/usr/bin/dash
dash — コマンドインタープリタ (shell)
SYNOPSIS
dash [-aCefnuvxIimqVEbp] [+aCefnuvxIimqVEbp] [-o option_name] [+o option_name] [command_file [argument ...]]dash -c [-aCefnuvxIimqVEbp] [+aCefnuvxIimqVEbp] [-o option_name] [+o option_name] command_string [command_name [argument ...]]dash -s [-aCefnuvxIimqVEbp] [+aCefnuvxIimqVEbp] [-o option_name] [+o option_name] [argument ...]
DESCRIPTION
dash はシステムの標準的なコマンドインタプリタである。現在の dash のバージョンは、シェルの仕様である POSIX 1003.2 および 1003.2a に準拠するよう変更されている最中である。 このバージョンは多くの機能を備えているため、Korn シェルと似ているように見える部分があるが Korn シェルのクローンではない (ksh を参照)。 POSIX で指定された機能と、いくつかの Berkeley 拡張機能だけがこのシェルに組み込まれている。 この man ページは、チュートリアルやシェルの完全な仕様書を意図したものではない。
Overview
シェルは、ファイルまたはターミナルから行を読み取り、それを解釈し、一般に他のコマンドを実行するコマンドである。 ユーザーがシステムにログインしたときに実行されるプログラムである (ただし、ユーザーは chsh コマンドで別のシェルを選択することができる)。 シェルは、フロー制御構造を持つ言語、データの保存に加えてさまざまな機能を提供するマクロ機能、および内蔵の履歴と行編集機能を実装している。 対話的な使用を助ける多くの機能が組み込まれており、対話的な使用と非対話的な使用（シェルスクリプト）の両方に解釈言語が共通であるという利点がある。 つまり、コマンドを実行中のシェルに直接入力したり、ファイルに記述して、そのファイルをシェルで直接実行することができるのである。
Invocation
args がなく、シェルの標準入力が端末に接続されていて (または -i フラグが設定されていて)、かつ -c オプションがない場合、そのシェルは対話型シェルとみなされる。 対話型シェルは一般に各コマンドの前にプロンプトを出し、プログラミングエラーやコマンドエラーの処理も (後述するように) 異なる。 初回起動時にシェルは引数 0 を検査し、それがダッシュ '-' で始まる場合、そのシェルもログインシェルとみなされる。 これは通常、ユーザーが最初にログインしたときにシステムによって自動的に行われる。 ログインシェルは、最初に /etc/profile と .profile ファイルが存在すれば、そこからコマンドを読み込む。 対話型シェルに入るときに環境変数 ENV が設定されているか、ログインシェルの .profile に設定されている場合、シェルは次に ENV で名付けられたファイルからコマンドを読み込む。 したがって、ログイン時にのみ実行されるコマンドは .profile ファイルに、対話型シェルごとに実行されるコマンドは ENV ファイルに記述する必要がある。 ENV変数に何らかのファイルを設定するには、ホームディレクトリの.profileに次のような行を記述すること。
- ENV=$HOME/.shinit; export ENV
- ".shinit "に任意のファイル名を代入。
オプション以外のコマンドライン引数が指定された場合、シェルは最初の引数をコマンドを読み込むファイル名（シェルスクリプト）として扱い、残りの引数はシェルの位置パラメータ（$1、$2など）として設定される。 そうでなければ、シェルは標準入力からコマンドを読み込む。
Argument List Processing
対応する名前を持つすべての一文字のオプションは、-o オプションの引数として使用することができる。 set -o の名前は、以下の説明で一文字のオプションの横に記載されている。 ダッシュ「-」を指定するとオプションがオンになり、プラス「+」を指定するとオプションがオフになる。 以下のオプションは、コマンドラインから、またはset組み込み関数（後述）を使って設定することができる。
- -a allexport
- 割り当てられたすべての変数をエクスポートする。
- -c
- 標準入力からではなく、command_string オペランドからコマンドを読み取る。 特殊パラメータ 0 は command_name オペランドから、位置パラメータ ($1, $2, など) は残りの引数オペランドから設定される。
- -C noclobber
- 既存のファイルを">"で上書きしないこと。
- -e errexit
- 対話型でない場合、テストされていないコマンドが失敗したら直ちに終了する。 コマンドの終了状態は、コマンドが if、elif、while、until を制御するために使用されている場合、またはコマンドが "&&" や "||" 演算子の左手オペランドである場合、明示的にテストされていると見なされる。
- -f noglob
- パス名展開を無効にする。
- -n noexec
- インタラクティブでない場合、コマンドは読むが、実行はしない。 これは、シェルスクリプトの構文をチェックするのに便利である。
- -u nounset
- 設定されていない変数を展開しようとしたときに標準エラーにメッセージを書き、シェルが対話型でない場合は即座に終了する。
- -v verbose
- シェルは、入力を読み込むと標準エラーに書き込む。 デバッグに便利である。
- -x xtrace
- 各コマンドを実行する前に標準エラーに書き出す（'+' が付く）。 デバッグに便利。
- -I ignoreeof
- 対話時に入力のEOFを無視する。
- -i interactive
- シェルを強制的に対話型に動作させる。
- -l
- dashをログインシェルとして起動したかのように動作させる。
- -m monitor
- ジョブ制御をオンにする（対話時に自動的に設定される）。
- -s stdin
- 標準入力からコマンドを読み込む (ファイル引数が存在しない場合は自動的に設定される)。 このオプションは、シェルがすでに実行を開始した後 (つまり set で) 設定しても、何の効果もない。
- -V vi
- 内蔵の vi コマンドラインエディタを有効にする (-E が設定されている場合は無効となる)。
- -E emacs
- 内臓のemacsコマンドラインエディタを有効にする (-Vが設定されている場合は無効となる)。
- -b notify
- バックグラウンドジョブの完了を非同期で通知するようにする。 (4.4alpha では UNIMPLEMENTED)
- -p priviliged
- uid と一致しない場合、実効 uid のリセットを試みない。これは、system(3) や popen(3) を経由した setuid root プログラムによる不正な使用を避けるために、 デフォルトでは設定されていない。
Lexical Structure
シェルはファイルから行単位で入力を読み込み、空白（ブランクやタブ）と、「演算子」と呼ばれるシェルにとって特別な文字の並びで単語を分割している。演算子には、制御演算子とリダイレクション演算子の2種類がある（意味は後述する）。 以下は、演算子の一覧:
- Control operators
- & && ( ) ; ;; | || <newline>
- Redirection operators
- < > >| << >> <& >& <<- <>
Quoting
クオートは、演算子、空白、キーワードなど、特定の文字や単語がシェルに与える特別な意味を取り除くために使用される。 クォートには、マッチド・シングルクォート、マッチド・ダブルクォート、バックスラッシュの3種類がある。
Backslash
バックスラッシュは、⟨ニューライン⟩を除いて、続く文字の文字通りの意味を保持する。 改行⟩の前のバックスラッシュは、行の継続として扱われる。
Single Quotes
Enclosing characters in single quotes preserves the literal meaning of all the characters (except sin‐
gle quotes, making it impossible to put single-quotes in a single-quoted string).
