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# Bash's behaviour

FIXME

incomplete

## Bash startup modes

### Login shell

As a "login shell", Bash reads and sets (executes) the user's profile from `/etc/profile` and one of `~/.bash_profile`, `~/.bash_login`, or `~/.profile` (in that order, using the first one that's readable!).

When a login shell exits, Bash reads and executes commands from the file `~/.bash_logout`, if it exists.

Why an extra login shell mode? There are many actions and variable sets that only make sense for the initial user login. That's why all UNIX® shells have (should have) a "login" mode.

Methods to start Bash as a login shell:

- the first character of `argv[0]` is `-` (a hyphen): traditional UNIX® shells start from the `login` binary
- Bash is started with the `-l` option
- Bash is started with the `--login` option

Methods to test for login shell mode:

- the shell option `login_shell` is set

Related switches:

- `--noprofile` disables reading of all profile files

### Interactive shell

When Bash starts as an interactive non-login shell, it reads and executes commands from `~/.bashrc`. This file should contain, for example, aliases, since they need to be defined in every shell as they're not inherited from the parent shell.

The feature to have a system-wide `/etc/bash.bashrc` or a similar system-wide rc-file is specific to vendors and distributors that ship their own, patched variant of Bash. The classic way to have a system-wide rc file is to `source /etc/bashrc` from every user's `~/.bashrc`.

Methods to test for interactive-shell mode:

- the special parameter `$-` contains the letter `i` (lowercase I)

Related switches:

- `-i` forces the interactive mode
- `--norc` disables reading of the startup files (e.g. `/etc/bash.bashrc` if supported) and `~/.bashrc`
- `--rcfile` defines another startup file (instead of `/etc/bash.bashrc` and `~/.bashrc`)

### SH mode

When Bash starts in SH compatiblity mode, it tries to mimic the startup behaviour of historical versions of `sh` as closely as possible, while conforming to the POSIX® standard as well. The profile files read are `/etc/profile` and `~/.profile`, if it's a login shell.

If it's not a login shell, the environment variable ENV is evaluated and the resulting filename is used as the name of the startup file.

After the startup files are read, Bash enters the POSIX® compatiblity mode (for running, not for starting!).

Bash starts in `sh` compatiblity mode when:

- the base filename in `argv[0]` is `sh` (:!: NB: `/bin/sh` may be linked to `/bin/bash`, but that doesn't mean it acts like `/bin/bash` :!:)

### POSIX mode

When Bash is started in POSIX® mode, it follows the POSIX® standard for startup files. In this mode, interactive shells expand the ENV variable and commands are read and executed from the file whose name is the expanded value. No other startup files are read. Hence, a non-interactive shell doesn't read any startup files in POSIX® mode.

Bash starts in POSIX® mode when:

- the commandline option `--posix` is specified
- the environment variable POSIXLY_CORRECT is set

### Quick startup file reference

- Eventual system-wide rc-files are usually read when `~/.bashrc` would be read (at least Debian GNU/Linux behaves like that)
- Regardless of the system-wide files in `/etc` which are always read, Bash usually reads the first file found, when multiple choices are given (for user files in `~/`)

Mode | `/etc/profile` | `~/.bash_profile` | `~/.bash_login` | `~/.profile` | `~/.bashrc` | `${ENV}`
--- | --- | --- | --- | --- | --- | ---
Login shell | ✅ | ✅ | ✅ | ✅ | 🚫 | 🚫
Interactive shell | 🚫 | 🚫 | 🚫 | 🚫 | ✅ | 🚫
SH compatible login | ✅ | 🚫 | 🚫 | ✅ | 🚫 | 🚫
SH compatible | 🚫 | 🚫 | 🚫 | 🚫 | 🚫 | ✅
POSIX® compatiblity | 🚫 | 🚫 | 🚫 | 🚫 | 🚫 | ✅

## Bash run modes

### Normal Bash

### POSIX run mode

In POSIX® mode, Bash follows the POSIX® standard regarding behaviour and parsing (excerpt from a Bash maintainer's document):

    Starting Bash with the `--posix' command-line option or executing `set
    -o posix' while Bash is running will cause Bash to conform more closely
    to the POSIX standard by changing the behavior to match that specified
    by POSIX in areas where the Bash default differs.

