Before a command is executed, its input and output may be redirected using a special notation interpreted
by the shell. Redirection may also be used to open and close files for the current shell execution
environment. The following redirection operators may precede or appear anywhere within a simple command
or may follow a command. Redirections are processed in the order they appear, from left to right.
This type of redirection instructs the shell to read input from the current source until a line
containing only delimiter (with no trailing blanks) is seen. All of the lines read up to that point are
then used as the standard input for a command.
The format of here-documents is:
No parameter expansion, command substitution, arithmetic expansion, or pathname expansion is performed on
word. If any characters in word are 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. In the latter case,
the character sequence \<newline> is ignored, and \ must be used to quote the characters \, $, and `.
If the redirection operator is <<-, then all leading tab characters are stripped from input lines and the
line containing delimiter. This allows here-documents within shell scripts to be indented in a natural
A variant of here documents, the format is:
The word is expanded and supplied to the command on its standard input.
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Commands separated by a ; are executed sequentially; the shell waits for each command to terminate in turn. The
return status is the exit status of the last command executed.
write arguments to standard output
A pipeline is a sequence of one or more commands separated by one of the control operators | or |&. The
format for a pipeline is:
[time [-p]] [ ! ] command [ [|⎪|&] command2 ... ]
The standard output of command is connected via a pipe to the standard input of command2. This
connection is performed before any redirections specified by the command (see REDIRECTION below). If |&
is used, the standard error of command is connected to command2's standard input through the pipe; it is
shorthand for 2>&1 |. This implicit redirection of the standard error is performed after any
redirections specified by the command.
The return status of a pipeline is the exit status of the last command, unless the pipefail option is
enabled. 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. If the reserved word
! precedes a pipeline, the exit status of that pipeline is the logical negation of the exit status as
described above. The shell waits for all commands in the pipeline to terminate before returning a value.
If the time reserved word precedes a pipeline, the elapsed as well as user and system time consumed by
its execution are reported when the pipeline terminates. The -p option changes the output format to that
specified by POSIX. When the shell is in posix mode, it does not recognize time as a reserved word if
the next token begins with a `-'. The TIMEFORMAT variable may be set to a format string that specifies
how the timing information should be displayed; see the description of TIMEFORMAT under Shell Variables
When the shell is in posix mode, time may be followed by a newline. In this case, the shell displays the
total user and system time consumed by the shell and its children. The TIMEFORMAT variable may be used
to specify the format of the time information.
Each command in a pipeline is executed as a separate process (i.e., in a subshell).
duplicate standard input
If a command is terminated by the control operator &, the shell executes the command in the background in
a subshell. The shell does not wait for the command to finish, and the return status is 0.
delay for a specified amount of time
transfer a URL
(HTTP/FTP/FILE) Fetch the HTTP-header only! HTTP-servers feature the command HEAD which this uses
to get nothing but the header of a document. When used on a FTP or FILE file, curl displays the
file size and last modification time only.
change the working directory
An absolute or relative pathname of the directory that shall become the new working directory. The
interpretation of a relative pathname by cd depends on the -L option and the CDPATH and PWD
environment variables. If directory is an empty string, the results are unspecified.
- When a hyphen is used as the operand, this shall be equivalent to the command:
cd "$OLDPWD" && pwd
which changes to the previous working directory and then writes its name.