ETSH(1)

NAME
etsh - enhanced Thompson shell (command interpreter)

SYNOPSIS
etsh [-V | -VV]
etsh [-nv] [- | -c [string] | -i | -l | -t | file [arg1 ...]]

DESCRIPTION
Etsh is an enhanced, backward-compatible port of the standard command interpreter from Version 6 (V6) UNIX. It may be used either as an interactive shell or as a non-interactive shell. Throughout this manual, `(+)' indicates those cases where etsh is known to differ from the original sh(1), as it appeared in Version 6 (V6) UNIX.

The options are as follows:

-   The shell reads and executes command lines from the standard input until end-of-file or exit.

-c [string]
    If a string is specified, the shell executes it as a command line and exits. Otherwise, the shell treats it as the - option.

-i  (+) The shell behaves as an interactive shell by reading and executing commands from the appropriate rc files if possible (see Startup and shutdown below) before prompting the user, reading, and executing command lines from the standard input. The shell prints a diagnostic and exits with a non-zero status if it is not connected to a terminal.

-l  (+) The shell behaves as a login shell by reading and executing commands from the appropriate rc files if possible (see Startup and shutdown below) before prompting the user, reading, and executing command lines from the standard input. The shell prints a diagnostic and exits with a non-zero status if it is not connected to a terminal.

-n  (+) noexec: The shell performs no command-line execution. It only checks the syntax of each command line, after performing parameter substitution and word splitting. The shell ignores this option for interactive and login shells.

-t  The shell reads a single command line from the standard input, executes it, and exits.

-v  (+) verbose: The shell prints the words of each command line to the standard error, after performing parameter substitution and word splitting, but before executing the resulting command line.

-V  (+) The shell prints its version from $v to the standard output and exits.

-VV (+) The shell prints its version from $v, and build-time system information from `uname -srm' (see uname(1)), to the standard output and exits.

The shell may also be invoked non-interactively to read, interpret, and execute a command file. The specified file and any arguments are treated as positional parameters (see Parameter substitution below) during execution of the command file.

Otherwise, if no arguments except for -v are specified and if both the standard input and standard error are connected to a terminal, the shell is interactive. By default, an interactive shell prompts a regular user with a `% ' or with a `# ' for the superuser before reading each command line from the terminal. (+) Notice that the user can set a non-default shell prompt if desired (see Prompt below).

(+) When an interactive shell starts, it reads and executes commands from the appropriate rc files if possible (see Startup and shutdown below) before reading and executing command lines from the terminal.

  Metacharacters
A syntactic metacharacter is any one of the following:

    | ^ ; & ( ) < > space tab

When such a character is unquoted, it has special meaning to the shell. The shell uses it to separate words (see Commands and Command lines below). A quoting metacharacter is any one of the following:

    " ' \

See Quoting below. The substitution metacharacter is a:

    $

See Parameter substitution and Variable substitution below. Finally, a pattern metacharacter is any one of the following:

    * ? [

See File name generation below.

  Commands
Each command is a sequence of non-blank command arguments, or words, separated by one or more blanks (spaces or tabs). The first argument specifies the name of a command to be executed. Except for certain special arguments described below, the arguments other than the command name are passed without interpretation to the invoked command.

If the first argument names a special command, the shell executes it (see Special commands below). (+) Otherwise, the shell treats it either as an alias (see Aliases below) or as an external command, which is located as follows.

(+) If the command name contains no `/' characters, the sequence of directories in the environment variable PATH is searched for the first occurrence of an executable file by that name, which the shell attempts to execute. However, if the command name contains one or more `/' characters, the shell attempts to execute it without performing any PATH search.

(+) If an executable file does not begin with the proper magic number or a `#!shell' sequence, it is assumed to be a shell command file, and a new shell is automatically invoked to execute it. The environment variable EXECSHELL specifies the shell which is invoked to execute such a file.

If a command cannot be found or executed, a diagnostic is printed.

  Command lines
Commands separated by | or ^ constitute a chain of filters, or a pipeline. The standard output of each command but the last is taken as the standard input of the next command. Each command is run as a separate process, connected by pipes (see pipe(2)) to its neighbors.

A command line, or list, consists of one or more pipelines separated, and perhaps terminated by ; or &. The semicolon designates sequential execution. The ampersand designates asynchronous execution, which causes the preceding pipeline to be executed without waiting for it to finish. The process ID of each command in such a pipeline is reported, so that it may be used if necessary for a subsequent kill(1).