    When invoked as `sh', Bash enters POSIX mode after reading the startup
    files.

    The following lists what's changed when Bash is in `POSIX mode':

      1. When a command in the hash table no longer exists, Bash will
         re-search `$PATH' to find the new location.  This is also
         available with `shopt -s checkhash'.

      2. The message printed by the job control code and builtins when a job
         exits with a non-zero status is `Done(status)'.

      3. The message printed by the job control code and builtins when a job
         is stopped is `Stopped(SIGNAME)', where SIGNAME is, for example,
         `SIGTSTP'.

      4. The `bg' builtin uses the required format to describe each job
         placed in the background, which does not include an indication of
         whether the job is the current or previous job.

      5. Reserved words appearing in a context where reserved words are
         recognized do not undergo alias expansion.

      6. The POSIX `PS1' and `PS2' expansions of `!' to the history number
         and `!!' to `!' are enabled, and parameter expansion is performed
         on the values of `PS1' and `PS2' regardless of the setting of the
         `promptvars' option.

      7. The POSIX startup files are executed (`$ENV') rather than the
         normal Bash files.

      8. Tilde expansion is only performed on assignments preceding a
         command name, rather than on all assignment statements on the line.

      9. The default history file is `~/.sh_history' (this is the default
         value of `$HISTFILE').

     10. The output of `kill -l' prints all the signal names on a single
         line, separated by spaces, without the `SIG' prefix.

     11. The `kill' builtin does not accept signal names with a `SIG'
         prefix.

     12. Non-interactive shells exit if FILENAME in `.' FILENAME is not
         found.

     13. Non-interactive shells exit if a syntax error in an arithmetic
         expansion results in an invalid expression.

     14. Redirection operators do not perform filename expansion on the word
         in the redirection unless the shell is interactive.

     15. Redirection operators do not perform word splitting on the word in
         the redirection.

     16. Function names must be valid shell names.  That is, they may not
         contain characters other than letters, digits, and underscores, and
         may not start with a digit.  Declaring a function with an invalid
         name causes a fatal syntax error in non-interactive shells.

     17. POSIX special builtins are found before shell functions during
         command lookup.

     18. If a POSIX special builtin returns an error status, a
         non-interactive shell exits.  The fatal errors are those listed in
         the POSIX standard, and include things like passing incorrect
         options, redirection errors, variable assignment errors for
         assignments preceding the command name, etc.

     19. If `CDPATH' is set, the `cd' builtin will not implicitly append
         the current directory to it.  This means that `cd' will fail if no
         valid directory name can be constructed from any of the entries in
         `$CDPATH', even if the a directory with the same name as the name
         given as an argument to `cd' exists in the current directory.

     20. A non-interactive shell exits with an error status if a variable
         assignment error occurs when no command name follows the assignment
         statements.  A variable assignment error occurs, for example, when
         trying to assign a value to a readonly variable.

     21. A non-interactive shell exits with an error status if the iteration
         variable in a `for' statement or the selection variable in a
         `select' statement is a readonly variable.

     22. Process substitution is not available.

     23. Assignment statements preceding POSIX special builtins persist in
         the shell environment after the builtin completes.

     24. Assignment statements preceding shell function calls persist in the
         shell environment after the function returns, as if a POSIX
         special builtin command had been executed.

     25. The `export' and `readonly' builtin commands display their output
         in the format required by POSIX.

     26. The `trap' builtin displays signal names without the leading `SIG'.

     27. The `trap' builtin doesn't check the first argument for a possible
         signal specification and revert the signal handling to the original
         disposition if it is, unless that argument consists solely of
         digits and is a valid signal number.  If users want to reset the
         handler for a given signal to the original disposition, they
         should use `-' as the first argument.

     28. The `.' and `source' builtins do not search the current directory
         for the filename argument if it is not found by searching `PATH'.

     29. Subshells spawned to execute command substitutions inherit the
         value of the `-e' option from the parent shell.  When not in POSIX
         mode, Bash clears the `-e' option in such subshells.