Double Quotes
Enclosing characters within double quotes preserves the literal meaning of all characters except dol‐
larsign ($), backquote (`), and backslash (\). The backslash inside double quotes is historically
weird, and serves to quote only the following characters:
$ ` " \ <newline>.
Otherwise it remains literal.
Reserved Words
Reserved words are words that have special meaning to the shell and are recognized at the beginning of
a line and after a control operator. The following are reserved words:
! elif fi while case
else for then { }
do done until if esac
Their meaning is discussed later.
Aliases
An alias is a name and corresponding value set using the alias(1) builtin command. Whenever a re‐
served word may occur (see above), and after checking for reserved words, the shell checks the word to
see if it matches an alias. If it does, it replaces it in the input stream with its value. For exam‐
ple, if there is an alias called “lf” with the value “ls -F”, then the input:
lf foobar ⟨return⟩
would become
ls -F foobar ⟨return⟩
Aliases provide a convenient way for naive users to create shorthands for commands without having to
learn how to create functions with arguments. They can also be used to create lexically obscure code.
This use is discouraged.
Commands
The shell interprets the words it reads according to a language, the specification of which is outside
the scope of this man page (refer to the BNF in the POSIX 1003.2 document). Essentially though, a
line is read and if the first word of the line (or after a control operator) is not a reserved word,
then the shell has recognized a simple command. Otherwise, a complex command or some other special
construct may have been recognized.
Simple Commands
単純なコマンドが認識された場合、シェルは以下の動作を行う:
- "name=value"の形の先頭の単語を取り除き、単純なコマンドの環境に割り当てる。 リダイレクション演算子およびその引数（後述）は削除され、処理のために保存される。
- 残りの単語は、「展開」の項で説明したように展開され、最初に残った単語をコマンド名とみなして、コマンドを配置する。 残りの単語は、コマンドの引数とみなされる。 コマンド名が得られなかった場合、項目1で認識された"name=value"変数の割り当てが現在のシェルに影響を与える。
- リダイレクションは、次のセクションで説明するように実行される。
Redirections
Redirections are used to change where a command reads its input or sends its output. In general,
redirections open, close, or duplicate an existing reference to a file. The overall format used for
redirection is:
[n] redir-op file
where redir-op is one of the redirection operators mentioned previously. Following is a list of the
possible redirections. The [n] is an optional number between 0 and 9, as in ‘3’ (not ‘[3]’), that
refers to a file descriptor.
[n]> file Redirect standard output (or n) to file.
[n]>| file Same, but override the -C option.
[n]>> file Append standard output (or n) to file.
[n]< file Redirect standard input (or n) from file.
[n1]<&n2 Copy file descriptor n2 as stdout (or fd n1). fd n2.
[n]<&- Close standard input (or n).
[n1]>&n2 Copy file descriptor n2 as stdin (or fd n1). fd n2.
[n]>&- Close standard output (or n).
[n]<> file Open file for reading and writing on standard input (or n).
The following redirection is often called a “here-document”.
[n]<< delimiter
here-doc-text ...
delimiter
All the text on successive lines up to the delimiter is saved away and made available to the command
on standard input, or file descriptor n if it is specified. If the delimiter as specified on the ini‐
tial line is quoted, then the here-doc-text is treated literally, otherwise the text is subjected to
parameter expansion, command substitution, and arithmetic expansion (as described in the section on
“Expansions”). If the operator is “<<-” instead of “<<”, then leading tabs in the here-doc-text are
stripped.
Search and Execution
There are three types of commands: shell functions, builtin commands, and normal programs – and the
command is searched for (by name) in that order. They each are executed in a different way.
When a shell function is executed, all of the shell positional parameters (except $0, which remains
unchanged) are set to the arguments of the shell function. The variables which are explicitly placed
in the environment of the command (by placing assignments to them before the function name) are made
local to the function and are set to the values given. Then the command given in the function defini‐
tion is executed. The positional parameters are restored to their original values when the command
completes. This all occurs within the current shell.
Shell builtins are executed internally to the shell, without spawning a new process.
Otherwise, if the command name doesn't match a function or builtin, the command is searched for as a
normal program in the file system (as described in the next section). When a normal program is exe‐
cuted, the shell runs the program, passing the arguments and the environment to the program. If the
program is not a normal executable file (i.e., if it does not begin with the "magic number" whose
ASCII representation is "#!", so execve(2) returns ENOEXEC then) the shell will interpret the program
in a subshell. The child shell will reinitialize itself in this case, so that the effect will be as
if a new shell had been invoked to handle the ad-hoc shell script, except that the location of hashed
commands located in the parent shell will be remembered by the child.
Note that previous versions of this document and the source code itself misleadingly and sporadically
refer to a shell script without a magic number as a "shell procedure".
Path Search
When locating a command, the shell first looks to see if it has a shell function by that name. Then
it looks for a builtin command by that name. If a builtin command is not found, one of two things
happen:
1. Command names containing a slash are simply executed without performing any searches.
2. The shell searches each entry in PATH in turn for the command. The value of the PATH variable
should be a series of entries separated by colons. Each entry consists of a directory name. The
current directory may be indicated implicitly by an empty directory name, or explicitly by a sin‐
gle period.