A list contained within parentheses such as ( list ) is executed in a subshell and may appear in place of a simple command as a filter.

If a command line is syntactically incorrect, a diagnostic is printed.

  Termination reporting
All terminations other than exit and interrupt are considered to be abnormal. If a sequential process terminates abnormally, a message is printed. The termination report for an asynchronous process is given upon execution of the first sequential command subsequent to its termination, or when the wait special command is executed. The following is a list of the possible termination messages:

	Hangup
	Quit
	Illegal instruction
	Trace/BPT trap
	IOT trap
	EMT trap
	Floating exception
	Killed
	Bus error
	Memory fault
	Bad system call
	Broken pipe

For an asynchronous process, its process ID is prepended to the appropriate message. If a core image is produced, ` -- Core dumped' is appended to the appropriate message.

  I/O redirection
Each of the following argument forms is interpreted as a redirection by the shell itself. Such a redirection may appear anywhere among the arguments of a simple command, or before or after a parenthesized command list, and is associated with that command or command list.

A redirection of the form <arg causes the file arg to be used as the standard input (file descriptor 0) for the associated command.

A redirection of the form >arg causes the file arg to be used as the standard output (file descriptor 1) for the associated command. If arg does not already exist, it is created; otherwise, it is truncated at the outset.

A redirection of the form >>arg is the same as >arg, except if arg already exists the command output is always appended to the end of the file.

For example, either of the following command lines:

	% date >index.txt ; pwd >>index.txt ; ls -l >>index.txt
	% ( date ; pwd ; ls -l ) >index.txt

creates on the file `index.txt', the current date and time, followed by the name and a long listing of the current working directory.

(+) A <- redirection causes input for the associated command to be redirected from the standard input which existed when the shell was invoked. This allows a command file to be used as a filter.

A >arg or >>arg redirection associated with any but the last command of a pipeline is ineffectual, as is a <arg redirection with any but the first.

The standard error (file descriptor 2) is never subject to redirection by the shell itself. Thus, commands may write diagnostics to a location where they have a chance to be seen. However, fd2 provides a way to redirect the diagnostic output to another location.

If the file for a redirection cannot be opened or created, a diagnostic is printed.

  Quoting
The shell treats all single (') and backslash (\) quoted characters literally, including characters which have special meaning to the shell (see Metacharacters above). If such characters are quoted, they represent themselves and may be passed as part of arguments.

(+) Like the quoting behavior described above, double (") quotes cause the shell to treat characters literally. However, double quotes also allow the shell to perform parameter and variable substitution via the dollar ($) metacharacter, whereas single (') quotes and backslash (\) quotes do not.

Individual characters, and sequences of characters, are quoted when enclosed by a matched pair of double (") or single (') quotes. For example:

	% awk '{ print NR "\t" $0 }' README ^ more

causes awk(1) to write each line from the `README' file, preceded by its line number and a tab, to the standard output which is piped to more(1) for viewing. The outer single quotes prevent the shell from trying to interpret any part of the string, which is then passed as a single argument to awk.

An individual backslash (\) quotes, or escapes, the next individual character. A backslash followed by a newline is a special case which allows continuation of command-line input onto the next line. Each backslash-newline sequence in the input is translated into a blank.

If a double or single quote appears but is not part of a matched pair, a diagnostic is printed.

  Parameter substitution
When the shell is invoked with arguments besides -v, it has additional string processing capabilities which are not otherwise available. Such a shell may be invoked as follows:

	etsh [-v] name [arg1 ...]

If the first character of name is not -, it is taken as the name of a command file, or shell script, which is opened as the standard input for a new shell instance. Thus, the new shell reads and interprets command lines from the named file.

Otherwise, name is taken as one of the shell options, and a new shell instance is invoked to read and interpret command lines from its standard input. However, notice that the -c option followed by a string is the one case where the shell does not read and interpret command lines from its standard input. Instead, the string itself is taken as a command line and executed.

In each command line, an unquoted character sequence of the form $N, where N is a digit, is treated as a positional parameter by the shell. Each occurrence of a positional parameter in the command line is substituted with the value of the Nth argument to the invocation of the shell (argN). $0 is substituted with name.

In all shell instances, $$ is substituted with the process ID of the current shell. The value is represented as a 5-digit ASCII string, padded on the left with zeros when the process ID is less than 10000.

