is started, the first character of argument zero (the program name) is typically a hyphen (-). This tells bash to read startup files like /etc/profile and ~/.bash_profile. Programs like login traditionally do this.

The --dump-po-strings option outputs strings in GNU gettext PO format with each translatable string formatted as:
#: filename:line
msgid "original string"
msgstr ""

This creates a valid .po file suitable for translation tools.

bash -O histappend causes the history list to be appended to $HISTFILE when the shell exits, rather than overwriting the file. This preserves history from multiple concurrent shell sessions.

bash -O lithist causes newlines in multi-line commands to be saved in the history with embedded newlines rather than using semicolons. This preserves the visual structure of commands in history.

The -t option causes bash to exit after reading and executing one command. Equivalent to 'set -t' or 'set -o onecmd'. This is useful for single-command execution from a pipe or file.

The --posix option changes bash behavior to match the POSIX standard more strictly. This affects: default value of ENV variable, special variables handling, startup files, disabling non-POSIX extensions, and more. It can also be enabled with 'set -o posix' within a running shell.

bash -O direxpand causes directory names to be expanded during word completion. For example, Tab completion of ~/doc would expand to ~/Documents/ rather than keeping it as ~/doc.

set -o pipefail (or -T) causes a pipeline to return the exit status of the last command in the pipe that exited with a non-zero status, or zero if all commands exited successfully. Without this, the pipeline returns the exit status of the last command only.

bash -O extglob enables extended pattern matching features. This allows patterns like !(pattern) - match anything except pattern, @(pattern1|pattern2) - match exactly one, *(pattern) - match zero or more, +(pattern) - match one or more, ?(pattern) - match zero or one.

bash -O checkwinsize checks the window size after each command and updates LINES and COLUMNS if the window size has changed. This is enabled by default in interactive shells.

bash -O globasciiranges causes range expressions in glob patterns like [a-z] to be interpreted in ASCII alphabetical order rather than using the current locale's collation order. This is the default in POSIX mode.

bash -O cdspell corrects minor spelling errors in directory names for cd command. For example, cd /usr/locl might autocorrect to /usr/local. It only works when the corrected component exists.

Use 'set -o' without arguments to display all current shell options and their settings. Use 'set -o optionname' to enable an option, 'set +o optionname' to disable. Use '+o' to display options in a format that can be re-used as input.

bash -O histreedit causes a failed history substitution to be reloaded into the readline editing buffer for correction. Without this, a failed history substitution (like !$ when no previous command) results in an error.

bash -O progcomp_alias causes programmable completion to also attempt alias expansion when completing. This allows completion to work with aliases.

set -o history enables command history (enabled by default). This allows commands to be saved to the history list and recalled. Disabling with +o history or set +H turns off command history saving.

The -D option causes bash to list all double-quoted strings prefixed with $ to standard output. These are the strings that are subject to language translation when the current locale is not C or POSIX. This does not execute the script but extracts translatable strings.

The -c option causes bash to read and execute commands from the first non-option argument argument (which must be present), then exit. Any remaining arguments after the command string are assigned to positional parameters starting at $0. Example: bash -c 'echo hello' world

The -v option causes shell input lines to be echoed as they are read. This shows each line before it's executed, useful for debugging. Equivalent to 'set -v' or 'set -o verbose'. Different from -x which shows the expanded command.

set -o onecmd (or -t) causes bash to exit after reading and executing a single command. This is useful for processing one command at a time from input.

When used with set, -- signifies the end of options. After --, all remaining arguments are assigned to positional parameters ($1, $2, etc.) even if they start with -. This allows setting positional parameters that look like options.

The -i option specifies that the shell should be interactive. This forces bash to behave as an interactive shell even if standard input is not a terminal. An interactive shell enables job control, reads commands from stdin with readline, and displays primary and secondary prompts.

bash -O progcomp enables programmable completion. This is the default and allows bash to use custom completion functions specified with complete and compgen builtins.

bash -O dirspell attempts spelling correction on directory names during completion. Similar to cdspell but works during tab completion, not just cd command.

set -o errexit (-e) exits if a pipeline command returns non-zero, except when the exit status is being tested (if, while, until, !, ||, &&), part of compound command except last, or part of any command in a && or || list except the last. It's disabled in subshells by default unless ERR trap is set.

set +o without arguments displays all current shell options in a format that can be re-input to the shell to restore the current settings. This is useful for saving and restoring option states.