Command Exit Status
Each command has an exit status that can influence the behaviour of other shell commands. The para‐
digm is that a command exits with zero for normal or success, and non-zero for failure, error, or a
false indication. The man page for each command should indicate the various exit codes and what they
mean. Additionally, the builtin commands return exit codes, as does an executed shell function.
If a command consists entirely of variable assignments then the exit status of the command is that of
the last command substitution if any, otherwise 0.
Complex Commands
Complex commands are combinations of simple commands with control operators or reserved words, to‐
gether creating a larger complex command. More generally, a command is one of the following:
• simple command
• pipeline
• list or compound-list
• compound command
• function definition
Unless otherwise stated, the exit status of a command is that of the last simple command executed by
the command.
Pipelines
A pipeline is a sequence of one or more commands separated by the control operator |. The standard
output of all but the last command is connected to the standard input of the next command. The stan‐
dard output of the last command is inherited from the shell, as usual.
The format for a pipeline is:
[!] command1 [| command2 ...]
The standard output of command1 is connected to the standard input of command2. The standard input,
standard output, or both of a command is considered to be assigned by the pipeline before any redi‐
rection specified by redirection operators that are part of the command.
If the pipeline is not in the background (discussed later), the shell waits for all commands to com‐
plete.
If the reserved word ! does not precede the pipeline, the exit status is the exit status of the last
command specified in the pipeline. Otherwise, the exit status is the logical NOT of the exit status
of the last command. That is, if the last command returns zero, the exit status is 1; if the last
command returns greater than zero, the exit status is zero.
Because pipeline assignment of standard input or standard output or both takes place before redirect‐
ion, it can be modified by redirection. For example:
$ command1 2>&1 | command2
sends both the standard output and standard error of command1 to the standard input of command2.
A ; or ⟨newline⟩ terminator causes the preceding AND-OR-list (described next) to be executed sequen‐
tially; a & causes asynchronous execution of the preceding AND-OR-list.
Note that unlike some other shells, each process in the pipeline is a child of the invoking shell (un‐
less it is a shell builtin, in which case it executes in the current shell – but any effect it has on
the environment is wiped).
Background Commands – &
If a command is terminated by the control operator ampersand (&), the shell executes the command asyn‐
chronously – that is, the shell does not wait for the command to finish before executing the next com‐
mand.
The format for running a command in background is:
command1 & [command2 & ...]
If the shell is not interactive, the standard input of an asynchronous command is set to /dev/null.
Lists – Generally Speaking
A list is a sequence of zero or more commands separated by newlines, semicolons, or ampersands, and
optionally terminated by one of these three characters. The commands in a list are executed in the
order they are written. If command is followed by an ampersand, the shell starts the command and im‐
mediately proceeds onto the next command; otherwise it waits for the command to terminate before pro‐
ceeding to the next one.
Short-Circuit List Operators
“&&” and “||” are AND-OR list operators. “&&” executes the first command, and then executes the sec‐
ond command if and only if the exit status of the first command is zero. “||” is similar, but exe‐
cutes the second command if and only if the exit status of the first command is nonzero. “&&” and
“||” both have the same priority.
Flow-Control Constructs – if, while, for, case
The syntax of the if command is
if list
then list
[ elif list
then list ] ...
[ else list ]
fi
The syntax of the while command is
while list
do list
done
The two lists are executed repeatedly while the exit status of the first list is zero. The until com‐
mand is similar, but has the word until in place of while, which causes it to repeat until the exit
status of the first list is zero.
The syntax of the for command is
for variable [ in [ word ... ] ]
do list
done
The words following in are expanded, and then the list is executed repeatedly with the variable set to
each word in turn. Omitting in word ... is equivalent to in "$@".
The syntax of the break and continue command is
break [ num ]
continue [ num ]
Break terminates the num innermost for or while loops. Continue continues with the next iteration of
the innermost loop. These are implemented as builtin commands.
The syntax of the case command is
case word in
[(]pattern) list ;;
...
esac
The pattern can actually be one or more patterns (see Shell Patterns described later), separated by
“|” characters. The “(” character before the pattern is optional.
Grouping Commands Together
Commands may be grouped by writing either
(list)
or
{ list; }
The first of these executes the commands in a subshell. Builtin commands grouped into a (list) will
not affect the current shell. The second form does not fork another shell so is slightly more effi‐
cient. Grouping commands together this way allows you to redirect their output as though they were
one program:
{ printf " hello " ; printf " world\n" ; } > greeting
Note that “}” must follow a control operator (here, “;”) so that it is recognized as a reserved word
and not as another command argument.
Functions
The syntax of a function definition is
name () command
A function definition is an executable statement; when executed it installs a function named name and
returns an exit status of zero. The command is normally a list enclosed between “{” and “}”.
Variables may be declared to be local to a function by using a local command. This should appear as
the first statement of a function, and the syntax is
local [variable | -] ...
Local is implemented as a builtin command.
When a variable is made local, it inherits the initial value and exported and readonly flags from the
variable with the same name in the surrounding scope, if there is one. Otherwise, the variable is
initially unset. The shell uses dynamic scoping, so that if you make the variable x local to function
f, which then calls function g, references to the variable x made inside g will refer to the variable
x declared inside f, not to the global variable named x.
The only special parameter that can be made local is “-”. Making “-” local any shell options that are
changed via the set command inside the function to be restored to their original values when the func‐
tion returns.
The syntax of the return command is
return [exitstatus]
It terminates the currently executing function. Return is implemented as a builtin command.
Variables and Parameters
The shell maintains a set of parameters. A parameter denoted by a name is called a variable. When
starting up, the shell turns all the environment variables into shell variables. New variables can be
set using the form
name=value
Variables set by the user must have a name consisting solely of alphabetics, numerics, and underscores
- the first of which must not be numeric. A parameter can also be denoted by a number or a special
character as explained below.
Positional Parameters
A positional parameter is a parameter denoted by a number (n > 0). The shell sets these initially to
the values of its command line arguments that follow the name of the shell script. The set builtin
can also be used to set or reset them.
Special Parameters
A special parameter is a parameter denoted by one of the following special characters. The value of
the parameter is listed next to its character.
* Expands to the positional parameters, starting from one. When the expansion occurs
within a double-quoted string it expands to a single field with the value of each parame‐
ter separated by the first character of the IFS variable, or by a ⟨space⟩ if IFS is un‐
set.