(+) All shell instances attempt to set the special parameters in the following list. `(*)' indicates those which are always set. Otherwise, the parameter is unset when the shell cannot determine its value.

$# (*)
    The number of positional parameters currently available to the shell.

$*  The values of the positional parameters currently available to the shell, from $1 through the end of its argument list.

$? (*)
    The exit status of the last sequential command from the previous command line.

$d  The value of the environment variable ETSHDIR.

$e  The value of the environment variable EXECSHELL.

$g  The effective group name of the current user, as determined by getgrgid(3). The value (if any) is equivalent to that given by `id -gn'.

$h  The value of the environment variable HOME.

$i (*)
    The effective group ID of the current user, as determined by getegid(2). The value is equivalent to that given by `id -g'.

$k (*)
    The effective user ID of the current user, as determined by geteuid(2). The value is equivalent to that given by `id -u'.

$m  The value of the environment variable MANPATH.

$p  The value of the environment variable PATH.

$s  The value of the environment variable SHELL.

$t  The terminal name with which the standard input was associated when the shell was invoked, as determined by ttyname(3). The value (if any) is equivalent to that given by `tty <-'.

$u  The effective user name of the current user, as determined by getpwuid(3). The value (if any) is equivalent to that given by `id -un'.

$v (*)
    The version of the current shell represented as a one-word, read-only string.

$w  The value of the environment variable CWD. It is the absolute physical path name of the current working directory, without symbolic links, as determined by getcwd(3). On error, the value of $w is set to the empty string.

All substitution on a command line is performed before the line is interpreted. Thus, no action which alters the value of any parameter can have any effect on a reference to that parameter occurring on the same line.

A positional-parameter value may contain any number of metacharacters. Each one which is unquoted, or unescaped, within a positional-parameter value retains its special meaning when the value is substituted in a command line by the invoked shell.

Take the following two shell invocations for example:

	% etsh -c '$1' 'echo Hello World! >/dev/null'
	% etsh -c '$1' 'echo Hello World! \>/dev/null'
	Hello World! >/dev/null

In the first invocation, the > in the value substituted by $1 retains its special meaning. This causes all output from echo to be redirected to /dev/null. However, in the second invocation, the meaning of > is escaped by \ in the value substituted by $1. This causes the shell to pass `>/dev/null' as a single argument to echo instead of interpreting it as a redirection.

  Variable substitution (+)
The shell can substitute variables; a user may cause the shell to unset and set variables by using the unset and set special commands.

Variables may be used both in interactive shells and in non-interactive shells. However, notice that variables are not functional when a non-interactive shell is invoked either with the -c option followed by a string or with the -t option. Such a shell only executes one command line, but setting and using a variable requires executing two command lines in the same shell, one to set it and one to use it.

A variable can either be unset or set. When unset, a variable has no value. When set, a variable has both name and value. A valid variable name is a single ASCII character, which matches either the [A-Z] range or the [a-cfjlnoq-sx-z] range, inclusive. A valid variable value can be anything from an empty string, denoted by "" or '', to whatever you can input into a syntactically correct command line with less than or equal to the maximum command line length of characters, 2048 for the sake of brevity.

For example, `set C value' sets the variable C to value, as you can see in the following examples.

	% : Example One
	% unset C
	% set C
	% if $? -eq 1 -a "$C" = "C" echo 'C is unset.'
	C is unset.
	% : Example Two
	% set C ''
	% ( set C ) >/dev/null
	% if $? -eq 0 -a -z "$C" echo 'C == "'"$C"'"'
	C == ""
	% set W "Hello "
	% set W "$WWorld!"
	% if "$W" != "W" -a -n "$W" echo "$W"
	Hello World!
	% : Example Three
	% alias now "date '+%A, %Y-%m-%d, %T %Z';:"
	% alias loadavg "echo -n $H': ';\
	  uptime|sed 's/^.*user[s,][ ,] *//';:"
	% set C '( now ; loadavg )' ; : 'C == Command Line (or List)'
	% ( set C ) >/dev/null
	% if $? -eq 0 -a -n "$C" echo "C == `$C'"
	C == `( now ; loadavg )'
	% $C | awk '{ print NR "\t" $0 }'
	1	Saturday, 2018-02-24, 21:52:03 UTC
	2	refugio: load averages: 0.54, 0.22, 0.08

As with parameters (see Parameter substitution above), all substitution on a command line is performed before the line is interpreted. Thus, no action which alters the value of any variable can have any effect on a reference to that variable occurring on the same line.

