> ## Documentation Index
> Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Bash Pipeline
A Bash pipeline is an inter-process communication (IPC) mechanism that chains multiple commands together by connecting the standard output (`stdout`) of one process directly to the standard input (`stdin`) of the next process using anonymous pipes.
## Syntax
```bash theme={"dark"}
# Standard pipeline (stdout to stdin)
command1 | command2 | command3
# Standard error and standard output pipeline (stdout + stderr to stdin)
command1 |& command2
```
## Technical Mechanics
**File Descriptor Mapping**
* The standard pipe operator (`|`) connects File Descriptor 1 (FD 1, `stdout`) of the preceding command to File Descriptor 0 (FD 0, `stdin`) of the succeeding command.
* The `|&` operator is a Bash shorthand introduced in version 4.0. It connects both FD 1 (`stdout`) and FD 2 (`stderr`) of the preceding command to FD 0 (`stdin`) of the succeeding command. It is functionally equivalent to `command1 2>&1 | command2`.
**Execution Environment (Subshells)**
By default, Bash executes every command within a pipeline in its own separate subshell environment. Consequently, variable assignments, state changes, or directory changes (`cd`) made by any command within the pipeline will not persist in the parent shell once the pipeline terminates.
*Note: In Bash 4.2+, enabling `shopt -s lastpipe` allows the final command of a pipeline to execute in the current shell environment, provided job control is disabled.*
**Concurrency and I/O Blocking**
Commands in a pipeline are invoked concurrently, not sequentially. The operating system kernel allocates a bounded memory buffer for each anonymous pipe.
* The writing process will block (suspend execution) if the pipe buffer is full.
* The reading process will block if the pipe buffer is empty.
* The pipeline terminates when all constituent processes have terminated.
**Exit Status**
By default, the exit status (`$?`) of an entire pipeline is strictly the exit status of the *last* (rightmost) command in the chain. The exit statuses of preceding commands are discarded.
To alter this behavior, Bash provides the `pipefail` option:
```bash theme={"dark"}
set -o pipefail
```
When `pipefail` is enabled, the pipeline's exit status becomes the exit status of the rightmost command to exit with a non-zero status. If all commands in the pipeline exit successfully (status `0`), the pipeline returns `0`.
**Array of Exit Codes**
Bash retains the individual exit statuses of all commands executed in the most recent pipeline within the internal array variable `PIPESTATUS`.
```bash theme={"dark"}
command1 | command2 | command3
echo "${PIPESTATUS[@]}" # Outputs an array of three exit codes, e.g., "0 1 0"
```
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