Use 'shopt' without arguments to display all current shopt options and their settings. Use 'shopt -s optionname' to enable (set) an option, 'shopt -u optionname' to disable (unset). Use 'shopt -p' to display in a reusable format.

The --norc option prevents bash from reading the ~/.bashrc initialization file for an interactive shell. It's equivalent to setting --norc or skipping the standard personal rc file.

set -o noclobber (or -C) prevents redirection from overwriting existing files. Using > file will fail if file exists. To force overwrite, use >| instead. This protects against accidental data loss when redirecting output.

bash -O promptvars enables prompt string variable expansion and command substitution in prompt strings like PS1. This is enabled by default. Disabling treats prompts as literal strings without expanding $ variables.

bash -O hostcomplete enables hostname completion when completing words containing @. Tab completion attempts to complete hostnames from /etc/hosts. This is enabled by default in interactive shells.

set -o noglob (or -f) disables pathname expansion (globbing). Wildcards like *, ?, and [...] are not expanded to matching filenames.

When -a is used with set (or bash -a), it automatically export all variables that are subsequently defined or modified. This is equivalent to 'set -a' or 'set -o allexport'. Note: this is typically used with set, not as a direct bash invocation option.

Many bash invocation options can be toggled after startup using the 'set' builtin. For example, 'set -x' enables tracing like -x, 'set +x' disables it. 'set -o errexit' enables -e behavior. However, some options like --norc, --noprofile, --rcfile only affect startup and cannot be changed later.

bash -v (verbose) prints shell input lines as they are read, before any expansion. bash -x (xtrace) prints commands after expansion but before execution. So -v shows what's in the script file, while -x shows what bash will actually execute with variables expanded.

bash -O nullglob causes patterns that match no files to expand to a null string rather than the pattern itself. Without this, echo .nonexistent prints '.nonexistent'; with nullglob it prints nothing.

set -o nounset (or -u) causes unset variables to be treated as an error when performing parameter expansion. An error is printed to stderr and the shell exits.

bash +O optionname disables (unsets) a shopt option. For example, bash +O extglob script.sh disables extended globbing. The + prefix means 'disable' while -O means 'enable'.

The -P option causes bash to not follow symbolic links when executing commands like cd that change the current working directory. It uses the physical directory structure instead. Equivalent to 'set -P' or 'set -o physical'.

set -o vi enables Vi-style command-line editing in the interactive shell instead of Emacs mode. This uses key bindings similar to the vi editor (ESC to enter command mode, i for insert mode, etc.).

set -o functrace (or -T) causes DEBUG and RETURN traps to be inherited by shell functions and command substitutions. This enables debugging in function calls.

The -k option causes all arguments assignment statements to be placed in the environment for a command, not just those preceding the command name. This allows variable assignments anywhere in the command line. Equivalent to 'set -k' or 'set -o keyword'.

The -B option enables brace expansion (equivalent to set -o braceexpand). This is enabled by default and allows patterns like {a,b,c} to expand to 'a b c' or {1..5} to '1 2 3 4 5'.

shopt -q optionname suppresses output and returns exit status 0 if the option is enabled, 1 if disabled. This is useful for testing option state in scripts without printing anything.

The -C option prevents bash from overwriting existing files with > redirection. You can still override this with >| operator. Equivalent to 'set -C' or 'set -o noclobber'. This provides protection against accidental file overwrites.

bash -O checkhash causes bash to check if a command found in the hash table still exists before executing it. If the command has been deleted, it's searched for in PATH again. This is off by default.

The --norc option prevents bash from reading ~/.bashrc for an interactive shell. This skips the personal initialization file that would normally be sourced by interactive non-login shells.