Also, a variable value may contain any number of metacharacters. Each one which is unquoted, or unescaped, within a variable value retains its special meaning when the value is substituted in a command line.

If a variable name passed as an argument to set or unset is invalid, a diagnostic is printed. Similarly, if a variable value causes an error, a diagnostic is printed.

  File name generation
Prior to executing a command, the shell scans each argument for unquoted *, ?, or [ characters. If one or more of these characters appears, the argument is treated as a pattern and causes the shell to search for file names which match it. Otherwise, the argument is used as is.

The meaning of each pattern character is as follows:

o   The * character in a pattern matches any string of characters in a file name (including the null string).

o   The ? character in a pattern matches any single character in a file name.

o   The [...] brackets in a pattern specifies a class of characters which matches any single file-name character in the class. Within the brackets, each character is taken to be a member of the class. A pair of characters separated by an unquoted - specifies the class as a range which matches each character lexically between the first and second member of the pair, inclusive. A - matches itself when quoted or when first or last in the class.

Any other character in a pattern matches itself in a file name.

Notice that the `.' character at the beginning of a file name, or immediately following a `/', is always special in that it must be matched explicitly. The same is true of the `/' character itself.

If the pattern contains no `/' characters, the current directory is always used. Otherwise, the specified directory is the one obtained by taking the pattern up to the last `/' before the first unquoted *, ?, or [. The matching process matches the remainder of the pattern after this `/' against the files in the specified directory.

In any event, a list of file names is obtained from the current (or specified) directory which match the given pattern. This list is sorted in ascending ASCII order, and the new sequence of arguments replaces the given pattern. The same process is carried out for each of the given pattern arguments; the resulting lists are not merged. Finally, the shell attempts to execute the command with the resulting argument list.

If a pattern argument refers to a directory which cannot be opened, a `No directory' diagnostic is printed.

If a command has only one pattern argument, a `No match' diagnostic is printed if it fails to match any files. However, if a command has more than one pattern argument, a diagnostic is printed only when they all fail to match any files. Otherwise, each pattern argument failing to match any files is removed from the argument list.

  Startup and shutdown (+)
If the first character of the argv[0] used to invoke an interactive shell is `-' (e.g., -etsh), it is a login shell and tries to read and execute commands from the following four rc init files in sequence: /data/data/com.termux/files/usr/etc/etsh.login, /data/data/com.termux/files/usr/etc/etsh.etshrc, $h/.etsh.login, and $h/.etshrc. The same is true when the shell is invoked with the -l option, regardless of the value of argv[0].

In the case where an interactive shell is not a login shell according to its argv[0], it tries to read and execute commands from the following two rc init files in sequence: /data/data/com.termux/files/usr/etc/etsh.etshrc and $h/.etshrc. The same is true when the shell is invoked with the -i option, regardless of the value of argv[0].

In any case, after the shell finishes its startup actions, it then prompts the user, reads, and executes command lines from the standard input as usual.

If the shell is invoked as a login shell, it tries to read and execute commands from /data/data/com.termux/files/usr/etc/etsh.logout and $h/.etsh.logout in sequence upon logout. These two rc logout files may be used if necessary for cleanup upon termination of a login session by an EOT (see End of file below) or a SIGHUP signal (see Signals below).

Notice that the shell only performs the startup and shutdown actions described above for readable, regular rc files. If any rc file is not readable, the shell ignores it and continues as normal. If any rc file is not a regular file (or a link to a regular file), the shell ignores it, prints a diagnostic, and continues as normal.

In the normal case, a SIGINT or SIGQUIT signal received by the shell during execution of any rc file causes it to cease execution of that file without terminating. Thus, it may be desirable to use the trap special command to ignore these and other signals in some cases. For example, this is particularly true for /data/data/com.termux/files/usr/etc/etsh.login, /data/data/com.termux/files/usr/etc/etsh.etshrc, and /data/data/com.termux/files/usr/etc/etsh.logout.

The exit special command always causes the shell to terminate if it occurs in any rc file.