@ Expands to the positional parameters, starting from one. When the expansion occurs
within double-quotes, each positional parameter expands as a separate argument. If there
are no positional parameters, the expansion of @ generates zero arguments, even when @ is
double-quoted. What this basically means, for example, is if $1 is “abc” and $2 is “def
ghi”, then "$@" expands to the two arguments:
"abc" "def ghi"
# Expands to the number of positional parameters.
? Expands to the exit status of the most recent pipeline.
- (Hyphen.) Expands to the current option flags (the single-letter option names concatenated into a
string) as specified on invocation, by the set builtin command, or implicitly by the
shell.
$ Expands to the process ID of the invoked shell. A subshell retains the same value of $
as its parent.
! Expands to the process ID of the most recent background command executed from the current
shell. For a pipeline, the process ID is that of the last command in the pipeline.
0 (Zero.) Expands to the name of the shell or shell script.
Word Expansions
This clause describes the various expansions that are performed on words. Not all expansions are per‐
formed on every word, as explained later.
Tilde expansions, parameter expansions, command substitutions, arithmetic expansions, and quote re‐
movals that occur within a single word expand to a single field. It is only field splitting or path‐
name expansion that can create multiple fields from a single word. The single exception to this rule
is the expansion of the special parameter @ within double-quotes, as was described above.
The order of word expansion is:
1. Tilde Expansion, Parameter Expansion, Command Substitution, Arithmetic Expansion (these all occur
at the same time).
2. Field Splitting is performed on fields generated by step (1) unless the IFS variable is null.
3. Pathname Expansion (unless set -f is in effect).
4. Quote Removal.
The $ character is used to introduce parameter expansion, command substitution, or arithmetic evalua‐
tion.
Tilde Expansion (substituting a user's home directory)
A word beginning with an unquoted tilde character (~) is subjected to tilde expansion. All the char‐
acters up to a slash (/) or the end of the word are treated as a username and are replaced with the
user's home directory. If the username is missing (as in ~/foobar), the tilde is replaced with the
value of the HOME variable (the current user's home directory).
Parameter Expansion
The format for parameter expansion is as follows:
${expression}
where expression consists of all characters until the matching “}”. Any “}” escaped by a backslash or
within a quoted string, and characters in embedded arithmetic expansions, command substitutions, and
variable expansions, are not examined in determining the matching “}”.
The simplest form for parameter expansion is:
${parameter}
The value, if any, of parameter is substituted.
The parameter name or symbol can be enclosed in braces, which are optional except for positional pa‐
rameters with more than one digit or when parameter is followed by a character that could be inter‐
preted as part of the name. If a parameter expansion occurs inside double-quotes:
1. Pathname expansion is not performed on the results of the expansion.
2. Field splitting is not performed on the results of the expansion, with the exception of @.
In addition, a parameter expansion can be modified by using one of the following formats.
${parameter:-word} Use Default Values. If parameter is unset or null, the expansion of word is
substituted; otherwise, the value of parameter is substituted.
${parameter:=word} Assign Default Values. If parameter is unset or null, the expansion of word is
assigned to parameter. In all cases, the final value of parameter is substi‐
tuted. Only variables, not positional parameters or special parameters, can be
assigned in this way.
${parameter:?[word]} Indicate Error if Null or Unset. If parameter is unset or null, the expansion
of word (or a message indicating it is unset if word is omitted) is written to
standard error and the shell exits with a nonzero exit status. Otherwise, the
value of parameter is substituted. An interactive shell need not exit.
${parameter:+word} Use Alternative Value. If parameter is unset or null, null is substituted; oth‐
erwise, the expansion of word is substituted.
In the parameter expansions shown previously, use of the colon in the format results in a test for a
parameter that is unset or null; omission of the colon results in a test for a parameter that is only
unset.
${#parameter} String Length. The length in characters of the value of parameter.
The following four varieties of parameter expansion provide for substring processing. In each case,
pattern matching notation (see Shell Patterns), rather than regular expression notation, is used to
evaluate the patterns. If parameter is * or @, the result of the expansion is unspecified. Enclosing
the full parameter expansion string in double-quotes does not cause the following four varieties of
pattern characters to be quoted, whereas quoting characters within the braces has this effect.
${parameter%word} Remove Smallest Suffix Pattern. The word is expanded to produce a pattern. The
parameter expansion then results in parameter, with the smallest portion of the
suffix matched by the pattern deleted.
${parameter%%word} Remove Largest Suffix Pattern. The word is expanded to produce a pattern. The
parameter expansion then results in parameter, with the largest portion of the
suffix matched by the pattern deleted.
${parameter#word} Remove Smallest Prefix Pattern. The word is expanded to produce a pattern. The
parameter expansion then results in parameter, with the smallest portion of the
prefix matched by the pattern deleted.
${parameter##word} Remove Largest Prefix Pattern. The word is expanded to produce a pattern. The
parameter expansion then results in parameter, with the largest portion of the
prefix matched by the pattern deleted.
Command Substitution
Command substitution allows the output of a command to be substituted in place of the command name it‐
self. Command substitution occurs when the command is enclosed as follows:
$(command)
or (“backquoted” version):
`command`
The shell expands the command substitution by executing command in a subshell environment and replac‐
ing the command substitution with the standard output of the command, removing sequences of one or
more ⟨newline⟩s at the end of the substitution. (Embedded ⟨newline⟩s before the end of the output are
not removed; however, during field splitting, they may be translated into ⟨space⟩s, depending on the
value of IFS and quoting that is in effect.)
Arithmetic Expansion
Arithmetic expansion provides a mechanism for evaluating an arithmetic expression and substituting its
value. The format for arithmetic expansion is as follows:
$((expression))
The expression is treated as if it were in double-quotes, except that a double-quote inside the ex‐
pression is not treated specially. The shell expands all tokens in the expression for parameter ex‐
pansion, command substitution, and quote removal.
Next, the shell treats this as an arithmetic expression and substitutes the value of the expression.
White Space Splitting (Field Splitting)
After parameter expansion, command substitution, and arithmetic expansion the shell scans the results
of expansions and substitutions that did not occur in double-quotes for field splitting and multiple
fields can result.
