Pipelines
A pipeline is a sequence of one or more commands separated by | or |&.
The format for a pipeline is:
[!] command1 [ | command2 ] ...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 command1.
Each command in the pipeline runs asynchronously in the background. The pipeline waits for all commands to finish before it exits.
An example of a pipeline can be illustrated as follows:
cat file.txt | sort | uniq
Piping all output
When using the | popeline separator. Only standard output is connected to the pipe. To illustrate this, let's look at this example:
mkdir foo | tr
When the separator |& is used, command1's standard error, in addition to its standard output, is connected to command2's standard input through the pipe; it is shorthand for 2>&1 |. This implicit redirection of the standard error to the standard output is performed after any redirections specified by command1.
Example:
mkdir foo |& tr
Exit code
The exit status of a pipeline is the exit status of the last command in the pipeline. For example:
command1 | command2 | command3The exit code of this pipeline is the exit code of the command3 command. No matter what the exit code of other commands is.
Exit code negation
If the pipeline is preceeded by an exclamation mark !. the exit code of that pipeline is negated. It will be 0 if the last command in the pipeline exits with a non-zero exit code. and 1otherwise.
For example:
! commandThe exit code for this pipeline is zero 0 if the exit code of the command command is non zero. (1, 2 ...). If the exit code of the command command is zero 0. Then the pipeline's exit code is one 1.
In other words. the ! mark negates the exit code of the pipeline.
INFO
Note that a pipeline does not have to contain more than one command. The same rules that apply to pipelines with many commands apply to pipelines with one command.