     30. Alias expansion is always enabled, even in non-interactive shells.

     31. When the `alias' builtin displays alias definitions, it does not
         display them with a leading `alias ' unless the `-p' option is
         supplied.

     32. When the `set' builtin is invoked without options, it does not
         display shell function names and definitions.

     33. When the `set' builtin is invoked without options, it displays
         variable values without quotes, unless they contain shell
         metacharacters, even if the result contains nonprinting characters.

     34. When the `cd' builtin is invoked in LOGICAL mode, and the pathname
         constructed from `$PWD' and the directory name supplied as an
         argument does not refer to an existing directory, `cd' will fail
         instead of falling back to PHYSICAL mode.

     35. When the `pwd' builtin is supplied the `-P' option, it resets
         `$PWD' to a pathname containing no symlinks.

     36. The `pwd' builtin verifies that the value it prints is the same as
         the current directory, even if it is not asked to check the file
         system with the `-P' option.

     37. When listing the history, the `fc' builtin does not include an
         indication of whether or not a history entry has been modified.

     38. The default editor used by `fc' is `ed'.

     39. The `type' and `command' builtins will not report a non-executable
         file as having been found, though the shell will attempt to
         execute such a file if it is the only so-named file found in
         `$PATH'.

     40. The `vi' editing mode will invoke the `vi' editor directly when
         the `v' command is run, instead of checking `$FCEDIT' and
         `$EDITOR'.

     41. When the `xpg_echo' option is enabled, Bash does not attempt to
         interpret any arguments to `echo' as options.  Each argument is
         displayed, after escape characters are converted.

    There is other POSIX behavior that Bash does not implement by default
    even when in POSIX mode.  Specifically:

      1. The `fc' builtin checks `$EDITOR' as a program to edit history
         entries if `FCEDIT' is unset, rather than defaulting directly to
         `ed'.  `fc' uses `ed' if `EDITOR' is unset.

      2. As noted above, Bash requires the `xpg_echo' option to be enabled
         for the `echo' builtin to be fully conformant.

    Bash can be configured to be POSIX-conformant by default, by specifying
    the `--enable-strict-posix-default' to `configure' when building.

FIXME

help me to find out what breaks in POSIX® mode!

The POSIX® mode can be switched on by:

- Bash starting as `sh` (the basename of `argv[0]` is `sh`)
- starting Bash with the commandline option `--posix`
- on startup, the environment variable POSIXLY_CORRECT is set
- the command `set -o posix`

Tests for the POSIX® mode:

- the variable SHELLOPTS contains `posix` in its list

### Restricted shell

In restricted mode, Bash sets up (and runs) a shell environment that's far more controlled and limited than the standard shell mode. It acts like normal Bash with the following restrictions:

- the `cd` command can't be used to change directories
- the variables SHELL, PATH, ENV and BASH_ENV can't be set or unset
- command names that contain a `/` (slash) can't be called (hence you're limited to `PATH`)
- filenames containing a `/` (slash) can't be specified as argument to the `source` or `.` builtin command
- filenames containing a `/` (slash) can't be specified as argument to the `-p` option of the `hash` builtin command
- function definitions are not inherited from the environment at shell startup
- the environment variable SHELLOPTS is ignored at startup
- redirecting output using the `>`, `>|`, `<>`, `>&`, `&>`, and `>>` redirection operators isn't allowed
- the `exec` builtin command can't replace the shell with another process
- adding or deleting builtin commands with the `-f` and `-d` options to the enable builtin command is forbidden
- using the `enable` builtin command to enable disabled shell builtins doesn't work
- the `-p` option to the `command` builtin command doesn't work
- turning off restricted mode with `set +r` or `set +o restricted` is (of course) forbidden

The "-r" restrictions are turned on after Bash has read its startup files.

When the command that is run is a shell script, then the restrictions are turned off for the (sub-)shell that runs that shell script.

The restricted shell can be switched on by:

- calling Bash as `rbash` (the basename of `argv[0]` is `rbash`)
- calling Bash with the `-r` option
- calling Bash with the `--restricted` option

Tests for restricted mode:

- the special parameter `$-` contains the letter `r` (lowercase R)
- the shell option `restricted_shell` is set and can be checked by the `shopt` builtin command

November 7, 2024
November 7, 2024

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