  History (+)
If the shell is invoked as an interactive shell, it tries to open the $h/.etsh.history file to save the user's command-line history. Notice that the history file must already exist as a writable, regular file (or a link to a regular file) when the shell is invoked to save the user's command-line history. Otherwise, it will not do so.

An interactive shell reads each command line from its terminal and appends the words of each one to the history file as a line after performing parameter substitution and word splitting.

The shell does not read the history file or have any features that allow the user to make direct use of the saved history. Such features are available via standard external commands and also via the history command found in the /data/data/com.termux/files/usr/libexec/etsh-5.4.0/etsh directory. Execute `history -h' to read its documentation.

Notice that the shell never creates or removes the $h/.etsh.history file. It always leaves these actions to the user. For example:

	% history -r ; history -c ; exec etsh -l

causes history to remove the existing history file (if any), to create a new (empty) one, and causes the current shell to replace itself with a new login shell, while opening the new history file. This, and future, interactive shells then save the user's command-line history as long as the history file exists.

If desired, the user can use the history file to repeat any command line as a command substitution with sed(1) and etsh. Taking the following command line and history entry for example:

	% history -n 6171
	Number	Command Line
	------	------------
	6171		uname -s | if { fd2 -ef/dev/null \
			egrep '([ONF][a-z]{2,3}BSD|Darwin|Linux)' } \
			echo '(Open|Net|Free)BSD || (Mac) OS X || OS == GNU/Linux'

and then doing a:

	% sed -n 6171p <$h/.etsh.history | etsh
	OS == GNU/Linux || (Mac) OS X || (Free|Net|Open)BSD
	(Open|Net|Free)BSD || (Mac) OS X || OS == GNU/Linux

causes sed to output the 6171st command line from the history file via pipe for repetition as a command substitution by etsh.

  Aliases (+)
The shell can interpret command aliases set by the user. A user may cause the shell to set, print, and unset command aliases by using the alias and unalias special commands.

A command alias is a string that substitutes for a given command alias name set by the user. Command aliases provide a simple way to represent complex, long, or often-used commands as simple command names. Thus, if the first argument names an existing command alias, its alias string substitutes for the command alias name. Any remaining arguments are appended to the argument list.

Aliases may be used both in interactive shells and in non-interactive shells. However, notice that aliases are not functional when a non-interactive shell is invoked either with the -c option followed by a string or with the -t option. Such a shell only executes one command line, but setting and using an alias requires executing two command lines in the same shell, one to set it and one to execute it.

The shell parses each alias in a command line into a list of words from left to right, wraps it as a ( list ), re-parses it while parsing any nested aliases (up to three deep), and executes the resulting alias in a subshell on success. Three examples of alias usage follow:

	% : Example One
	% alias s 'echo $?;:' ; alias status 's'
	% alias s ; alias status
	(echo $?;:)
	(s)
	% false
	% status
	1
	% : Example Two
	% alias ll 'ls -AlF'
	% alias ll
	(ls -AlF)
	% ll -d [A-Z]* | wc -l | tr -d ' \t'
	10
	% : Example Three
	% alias loadavg "uname -n|sed 's/\([^.]*\).*/\1/'|tr -d \\n;\
	  echo -n ': ';uptime|sed 's/^.*user[s,][ ,] *//';:"
	% alias loadavg
	(uname -n|sed 's/\([^.]*\).*/\1/'|tr -d \\n;\
	 echo -n ': ';uptime|sed 's/^.*user[s,][ ,] *//';:)
	% loadavg | awk '{ print NR "\t" $0 }'
	1       serenity: load average: 0.49 0.39 0.29

If an alias, or its parsed result in a command line, is syntactically incorrect, a diagnostic is printed. If an alias loop error occurs, a `Too many nested aliases' diagnostic is printed.

  Prompt (+)

The state of the P variable determines how the shell prompt appears. The user can return to the default prompt by executing `unset P' whenever desired. Since P is a variable, see Variable substitution above, as the same documentation applies here too.

Notice that `set P string' sets the shell prompt to string. See SetP in the /data/data/com.termux/files/usr/libexec/etsh-5.4.0/etsh directory. You can source or . it if you wish to use it.