The shell treats each character of the IFS as a delimiter and uses the delimiters to split the results
of parameter expansion and command substitution into fields.
Pathname Expansion (File Name Generation)
Unless the -f flag is set, file name generation is performed after word splitting is complete. Each
word is viewed as a series of patterns, separated by slashes. The process of expansion replaces the
word with the names of all existing files whose names can be formed by replacing each pattern with a
string that matches the specified pattern. There are two restrictions on this: first, a pattern can‐
not match a string containing a slash, and second, a pattern cannot match a string starting with a pe‐
riod unless the first character of the pattern is a period. The next section describes the patterns
used for both Pathname Expansion and the case command.
Shell Patterns
A pattern consists of normal characters, which match themselves, and meta-characters. The meta-char‐
acters are “!”, “*”, “?”, and “[”. These characters lose their special meanings if they are quoted.
When command or variable substitution is performed and the dollar sign or back quotes are not double
quoted, the value of the variable or the output of the command is scanned for these characters and
they are turned into meta-characters.
An asterisk (“*”) matches any string of characters. A question mark matches any single character. A
left bracket (“[”) introduces a character class. The end of the character class is indicated by a
(“]”); if the “]” is missing then the “[” matches a “[” rather than introducing a character class. A
character class matches any of the characters between the square brackets. A range of characters may
be specified using a minus sign. The character class may be complemented by making an exclamation
point the first character of the character class.
To include a “]” in a character class, make it the first character listed (after the “!”, if any). To
include a minus sign, make it the first or last character listed.
Builtins
This section lists the builtin commands which are builtin because they need to perform some operation
that can't be performed by a separate process. In addition to these, there are several other commands
that may be builtin for efficiency (e.g. printf(1), echo(1), test(1), etc).
:
true A null command that returns a 0 (true) exit value.
. file
The commands in the specified file are read and executed by the shell.
alias [name[=string ...]]
If name=string is specified, the shell defines the alias name with value string. If just name
is specified, the value of the alias name is printed. With no arguments, the alias builtin
prints the names and values of all defined aliases (see unalias).
bg [job] ...
Continue the specified jobs (or the current job if no jobs are given) in the background.
command [-p] [-v] [-V] command [arg ...]
Execute the specified command but ignore shell functions when searching for it. (This is use‐
ful when you have a shell function with the same name as a builtin command.)
-p search for command using a PATH that guarantees to find all the standard utilities.
-V Do not execute the command but search for the command and print the resolution of the
command search. This is the same as the type builtin.
-v Do not execute the command but search for the command and print the absolute pathname of
utilities, the name for builtins or the expansion of aliases.
cd -
cd [-LP] [directory]
Switch to the specified directory (default HOME). If an entry for CDPATH appears in the envi‐
ronment of the cd command or the shell variable CDPATH is set and the directory name does not
begin with a slash, then the directories listed in CDPATH will be searched for the specified
directory. The format of CDPATH is the same as that of PATH. If a single dash is specified as
the argument, it will be replaced by the value of OLDPWD. The cd command will print out the
name of the directory that it actually switched to if this is different from the name that the
user gave. These may be different either because the CDPATH mechanism was used or because the
argument is a single dash. The -P option causes the physical directory structure to be used,
that is, all symbolic links are resolved to their respective values. The -L option turns off
the effect of any preceding -P options.
echo [-n] args...
Print the arguments on the standard output, separated by spaces. Unless the -n option is
present, a newline is output following the arguments.
If any of the following sequences of characters is encountered during output, the sequence is
not output. Instead, the specified action is performed:
\b A backspace character is output.
\c Subsequent output is suppressed. This is normally used at the end of the last argument
to suppress the trailing newline that echo would otherwise output.
\e Outputs an escape character (ESC).
\f Output a form feed.
\n Output a newline character.
\r Output a carriage return.
\t Output a (horizontal) tab character.
\v Output a vertical tab.
\0digits
Output the character whose value is given by zero to three octal digits. If there are
zero digits, a nul character is output.
\\ Output a backslash.
All other backslash sequences elicit undefined behaviour.
eval string ...
Concatenate all the arguments with spaces. Then re-parse and execute the command.
exec [command arg ...]
Unless command is omitted, the shell process is replaced with the specified program (which must
be a real program, not a shell builtin or function). Any redirections on the exec command are
marked as permanent, so that they are not undone when the exec command finishes.
exit [exitstatus]
Terminate the shell process. If exitstatus is given it is used as the exit status of the
shell; otherwise the exit status of the preceding command is used.
export name ...
export -p
The specified names are exported so that they will appear in the environment of subsequent com‐
mands. The only way to un-export a variable is to unset it. The shell allows the value of a
variable to be set at the same time it is exported by writing
export name=value
With no arguments the export command lists the names of all exported variables. With the -p
option specified the output will be formatted suitably for non-interactive use.
fc [-e editor] [first [last]]
fc -l [-nr] [first [last]]
fc -s [old=new] [first]
The fc builtin lists, or edits and re-executes, commands previously entered to an interactive
shell.
-e editor
Use the editor named by editor to edit the commands. The editor string is a command
name, subject to search via the PATH variable. The value in the FCEDIT variable is used
as a default when -e is not specified. If FCEDIT is null or unset, the value of the
EDITOR variable is used. If EDITOR is null or unset, ed(1) is used as the editor.
-l (ell)
List the commands rather than invoking an editor on them. The commands are written in
the sequence indicated by the first and last operands, as affected by -r, with each com‐
mand preceded by the command number.
-n Suppress command numbers when listing with -l.
-r Reverse the order of the commands listed (with -l) or edited (with neither -l nor -s).
-s Re-execute the command without invoking an editor.
first
last Select the commands to list or edit. The number of previous commands that can be ac‐
cessed are determined by the value of the HISTSIZE variable. The value of first or last
or both are one of the following:
[+]number
A positive number representing a command number; command numbers can be displayed
with the -l option.
-number
A negative decimal number representing the command that was executed number of
commands previously. For example, -1 is the immediately previous command.
string
A string indicating the most recently entered command that begins with that string. If
the old=new operand is not also specified with -s, the string form of the first operand
cannot contain an embedded equal sign.
