Simple Commands & Pipelines (zsh)

### 6.1 Simple Commands & Pipelines ¶

A simple command is a sequence of optional parameter assignments followed by blank-separated words, with optional redirections interspersed. For a description of assignment, see the beginning of Parameters.

The first word is the command to be executed, and the remaining words, if any, are arguments to the command. If a command name is given, the parameter assignments modify the environment of the command when it is executed. The value of a simple command is its exit status, or 128 plus the signal number if terminated by a signal. For example,

    echo foo

is a simple command with arguments.

A pipeline is either a simple command, or a sequence of two or more simple commands where each command is separated from the next by ‘`|`’ or ‘`|&`’. Where commands are separated by ‘`|`’, the standard output of the first command is connected to the standard input of the next. ‘`|&`’ is shorthand for ‘`2>&1 |`’, which connects both the standard output and the standard error of the command to the standard input of the next. The value of a pipeline is the value of the last command, unless the pipeline is preceded by ‘`!`’ in which case the value is the logical inverse of the value of the last command. For example,

    echo foo | sed 's/foo/bar/'

is a pipeline, where the output (‘`foo`’ plus a newline) of the first command will be passed to the input of the second.

If a pipeline is preceded by ‘`coproc`’, it is executed as a coprocess; a two-way pipe is established between it and the parent shell. The shell can read from or write to the coprocess by means of the ‘`>&p`’ and ‘`<&p`’ redirection operators or with ‘`print -p`’ and ‘`read -p`’. A pipeline cannot be preceded by both ‘`coproc`’ and ‘`!`’. If job control is active, the coprocess can be treated in other than input and output as an ordinary background job.

A sublist is either a single pipeline, or a sequence of two or more pipelines separated by ‘`&&`’ or ‘`||`’. If two pipelines are separated by ‘`&&`’, the second pipeline is executed only if the first succeeds (returns a zero status). If two pipelines are separated by ‘`||`’, the second is executed only if the first fails (returns a nonzero status). Both operators have equal precedence and are left associative. The value of the sublist is the value of the last pipeline executed. For example,

    dmesg | grep panic && print yes

is a sublist consisting of two pipelines, the second just a simple command which will be executed if and only if the `grep` command returns a zero status. If it does not, the value of the sublist is that return status, else it is the status returned by the `print` (almost certainly zero).

A list is a sequence of zero or more sublists, in which each sublist is terminated by ‘`;`’, ‘`&`’, ‘`&|`’, ‘`&!`’, or a newline. This terminator may optionally be omitted from the last sublist in the list when the list appears as a complex command inside ‘`(`...`)`’ or ‘`{`...`}`’. When a sublist is terminated by ‘`;`’ or newline, the shell waits for it to finish before executing the next sublist. If a sublist is terminated by a ‘`&`’, ‘`&|`’, or ‘`&!`’, the shell executes the last pipeline in it in the background, and does not wait for it to finish (note the difference from other shells which execute the whole sublist in the background). A backgrounded pipeline returns a status of zero.

More generally, a list can be seen as a set of any shell commands whatsoever, including the complex commands below; this is implied wherever the word ‘list’ appears in later descriptions. For example, the commands in a shell function form a special sort of list.