The interactive mode is a mode of the shell intended for direct interaction with a user. If yash is in the interactive mode, it is called an interactive shell.

Whether a shell runs in the interactive mode or not is determined in the invocation of the shell. After the shell has started up, the interactive mode cannot be switched on or off.

When the shell is interactive:

  • Initialization scripts are executed during invocation.

  • The shell checks for mail and prints a command prompt when it reads a command. Job status changes are also reported if job control is active. Line-editing may be used depending on the capability of the terminal.

  • Commands executed are automatically registered in command history.

  • If a command executed by the shell is killed by a signal other than SIGINT and SIGPIPE, the shell reports the fact to the standard error.

  • The filename token is subject to pathname expansion in file redirection.

  • The shell does not exit when it encounters a syntax or expansion error during command execution. (cf. Termination of the shell)

  • The shell does not exit when it receives the SIGINT, SIGTERM, or SIGQUIT signal.

  • A signal handler can be changed by the trap built-in even if the handler had been set to “ignore” when the shell was invoked.

  • The value of the - special parameter contains i.

  • The shell’s locale reflects the value of the LC_CTYPE variable whenever the value is changed (if the shell is not in the POSIXly-correct mode).

  • Commands are executed even when the exec option is off.

  • The ignore-eof option takes effect when enabled.

  • When the shell reaches the end of input or the exit built-in is executed, the shell checks if there is any stopped job. If so, the shell prints a warning and does not actually exit.

  • The suspend built-in by default cannot stop the shell if it is a session leader.

  • The shell does not exit when the dot built-in fails to find a script file to read.

  • The shell does not exit when the exec built-in fails to execute a command (if not in the POSIXly-correct mode).

  • When a job finished for which the wait built-in has been waiting, the fact is reported (only if job control is active and not in the POSIXly-correct mode).

  • A prompt is printed when the read built-in reads a second or following line.


The interactive shell prints a prompt just before it reads a command. The contents of the prompt is specified by the value of the PS1 and PS2 variables. The former is used for reading the first line of the command and the latter for other lines.

When the prompt is printed, the variable value is subjected to parameter expansion, command substitution, and arithmetic expansion (but note that the POSIX standard requires parameter expansion only). The result of the expansion is parsed by the rules below to make the actual prompt string, which is printed to the standard error.

In the POSIXly-correct mode, each exclamation mark (!) in the string is substituted with the command history number of the command that is being input. Two adjacent exclamation marks (!!) are printed as a single exclamation. Other characters are printed intact.

If the shell is not in the POSIXly-command mode, the following notations can be used to format the prompt string. Notations are replaced with the strings designated in the list below. Characters that are not interpreted as notations are printed intact.


Bell character (ASCII code: 7)


Escape character (ASCII code: 27)


The number of jobs in the shell.


Newline character (ASCII code: 10)


Carriage return character (ASCII code: 13)


The command history number of the command that is being input


# if the shell’s effective user ID is 0; $ otherwise.




These two notations can surround part of the prompt string that is not visible on the terminal. The surrounded part is ignored when the shell counts the number of characters that is displayed on the terminal, thus making characters correctly aligned on the terminal when the prompt string contains special invisible characters.


When line-editing is active, this notation is replaced with special characters to change font styles on the terminal if the terminal is capable of it. If line-editing is inactive or the terminal is incapable of changing font styles, this notation is silently ignored. One or more of the following can be used for fontspecs:


Change font color to black


Change font color to red


Change font color to green


Change font color to yellow


Change font color to blue


Change font color to magenta


Change font color to cyan


Change font color to white


Change background color to black


Change background color to red


Change background color to green


Change background color to yellow


Change background color to blue


Change background color to magenta


Change background color to cyan


Change background color to white


Make font color or background brighter (can only be used just after one of the characters above)


Change font and background colors to normal


Make font standout


Make font underlined


Make font and background colors reversed


Make font blink


Make font dim


Make font bold


Make font invisible


Make color and style normal

The actual colors of font and background are defined by the terminal. Different terminals may use different colors.

In addition to the normal prompt, a prompt string can be displayed to the right of the cursor if line-editing is active. Those prompts are called right prompts. The contents of right prompts are defined by the value of the PS1R and PS2R variables, each corresponding to the PS1 and PS2 variables.

Using the above-said notations, the font style of command strings the user inputs can be changed as well as that of prompts. The font style of command strings is defined by the value of the PS1S and PS2S variables, each corresponding to the PS1 and PS2 variables. The value can contain the \ffontspecs. notation only.

When the shell is not in the POSIXly-correct mode, the value of the PROMPT_COMMAND variable is executed before each prompt.

Command history

Command history is a feature of the shell that remembers executed commands to allow re-executing them later. Commands executed in the interactive mode are automatically saved in the command history. Saved commands can be edited and re-executed using line-editing and the fc and history built-ins.

Commands are saved line by line. Lines that do not contain any non-whitespace characters are not saved in the history. Lines that start with whitespaces are not saved when the hist-space option is on.

Command history is saved in a file. When history is first used after an interactive shell was started, the shell opens a file to save history in. The filename is specified by the value of the HISTFILE variable. If the file contains history data when opened, the data is restored to the shell’s history. The file contents are updated in real time as the user inputs commands into the shell. If the HISTFILE variable is not set or the file cannot be opened successfully, history is not saved in the file, but the history feature will be functional in all other respects.

The number of commands saved in history is specified by the value of the HISTSIZE variable. The shell automatically removes old history data so that the number of saved commands does not exceed the value. If the HISTSIZE variable is not set or its value is not a natural number, 500 items will be saved in history.

The shell looks at the value of the HISTFILE and HISTSIZE variables only when the history feature is first used after the shell was started. “The history feature is used” when:

  • the fc or history built-in is executed,

  • line-editing is used (regardless of whether or not history data is recalled in line-editing), or

  • a command is input to the shell

Therefore, the variables should be set in initialization scripts.

When more than one instance of yash shares a single history file, all the shells use the same history data. As a result, commands that have been executed by a shell instance can be recalled on another shell instance. Shells sharing the same history should have the same HISTSIZE value so that they manipulate history data properly.

Yash’s history data file has its own format that is incompatible with other kinds of shells.

The HISTRMDUP variable can be set to remove duplicate history items.

Mail checking

An interactive shell can notify receipt of email. The shell periodically checks the modification date/time of a file specified by the user. If the file has been modified since the previous check, the shell prints a notification message (except when the shell is not in the POSIXly-correct mode and the file is empty). By specifying a mailbox file to be checked, the shell will print a message when the file has been modified, that is, some mail has been received.

Check is done just before the shell prints a command line prompt. The interval of checks can be specified by the MAILCHECK variable in seconds. If the variable value is 0, check is done before every prompt. If the variable value is not a non-negative integer, no checks are done.

The file whose modification time is checked is specified by the MAIL variable. The variable value should be set to the pathname of the file.

If you want to check more than one file or customize the notification message, you can set the MAILPATH variable instead of the MAIL variable. When the MAILPATH variable is set, the MAIL variable is ignored. The value of the MAILPATH variable should be set to one or more colon-separated pathnames of files to be checked. Each pathname can be followed by a percent sign (%) and a custom notification message, which is printed when the corresponding file has been modified. If the pathname contains a percent sign, it should be quoted by a backslash. The specified message is subject to parameter expansion. For example, if the value of the MAILPATH variable is /foo/mail%New mail!:/bar/mailbox%You've got mail:/baz/mail\%data, the shell will print

  • New mail! when the file /foo/mail has been modified

  • You've got mail when the file /bar/mailbox has been modified

  • the default message when the file /baz/mail%data has been modified.