The following environment variables affect the execution of fc:
FCEDIT Name of the editor to use.
HISTSIZE The number of previous commands that are accessible.
fg [job]
Move the specified job or the current job to the foreground.
getopts optstring var
The POSIX getopts command, not to be confused with the Bell Labs -derived getopt(1).
The first argument should be a series of letters, each of which may be optionally followed by a
colon to indicate that the option requires an argument. The variable specified is set to the
parsed option.
The getopts command deprecates the older getopt(1) utility due to its handling of arguments
containing whitespace.
The getopts builtin may be used to obtain options and their arguments from a list of parame‐
ters. When invoked, getopts places the value of the next option from the option string in the
list in the shell variable specified by var and its index in the shell variable OPTIND. When
the shell is invoked, OPTIND is initialized to 1. For each option that requires an argument,
the getopts builtin will place it in the shell variable OPTARG. If an option is not allowed
for in the optstring, then OPTARG will be unset.
optstring is a string of recognized option letters (see getopt(3)). If a letter is followed by
a colon, the option is expected to have an argument which may or may not be separated from it
by white space. If an option character is not found where expected, getopts will set the vari‐
able var to a “?”; getopts will then unset OPTARG and write output to standard error. By spec‐
ifying a colon as the first character of optstring all errors will be ignored.
After the last option getopts will return a non-zero value and set var to “?”.
The following code fragment shows how one might process the arguments for a command that can
take the options [a] and [b], and the option [c], which requires an argument.
while getopts abc: f
do
case $f in
a | b) flag=$f;;
c) carg=$OPTARG;;
\?) echo $USAGE; exit 1;;
esac
done
shift `expr $OPTIND - 1`
This code will accept any of the following as equivalent:
cmd -acarg file file
cmd -a -c arg file file
cmd -carg -a file file
cmd -a -carg -- file file
hash -rv command ...
The shell maintains a hash table which remembers the locations of commands. With no arguments
whatsoever, the hash command prints out the contents of this table. Entries which have not
been looked at since the last cd command are marked with an asterisk; it is possible for these
entries to be invalid.
With arguments, the hash command removes the specified commands from the hash table (unless
they are functions) and then locates them. With the -v option, hash prints the locations of
the commands as it finds them. The -r option causes the hash command to delete all the entries
in the hash table except for functions.
pwd [-LP]
builtin command remembers what the current directory is rather than recomputing it each time.
This makes it faster. However, if the current directory is renamed, the builtin version of pwd
will continue to print the old name for the directory. The -P option causes the physical value
of the current working directory to be shown, that is, all symbolic links are resolved to their
respective values. The -L option turns off the effect of any preceding -P options.
read [-p prompt] [-r] variable [...]
The prompt is printed if the -p option is specified and the standard input is a terminal. Then
a line is read from the standard input. The trailing newline is deleted from the line and the
line is split as described in the section on word splitting above, and the pieces are assigned
to the variables in order. At least one variable must be specified. If there are more pieces
than variables, the remaining pieces (along with the characters in IFS that separated them) are
assigned to the last variable. If there are more variables than pieces, the remaining vari‐
ables are assigned the null string. The read builtin will indicate success unless EOF is en‐
countered on input, in which case failure is returned.
By default, unless the -r option is specified, the backslash “\” acts as an escape character,
causing the following character to be treated literally. If a backslash is followed by a new‐
line, the backslash and the newline will be deleted.
readonly name ...
readonly -p
The specified names are marked as read only, so that they cannot be subsequently modified or
unset. The shell allows the value of a variable to be set at the same time it is marked read
only by writing
readonly name=value
With no arguments the readonly command lists the names of all read only variables. With the -p
option specified the output will be formatted suitably for non-interactive use.
printf format [arguments ...]
printf formats and prints its arguments, after the first, under control of the format. The
format is a character string which contains three types of objects: plain characters, which are
simply copied to standard output, character escape sequences which are converted and copied to
the standard output, and format specifications, each of which causes printing of the next suc‐
cessive argument.
The arguments after the first are treated as strings if the corresponding format is either b, c
or s; otherwise it is evaluated as a C constant, with the following extensions:
• A leading plus or minus sign is allowed.
• If the leading character is a single or double quote, the value is the ASCII code of
the next character.
The format string is reused as often as necessary to satisfy the arguments. Any extra format
specifications are evaluated with zero or the null string.
Character escape sequences are in backslash notation as defined in ANSI X3.159-1989
(“ANSI C89”). The characters and their meanings are as follows:
\a Write a <bell> character.
\b Write a <backspace> character.
\e Write an <escape> (ESC) character.
\f Write a <form-feed> character.
\n Write a <new-line> character.
\r Write a <carriage return> character.
\t Write a <tab> character.
\v Write a <vertical tab> character.
\\ Write a backslash character.
\num Write an 8-bit character whose ASCII value is the 1-, 2-, or 3-digit octal number
num.
Each format specification is introduced by the percent character (``%). The remainder of the
format specification includes, in the following order:
Zero or more of the following flags:
# A `#' character specifying that the value should be printed in an ``alternative
form. For b, c, d, and s formats, this option has no effect. For the o for‐
mat the precision of the number is increased to force the first character of
the output string to a zero. For the x (X) format, a non-zero result has the
string 0x (0X) prepended to it. For e, E, f, g, and G formats, the result will
always contain a decimal point, even if no digits follow the point (normally, a
decimal point only appears in the results of those formats if a digit follows
the decimal point). For g and G formats, trailing zeros are not removed from
the result as they would otherwise be.
- A minus sign `-' which specifies left adjustment of the output in the indicated
field;
+ A `+' character specifying that there should always be a sign placed before the
number when using signed formats.
‘ ’ A space specifying that a blank should be left before a positive number for a
signed format. A `+' overrides a space if both are used;
0 A zero `0' character indicating that zero-padding should be used rather than
blank-padding. A `-' overrides a `0' if both are used;
Field Width:
An optional digit string specifying a field width; if the output string has fewer char‐
acters than the field width it will be blank-padded on the left (or right, if the left-
adjustment indicator has been given) to make up the field width (note that a leading
zero is a flag, but an embedded zero is part of a field width);
Precision:
An optional period, ‘.’, followed by an optional digit string giving a precision which
specifies the number of digits to appear after the decimal point, for e and f formats,
or the maximum number of bytes to be printed from a string (b and s formats); if the
digit string is missing, the precision is treated as zero;
Format:
A character which indicates the type of format to use (one of diouxXfwEgGbcs).