Four examples of setting your prompt follow:

	% : Example One
	% : " This is the default prompt, but let's " ; unset P ;\
	  : " it to be sure. I am not root now, as you can see. "
	%
	% : Example Two
	% : " OK, let's see others. "
	% set P '=\^\) hello \(\^=\>% '
	=^) hello (^=>%
	% : Example Three
	=^) hello (^=>% : " Silly! Others? OK. "
	=^) hello (^=>% . SetP "$u:p2\>"
	jneitzel:p2>%
	% : Example Four
	jneitzel:p2>% : " Or with $w and 2 more lines? OK. "
	jneitzel:p2>% 		. $h/.etsh.prompt
	~/src/git/v6shell
	jneitzel@refugio
	p2>%
	~/src/git/v6shell
	jneitzel@refugio
	p2>% : " In effect, the '. $h/etsh.prompt' above prepares \
	         things before doing a special '. SetP string'. "
	~/src/git/v6shell
	jneitzel@refugio
	p2>%

Notice that an $h/.etsh.prompt file is not required in order to set and use a personal etsh prompt. That said, it can make things simpler or easier, in the same way that having an $h/.etshrc file can do so. In any case, I trust you noticed that "Example Two" above did not mention or use $h/.etsh.prompt at all; it was completely manual.

Now, while the included .etsh.prompt file is admittedly too complex with 74 lines, a simpler and shorter file can be quite effective if your prompt objective is more focused than mine is, or was, when I wrote it.

Documenting all of the appropriate bits here in the manual was quite a challenge. Hopefully, I have not added unnecessary complexity and confusion. That said, if you find room for improvement, I invite you to send me your suggestions.

  End of file
An end-of-file in the shell's input causes it to exit. If the shell is interactive, this means it exits by default when the user types an EOT (^D) at the prompt. If desired, the user may change or disable interactive shell EOT exit behavior with stty(1).

  Special commands
The shell treats the following built-in commands specially.

: [arg ...]
    Does nothing and sets the exit status to zero.

alias [name [string]] (+)
    Sets the alias name to string in the current shell if both name and string are specified. Prints the current value of the alias name's string if name is specified and set in the current shell. Prints the name and string of each alias set in the current shell if no arguments are specified.

cd [dir ...] (+)
    Is a synonym for the chdir special command.

chdir [dir ...]
    Changes the shell's current working directory to dir. (+) If dir is an unquoted -, the shell's previous working directory is used instead. Otherwise, if dir is not specified, the user's home directory is used by default.

echo [-n] [string ...] (+)
    Writes its string arguments (if any) separated by blanks and terminated by a newline to the standard output. If -n is specified, the terminating newline is not written.

exec command [arg ...] (+)
    Replaces the current shell with an instance of command, which must be external to the shell.

exit
    Causes the shell to cease execution of a file. This means exit has no effect at the prompt of an interactive shell.

fd2 [-e] [-f file] [--] command [arg ...] (+)
    Redirects from/to file descriptor 2 for command. See the fd2(1) manual page for full details.

goto label [...] (+)
    Transfers shell control to the `: label' line of the current command file. See the goto(1) manual page for full details.

history [-c | -h | -l | -n number1[,number2] | -p pattern | -r] (+)
    Manages, prints, and searches the user's $h/.etsh.history file. If no options are specified, the history command prints all history entries from the user's etsh history file to the standard output. Execute `history -h' to read its documentation.

if [expression [command [arg ...]]] (+)
    Evaluates expression, conditionally executes command, and sets the exit status to zero or non-zero as appropriate. See the if(1) manual page for full details.

login [arg ...]
    Replaces the current interactive shell with login(1).

newgrp [arg ...]
    Replaces the current interactive shell with newgrp(1).

prompt [on | debug | off] (+)
    Sets the shell's current prompt state to on, debug, or off, or prints its current value. The prompt command helps manage the shell's different prompt behaviors on the user's behalf. When on, it enables personal, non-default, shell prompts. When set to debug, it enables some extra functionality that can help in seeing what the shell is doing to convert $P string into a personal prompt. When off, the shell uses its default prompts. An exit status of 2 indicates error.

pwd (+)
    Prints the absolute physical pathname of the current working directory to the standard output. An exit status of 2 indicates that getcwd(3) detected an error, or that pwd itself detected a usage error.

set [name [value]] (+)
    Sets the variable name to the string value in the current shell if both name and value are specified. Prints the current value of variable name if name is specified and set in the current shell. Prints the name and value of each variable set in the current shell if no arguments are specified.

setenv name value (+)
    Sets the environment variable name to the string value.

shift
    Shifts all positional-parameter values to the left by 1, so that the old value of $2 becomes the new value of $1 and so forth. The value of $0 does not shift.