When invoked as 'sh', bash reads /etc/profile and then ~/.profile if it's a login shell. For non-interactive shells, it doesn't read any startup files. It does NOT read ~/.bashrc or /etc/bash.bashrc when invoked as sh.

set -o ignoreeof prevents an interactive shell from exiting upon receiving EOF (Ctrl+D). The user must use 'exit' or 'logout' instead. This prevents accidental shell termination from pressing Ctrl+D.

The --version option displays version information about this instance of bash and then exits successfully. This shows the bash version number, release status, and copyright information.

When using bash -c 'command_string', arguments after the command string are assigned to positional parameters starting at $0. For example: bash -c 'echo $0 $1 $2' arg0 arg1 arg2 prints 'arg0 arg1 arg2'. The command string itself is not assigned to any parameter.

When bash is invoked with the name 'sh' (e.g., via a symlink), it tries to mimic the startup behavior of historical versions of sh as closely as possible while conforming to POSIX. In this mode, bash doesn't read ~/.bashrc and looks for ENV variable for its interactive startup file.

The +T option disables the DEBUG and RETURN trap inheritance (disables functrace). This is the opposite of -T which enables it.

set -o huponexit causes bash to send SIGHUP to all jobs when an interactive login shell exits. This terminates background jobs when you log out. Disabled by default.

set -o physical (or -P) causes bash to not follow symbolic links when changing directories with cd. It uses the physical directory structure instead of the logical one with symlinks.

bash -O execfail causes a non-interactive shell to not exit if it cannot execute the file specified as an argument (e.g., the script file doesn't exist or isn't executable). The shell just reports the error and continues. Default is to exit.

bash -O sourcepath enables the source builtin to use PATH to find the file to source. This is enabled by default. If disabled, source only looks in the current directory.

The -p option turns on privileged mode. In this mode, the $BASH_ENV and $ENV files are not processed, shell functions are not inherited from the environment, and SHELLOPTS, BASHOPTS, CDPATH, and GLOBIGNORE are ignored if set in the environment. This is used when bash runs with real/effective UID/GID differences.

The -b option causes bash to report termination status of background jobs immediately, rather than waiting until just before printing the next primary prompt. This is equivalent to 'set -b' or 'set -o notify'.

bash -O nocasematch causes pattern matching in [[ ... ]] and case statements to be case-insensitive. For example, [[ $name == 'test' ]] matches 'test', 'TEST', 'TeSt', etc.

set -o histexpand (or -H) enables history expansion using ! character. This allows !! (last command), !$ (last argument), !n (command number n), etc. Disabling with +o histexpand turns this feature off.

When bash is invoked without options on a terminal (stdin is a tty), it starts as an interactive, non-login shell. It reads ~/.bashrc for initialization. It enables job control, readline editing, and displays prompts.

The --debugger option arranges for the debugger profile to be executed before the shell starts. This allows debugging facilities to be enabled at shell startup. It is equivalent to --debugger or -T but with more explicit naming.

The --noediting option prevents bash from using the GNU Readline library for command-line editing when running interactively. This disables command history, line editing, and tab completion in interactive sessions.

bash -O mailwarn causes bash to print a warning message when a mail file is accessed after the shell has first noticed that mail exists in it. This warns if mail was already read.

The --rcfile option takes a filename as an argument and causes bash to execute commands from that file instead of ~/.bashrc for an interactive shell. Example: bash --rcfile /my/custom/rcfile

The -r option makes the shell restricted. A restricted shell cannot: change directories with cd, change the value of SHELL, PATH, ENV, or BASH_ENV, specify command names containing /, redirect output using >, >|, <>, >&, &>, or >>, or turn off restricted mode with set +r or set +o restricted.

set -o hashall (or -h) enables command hashing (default). Bash remembers the location of commands in a hash table to avoid PATH searches. Disabling with +o hashall turns off this optimization.

The -G option causes the pattern '**' to match all files and zero or more directories and subdirectories when used in pathname expansion. This enables recursive globbing (requires globstar).

An interactive non-login shell (not started with -l or name not starting with -) reads ~/.bashrc for initialization. This happens unless invoked with --norc. The system-wide /etc/bash.bashrc may also be read if configured.