A field width or precision may be ‘*’ instead of a digit string. In this case an argument sup‐
plies the field width or precision.
The format characters and their meanings are:
diouXx The argument is printed as a signed decimal (d or i), unsigned octal, unsigned dec‐
imal, or unsigned hexadecimal (X or x), respectively.
f The argument is printed in the style [-]ddd.ddd where the number of d's after the
decimal point is equal to the precision specification for the argument. If the
precision is missing, 6 digits are given; if the precision is explicitly 0, no dig‐
its and no decimal point are printed.
eE The argument is printed in the style [-]d.ddde±dd where there is one digit before
the decimal point and the number after is equal to the precision specification for
the argument; when the precision is missing, 6 digits are produced. An upper-case
E is used for an `E' format.
gG The argument is printed in style f or in style e (E) whichever gives full precision
in minimum space.
b Characters from the string argument are printed with backslash-escape sequences ex‐
panded.
The following additional backslash-escape sequences are supported:
\c Causes dash to ignore any remaining characters in the string operand con‐
taining it, any remaining string operands, and any additional characters in
the format operand.
\0num Write an 8-bit character whose ASCII value is the 1-, 2-, or 3-digit octal
number num.
c The first character of argument is printed.
s Characters from the string argument are printed until the end is reached or until
the number of bytes indicated by the precision specification is reached; if the
precision is omitted, all characters in the string are printed.
% Print a `%'; no argument is used.
In no case does a non-existent or small field width cause truncation of a field; padding takes
place only if the specified field width exceeds the actual width.
set [{ -options | +options | -- }] arg ...
The set command performs three different functions.
With no arguments, it lists the values of all shell variables.
If options are given, it sets the specified option flags, or clears them as described in the
section called Argument List Processing. As a special case, if the option is -o or +o and no
argument is supplied, the shell prints the settings of all its options. If the option is -o,
the settings are printed in a human-readable format; if the option is +o, the settings are
printed in a format suitable for reinput to the shell to affect the same option settings.
The third use of the set command is to set the values of the shell's positional parameters to
the specified args. To change the positional parameters without changing any options, use “--”
as the first argument to set. If no args are present, the set command will clear all the posi‐
tional parameters (equivalent to executing “shift $#”.)
shift [n]
Shift the positional parameters n times. A shift sets the value of $1 to the value of $2, the
value of $2 to the value of $3, and so on, decreasing the value of $# by one. If n is greater
than the number of positional parameters, shift will issue an error message, and exit with re‐
turn status 2.
test expression
[ expression ]
The test utility evaluates the expression and, if it evaluates to true, returns a zero (true)
exit status; otherwise it returns 1 (false). If there is no expression, test also returns 1
(false).
All operators and flags are separate arguments to the test utility.
The following primaries are used to construct expression:
-b file True if file exists and is a block special file.
-c file True if file exists and is a character special file.
-d file True if file exists and is a directory.
-e file True if file exists (regardless of type).
-f file True if file exists and is a regular file.
-g file True if file exists and its set group ID flag is set.
-h file True if file exists and is a symbolic link.
-k file True if file exists and its sticky bit is set.
-n string True if the length of string is nonzero.
-p file True if file is a named pipe (FIFO).
-r file True if file exists and is readable.
-s file True if file exists and has a size greater than zero.
-t file_descriptor
True if the file whose file descriptor number is file_descriptor is open and is
associated with a terminal.
-u file True if file exists and its set user ID flag is set.
-w file True if file exists and is writable. True indicates only that the write flag is
on. The file is not writable on a read-only file system even if this test indi‐
cates true.
-x file True if file exists and is executable. True indicates only that the execute flag
is on. If file is a directory, true indicates that file can be searched.
-z string True if the length of string is zero.
-L file True if file exists and is a symbolic link. This operator is retained for com‐
patibility with previous versions of this program. Do not rely on its existence;
use -h instead.
-O file True if file exists and its owner matches the effective user id of this process.
-G file True if file exists and its group matches the effective group id of this process.
-S file True if file exists and is a socket.
file1 -nt file2
True if file1 and file2 exist and file1 is newer than file2.
file1 -ot file2
True if file1 and file2 exist and file1 is older than file2.
file1 -ef file2
True if file1 and file2 exist and refer to the same file.
string True if string is not the null string.
s1 = s2 True if the strings s1 and s2 are identical.
s1 != s2 True if the strings s1 and s2 are not identical.
s1 < s2 True if string s1 comes before s2 based on the ASCII value of their characters.
s1 > s2 True if string s1 comes after s2 based on the ASCII value of their characters.
n1 -eq n2 True if the integers n1 and n2 are algebraically equal.
n1 -ne n2 True if the integers n1 and n2 are not algebraically equal.
n1 -gt n2 True if the integer n1 is algebraically greater than the integer n2.
n1 -ge n2 True if the integer n1 is algebraically greater than or equal to the integer n2.
n1 -lt n2 True if the integer n1 is algebraically less than the integer n2.
n1 -le n2 True if the integer n1 is algebraically less than or equal to the integer n2.
These primaries can be combined with the following operators:
! expression True if expression is false.
expression1 -a expression2
True if both expression1 and expression2 are true.
expression1 -o expression2
True if either expression1 or expression2 are true.
(expression) True if expression is true.
The -a operator has higher precedence than the -o operator.
times Print the accumulated user and system times for the shell and for processes run from the shell.
The return status is 0.
trap [action signal ...]
Cause the shell to parse and execute action when any of the specified signals are received.
The signals are specified by signal number or as the name of the signal. If signal is 0 or
EXIT, the action is executed when the shell exits. action may be empty (), which causes the
specified signals to be ignored. With action omitted or set to `-' the specified signals are
set to their default action. When the shell forks off a subshell, it resets trapped (but not
ignored) signals to the default action. The trap command has no effect on signals that were
ignored on entry to the shell. trap without any arguments cause it to write a list of signals
and their associated action to the standard output in a format that is suitable as an input to
the shell that achieves the same trapping results.