. file [arg1 ...] (+)
    Is a synonym for the source special command.

source file [arg1 ...] (+)
    Causes the shell to read and execute commands from the specified file and return. If the file name contains no `/' characters, the sequence of directories in the environment variable PATH is searched for the first occurrence of a file by that name. However, if the file name contains one or more `/' characters, the shell attempts to source it without performing any PATH search. Notice that the file does not need to be executable. The file and any arguments are treated as positional parameters (see Parameter substitution above) during execution of the file. The source command may be nested. As with command files, most shell-detected errors cause the shell to cease execution of the file. If the source command is nested and such an error occurs, all nested source commands terminate.

trap ['' | : | - signal_number ...] (+)
    '' or : causes the specified signals to be ignored if possible, and - causes the specified signals to be reset to the default action if possible. If a signal was already ignored when the shell was invoked, it cannot be reset with -. If no arguments are specified, a list is printed of those signals which are ignored by trap in the current shell context.

umask [mask] (+)
    Sets the file creation mask (see umask(2)) to the octal value specified by mask. If the mask is not specified, its current value is printed.

unalias name ... (+)
    Removes each of the specified alias names from the current shell instance.

unset name ... (+)
    Removes each of the specified variable names from the current shell instance.

unsetenv name ... (+)
    Removes each of the specified environment variable names from the environment of the current shell instance.

verbose [true | false] (+)
    Sets the shell's current verbose state to true or false, or prints its current value. When true, the shell is verbose, as it is when invoked with the -v option. Otherwise, it is not verbose. Its exit status indicates the current value of 0 for true or 1 for false, with 2 indicating error. This may be tested with the if command.

wait
    Waits for all asynchronous processes to terminate, reporting on abnormal terminations.

  Signals (+)
An interactive or login shell always ignores the SIGINT, SIGQUIT, and SIGTERM signals (see signal(3)). A login shell also handles the SIGHUP signal, the receipt of which causes the shell to terminate the login session and to read and execute its rc logout files if possible.

If SIGHUP, SIGINT, SIGQUIT, or SIGTERM is already ignored when the shell starts, it is also ignored by the current shell and all of its child processes. Otherwise, SIGINT and SIGQUIT are reset to the default action for sequential child processes, whereas SIGHUP and SIGTERM are reset to the default action for all child processes.

When a non-interactive shell executes a command file, it does not handle or ignore any signal by default. Any other non-interactive shell ignores SIGINT and SIGQUIT.

For any signal not mentioned above, the shell inherits the signal action (default or ignore) from its parent process and passes it to its child processes. Remember that the trap special command may be used to ignore signals when the shell does not do so by default.

Asynchronous child processes always ignore both SIGINT and SIGQUIT. Also, if such a process has not redirected its input with a <, |, or ^, the shell automatically redirects it to come from /dev/null.

EXIT STATUS (+)
The exit status of the shell is generally that of the last command executed prior to end-of-file or exit.

However, if the shell is interactive and detects an error, it exits with a non-zero status if the user types an EOT at the next prompt.

Otherwise, if the shell is non-interactive and is reading commands from a file, any shell-detected error causes the shell to cease execution of that file. This results in a non-zero exit status.

A non-zero exit status returned by the shell itself is always one of the values described in the following list, each of which may be accompanied by an appropriate diagnostic:

2   The shell detected a syntax, redirection, or other error not described in this list.

125 An external command was found but did not begin with the proper magic number or a `#!shell' sequence, and a valid shell was not specified by EXECSHELL with which to execute it.

126 An external command was found but could not be executed.

127 An external command was not found.

>128
    An external command was terminated by a signal.

ENVIRONMENT (+)
Notice that the concept of `user environment' was not defined in Version 6 (V6) UNIX. Thus, use of the following environment variables by this port of the shell is an enhancement:

CWD The shell sets the value of this variable while setting the $w special parameter. Both represent the absolute physical path name of the current working directory, without symbolic links. On error, CWD is removed from the environment.

ETSHDIR
    If set to a non-empty string, the value of this variable is taken as the path name of a directory which may be used for temporary files. Its value is available to the shell via the $d special parameter.

EXECSHELL
    If set to a non-empty string, the value of this variable is taken as the path name of the shell which is invoked to execute an external command when it does not begin with the proper magic number or a `#!shell' sequence. Its value is available to the shell via the $e special parameter.

