Bash 매뉴얼 간단 요약

[codeyoma]

built_in

• echo
  The return status is 0 unless a write error occurs.

• cd
  If a non-empty directory name from CDPATH is used, or if ‘-’ is the first argument, and the directory change is successful, the absolute pathname of the new working directory is written to the standard output.

• pwd
  The return status is zero unless an error is encountered while determining the name of the current directory or an invalid option is supplied.

• export
  The return status is zero unless an invalid option is supplied, one of the names is not a valid shell variable name, or -f is supplied with a name that is not a shell function.

• unset
  If no options are supplied, each name refers to a variable; if there is no variable by that name, a function with that name, if any, is unset.
  The return status is zero unless a name is readonly.

• exit
  Exit the shell, returning a status of n to the shell’s parent. If n is omitted, the exit status is that of the last command executed.
  +. 서브쉘이거나, 파이프 열리면 exit 문구 출력 X, 끝의 exit_status 받아옴

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Simple Commands

• The first word generally specifies a command to be executed, with the rest of the words being that command’s arguments.
• The return status of a simple command is its exit status as provided by the POSIX 1003.1 waitpid function, or 128+n if the command was terminated by signal n.

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$_ : $_지난 명령의 마지막 인자로 설정된 특수 변수

• ($_, an underscore.) At shell startup, set to the pathname used to invoke the shell or shell script being executed as passed in the environment or argument list.
• Subsequently, expands to the last argument to the previous simple command executed in the foreground, after expansion.
• Also set to the full pathname used to invoke each command executed and placed in the environment exported to that command.
• When Bash invokes an external command, the variable ‘$_’ is set to the full pathname of the command and passed to that command in its environment.

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( cmd ) → subshell

• Since the list  is executed in a subshell, variable assignments do not remain in effect after the subshell completes.

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environment variables

• ‘+=’ In the context where an assignment statement is assigning a value to a shell variable or array index, the ‘+=’ operator can be used to append to or add to the variable’s previous value. -> Shell Arithmetic Assignment

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signal

• When a command terminates on a fatal signal whose number is N, Bash uses the value 128+N as the exit status.
• If a command is not found, the child process created to execute it returns a status of 127. If a command is found but is not executable, the return status is 126.

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here doc*

• No parameter and variable expansion, command substitution, arithmetic expansion, or filename expansion is performed on word.
• If any part of word is quoted, the delimiter is the result of quote removal on word, and the lines in the here-document are not expanded.
• If word is unquoted, all lines of the here-document are subjected to parameter expansion, command substitution, and arithmetic expansion, the character sequence \newline is ignored, and ‘\’ must be used to quote the characters ‘\’, ‘$’, and ‘`’.

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redirections*

• The word following the redirection operator in the following descriptions, unless otherwise noted, is subjected to brace expansion, tilde expansion, parameter expansion, command substitution, arithmetic expansion, quote removal, filename expansion, and word splitting.
• If it expands to more than one word, Bash reports an error.

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pipeline

• The output of each command in the pipeline is connected via a pipe to the input of the next command. That is, each command reads the previous command’s output. This connection is performed before any redirections specified by the command.
• If the pipeline is not executed asynchronously (see Lists), the shell waits for all commands in the pipeline to complete.
• Each command in a pipeline is executed in its own subshell, which is a separate process (see Command Execution Environment).
• The exit status of a pipeline is the exit status of the last command in the pipeline, unless the pipefail option is enabled (see The Set Builtin). If pipefail is enabled, the pipeline’s return status is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands exit successfully.
• The shell waits for all commands in the pipeline to terminate before returning a value.

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