Examples:
trap
List trapped signals and their corresponding action
trap INT QUIT tstp 30
Ignore signals INT QUIT TSTP USR1
trap date INT
Print date upon receiving signal INT
type [name ...]
Interpret each name as a command and print the resolution of the command search. Possible res‐
olutions are: shell keyword, alias, shell builtin, command, tracked alias and not found. For
aliases the alias expansion is printed; for commands and tracked aliases the complete pathname
of the command is printed.
ulimit [-H | -S] [-a | -tfdscmlpnv [value]]
Inquire about or set the hard or soft limits on processes or set new limits. The choice be‐
tween hard limit (which no process is allowed to violate, and which may not be raised once it
has been lowered) and soft limit (which causes processes to be signaled but not necessarily
killed, and which may be raised) is made with these flags:
-H set or inquire about hard limits
-S set or inquire about soft limits. If neither -H nor -S is specified, the soft
limit is displayed or both limits are set. If both are specified, the last one
wins.
The limit to be interrogated or set, then, is chosen by specifying any one of these flags:
-a show all the current limits
-t show or set the limit on CPU time (in seconds)
-f show or set the limit on the largest file that can be created (in 512-byte blocks)
-d show or set the limit on the data segment size of a process (in kilobytes)
-s show or set the limit on the stack size of a process (in kilobytes)
-c show or set the limit on the largest core dump size that can be produced (in
512-byte blocks)
-m show or set the limit on the total physical memory that can be in use by a process
(in kilobytes)
-l show or set the limit on how much memory a process can lock with mlock(2) (in kilo‐
bytes)
-p show or set the limit on the number of processes this user can have at one time
-n show or set the limit on the number files a process can have open at once
-v show or set the limit on the total virtual memory that can be in use by a process
(in kilobytes)
-r show or set the limit on the real-time scheduling priority of a process
If none of these is specified, it is the limit on file size that is shown or set. If value is
specified, the limit is set to that number; otherwise the current limit is displayed.
Limits of an arbitrary process can be displayed or set using the sysctl(8) utility.
umask [mask]
Set the value of umask (see umask(2)) to the specified octal value. If the argument is omit‐
ted, the umask value is printed.
unalias [-a] [name]
If name is specified, the shell removes that alias. If -a is specified, all aliases are re‐
moved.
unset [-fv] name ...
The specified variables and functions are unset and unexported. If -f or -v is specified, the
corresponding function or variable is unset, respectively. If a given name corresponds to both
a variable and a function, and no options are given, only the variable is unset.
wait [job]
Wait for the specified job to complete and return the exit status of the last process in the
job. If the argument is omitted, wait for all jobs to complete and return an exit status of
zero.
Command Line Editing
When dash is being used interactively from a terminal, the current command and the command history
(see fc in Builtins) can be edited using vi-mode command-line editing. This mode uses commands, de‐
scribed below, similar to a subset of those described in the vi man page. The command ‘set -o vi’ en‐
ables vi-mode editing and places sh into vi insert mode. With vi-mode enabled, sh can be switched be‐
tween insert mode and command mode. It is similar to vi: typing ⟨ESC⟩ enters vi command mode. Hit‐
ting ⟨return⟩ while in command mode will pass the line to the shell.
EXIT STATUS
構文エラーなど、シェルが検出したエラーは、シェルが0以外の終了ステータスで終了する。 シェルが対話型シェルでない場合、シェルファイルの実行は中断される。 それ以外の場合、シェルは最後に実行されたコマンドの終了ステータスを返すか、またはexit組み込み関数が数値引数とともに使用されている場合、その引数を返す。
ENVIRONMENT
- HOME
- パスワードファイル (passwd) の中のユーザのログインディレクトリから login によって自動的に設定される。 この環境変数は、cd 組み込み関数に対するデフォルトの引数としても機能する。
- PATH
- 実行可能ファイルのデフォルトの検索パスである。 上記の「パスの検索」を参照。
- CDPATH
- cd 組み込み関数で使われる検索パス。
- 新しいメールの到着をチェックするメールファイルの名前。 MAILPATH で上書きされる。
- MAILCHECK
- MAILPATH または MAIL ファイルで指定されたファイルにメールが到着しているかどうか シェルがチェックする頻度を秒数で指定する。 0 に設定すると、各プロンプトでチェックが行われる。
- MAILPATH
- シェルがメールの到着を確認するためのファイル名のコロン ":" で区切られたリスト。 この環境設定は、MAILの設定より優先される。 一度に監視できるメールボックスの数は最大10個である。
- PS1
- 主なプロンプト文字列。デフォルトは "$" で、スーパーユーザーでない場合は "#" になる。
PS2
- 二次プロンプトの文字列。デフォルトは ">" である。
- PS4
- 実行トレース (set -x) が有効な場合に、各行の前に出力する文字列。
- IFS
- 入力フィールドの区切り文字。 これは通常、⟨space⟩、⟨tab⟩、⟨newline⟩に設定されている。 詳細は空白の分割のセクションを参照。
- TERM
- シェルのデフォルトの端末設定。 これはシェルの子にも継承され、履歴編集モードで使用される。
- HISTSIZE
- シェルのヒストリバッファの行数。
- PWD
- 現在の作業ディレクトリの論理値。 cd コマンドで設定する。
- OLDPWD
- 現在の作業ディレクトリの直前の論理値。 cdコマンドで設定される。
- PPID
- シェルの親プロセスのプロセスID。
FILES
- $HOME/.profile
SEE ALSO
csh, echo, getopt, ksh, login, printf, test, getopt(3), passwd, environ(7), sysctl
HISTORY
dash は POSIX に準拠した /bin/sh の実装で、できるだけ小さくなることを目指している。 dash は NetBSD 版 ash (the Almquist SHell) の直系の子孫で、1997 年初めに Linux に移植された。2002年に dash に改名された。
BUGS
Setuid シェルスクリプトは、重大なセキュリティリスクとなるため、絶対に避けるべきである。
PS1、PS2、PS4は、表示する前にパラメータ展開する必要がある。
External link
 | この記事は、Debianのmanpageの項目を翻訳一部改変しております。 |