HOME
    If set to a non-empty string, the value of this variable is taken as the user's home directory. Its value is available to the shell via the $h special parameter and is the default directory for the chdir special command.

MANPATH
    If set, the value of this variable is taken as the sequence of directories used by man(1) to search for manual page files. Its value is available to the shell via the $m special parameter.

PATH
    If set to a non-empty string, the value of this variable is taken as the sequence of directories used by the shell to search both for external commands and for files to be executed by the source special command. Its value is available to the shell via the $p special parameter. Notice that the Version 6 (V6) UNIX shell always used the equivalent of `.:/bin:/usr/bin', not PATH.

SHELL
    If set to a non-empty string, the value of this variable is taken as the full path name of the user's login shell. Its value is available to the shell via the $s special parameter.

FILES

/dev/null
    default source of input for asynchronous processes

/data/data/com.termux/files/usr/etc/etsh.login (+)
    system-wide rc init file for login shells

/data/data/com.termux/files/usr/etc/etsh.etshrc (+)
    system-wide rc init file for all interactive shells

$h/.etsh.history (+)
    user history file for all interactive shells

$h/.etsh.login (+)
    user rc init file for login shells

$h/.etshrc (+)
    user rc init file for all interactive shells

/data/data/com.termux/files/usr/etc/etsh.logout (+)
    system-wide rc logout file for login shells

$h/.etsh.logout (+)
    user rc logout file for login shells

$h/.etsh.prompt (+)
    user etsh prompt preprocessor for interactive etsh instances. It supports the prompt special command. In effect, it serves as an interface between the shell and the SetP script.

SEE ALSO
awk(1), env(1), expr(1), fd2(1), goto(1), grep(1), if(1), kill(1), login(1), man(1), newgrp(1), sed(1), stty(1), tsh(1), uname(1)

Etsh home page: https://etsh.nl/

`The UNIX Time-Sharing System' (CACM, July, 1974):

	https://etsh.nl/history/unix/

gives the theory of operation of both the system and the shell.

HISTORY
A sh command appeared as /bin/sh in Version 1 (V1) UNIX.

The Thompson shell was used as the standard command interpreter through Version 6 (V6) UNIX. Then, in Version 7 (V7) UNIX, it was replaced by the Bourne shell. However, the Thompson shell was still distributed with the system as osh because of known portability problems with the Bourne shell's memory management in Version 7 (V7) UNIX.

AUTHORS
This port of the Thompson shell is derived from Version 6 (V6) UNIX /usr/source/s2/sh.c, which was principally written by Ken Thompson of Bell Labs. Jeffrey Allen Neitzel <jan@etsh.nl> ported and maintains it as tsh(1). Jeffrey also ported and maintains this enhanced port of the Thompson shell as etsh(1).

LICENSE
See either the LICENSE file which is distributed with etsh or https://etsh.nl/license/ for full details.

CAVEATS
Unlike the original, this port of the shell can handle 8-bit character sets, as well as the UTF-8 encoding. The original, on the other hand, can only handle 7-bit ASCII.

Notice that certain shell oddities were historically undocumented in this manual page. Particularly noteworthy is the fact that there is no such thing as a usage error. Thus, the following shell invocations are all perfectly valid:

	etsh -cats_are_nice!!! ': "Good kitty =)"'
	etsh -tabbies_are_too!
	etsh -s

The first two cases correspond to the -c and -t options respectively; the third case corresponds to the - option.

BUGS (+)
The shell differentiates between an end-of-file and an error returned by the read(2) system call. This allows the shell to print a diagnostic and exit with a non-zero status if appropriate. Previously, it did not recover from read(2) errors and exited when this system call failed, and this is still true today.

SECURITY CONSIDERATIONS
This port of the Thompson shell does not support set-ID execution. If the effective user (group) ID of the shell process is not equal to its real user (group) ID when an instance of it starts up, the shell prints the following diagnostic and exits with a non-zero status.

	Set-ID execution denied

If the shell did support set-ID execution, it could possibly allow a user to violate the security policy on a host where the shell is used. For example, if the shell were running a setuid-root command file, a regular user could possibly invoke an interactive root shell as a result.

This is not a bug. It is simply how the shell works. Thus, etsh does not support set-ID execution. This is a proactive measure to avoid problems, nothing more.