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bash Cheat Sheet

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Function names and special characters

The following are all permitted in function names in addition to [A-Z0-9a-z_]:

- "+", ".", "/", ":", "=", "?", "@", "", "", "^", "_", "{", "}", "~", and even some Unicode symbols

However, for portability with POSIX or other shell implementations it is not recommended.

Shell Options (shopt)

Shell options turn features within bash on and off interactively.

  • shopt -s - Set the option (enable it)
  • shopt -u - Unset the option (disable it)

Shell interactivity

  • cdable_vars - Allows cd HOME (assumes any non-directory is the variable name) (Recommend off)
  • cdspell - Attempt to fix typos in directory names on the command line (Recommend on)
  • interactive_comments- Command lines that begin with # are comments and should be ignored. (Recommend on)
  • promptvars - Enable variable substitutions in prompt displays (Recommend on)

Execution path

  • checkhash - Check whether commands found in the hash table exists before executing it. (Recommend on)

Window size

  • checkwinsize - If set, bash checks the window size after each command and, if necessary, updates the values of LINES and COLUMNS. (Recommend on)

History

  • cmdhist - Save command history to .bash_history (Recommend ison)
  • histappend - When updating bash history, append to the file instead of overwriting it (Recommend on)
  • histreedit - User can edit failed history substitutions when readline is used (Recommend off)
  • histverify - Allow additional editing of history results prior to execution using readline (Recommend off)
  • lithist - Embed newlines in history entries to ensure commands are stored as one line (Recommend on)

Completions

  • force_fignore - Ignore FIGNORE expansions even when it is the only match (Recommend on)
  • hostcomplete - When user types @ on the command line attempt to complete the hostname (Recommend off)
  • no_empty_cmd_completion- Do not attempt a completion for an empty line (Recommend on)
  • progcomp - Programmable completion is enabled (Recommend on)

Glob behavior

  • dotglob - Files that start with . are included in pathname expansion (Recommend on)
  • extglob - Extended pattern matching for pathname expansion is enabled (Recommend off)
  • failglob - Bash will produce an error when a pathname expansion matches no files (Recommend off)
  • nocaseglob - Pathname expansion is case-insensitive (Recommend off)
  • nullglob - When a pathname expansion matches no files, return an empty string instead of the pattern (Recommend on)

Shell behavior

  • extquote - Extended quotes will double-quote $'string' and $"string" within a $parameter expansion. ( Recommend on. Default is on.)
  • huponexit - If set, bash will send SIGHUP to all jobs when an interactive login shell exits. (Recommend off)
  • execfail - If set, a non-interactive shell will not exit if it cannot execute the file specified as an argument to the exec builtin command. An interactive shell does not exit if exec fails. (Recommend off)
  • mailwarn - Shows a message about mail on login (Recommend off)
  • expand_aliases - If set, aliases are expanded as described above under ALIASES. This option is enabled by default for interactive shells. (Recommend on)
  • gnu_errfmt - Use GNU's error format for error messages (Recommend off)
  • extdebug - Debugging features (Recommend off)

Language behavior

  • nocasematch - case statements are case-insensitive (Recommend off)
  • sourcepath - When enabled, the . builtin uses the $PATH environment to load the file. (Recommend on. Default is on.)
  • shift_verbose - shift displays an error when shifting beyond the arguments available (Recommend on)
  • xpg_echo - echo expands backslash-escape sequences by default (Recommend off)
  • compat31 - Shell 3.1 compatibility (with =~ and quotes) (Recommend off)

Shell flags

  • login_shell - Set by bash automatically if this shell is a login shell (Can not change)
  • restricted_shell- This shell is restricted. (Can not change)

Set Flags

See set cheatsheet.

Privileged Mode

In this mode, the $ENV and $BASH_ENV files are not processed, shell functions are not inherited from the environment, and the SHELLOPTS variable, if it appears in the environment, is ignored. If the shell is started with the effective user (group) id not equal to the real user (group) id, and the -p option is not supplied, these actions are taken and the effective user id is set to the real user id. If the -p option is supplied at startup, the effective user id is not reset. Turning this option off causes the effective user and group ids to be set to the real user and group ids.

Bash Problems

  • ${!variable} and ${!variable[@]} have different semantic meanings
  • No (easy) way to deference an array by another variable name
  • The lack of default (empty) arrays leads to errors when doing "${__emptyArray[@]}" with set -u (and leads to ugly code like "${__emptyArray[@]+"${__emptyArray[@]}"}")
  • read exits 0 on end of file but actually may return a string leading to issues with missing newlines causing problems; use the pattern: local finished=false; while ! $finished; do read -r line || finished=true; ... and then test the value of line for non-empty values and handle when $finished becomes true. Source
  • Problems with && or || precedence leads to errors

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