lastpipe

Lastpipe is a Bash shell option that controls where the final component of a pipeline is executed. In the standard pipeline model, each element runs in its own subshell, which means that changes to shell variables made by any but the last stage are not visible to the parent shell after the pipeline finishes. When the lastpipe option is enabled, the last command in a pipeline is executed in the current shell instead of a subshell, allowing variable assignments and other side effects to persist beyond the pipeline.

Usage and behavior

Lastpipe is managed with the built-in shopt command. Enable it with shopt -s lastpipe and disable it with shopt -u lastpipe. The option applies only to pipelines in the current shell; it does not affect pipelines in command substitutions or when the shell is invoked in a way that isolates the pipeline in a subshell. It is a Bash-specific feature and is not portable to POSIX sh or other shells.

Examples

Without lastpipe, a common pattern to accumulate a value in a pipeline may fail to update the variable in the outer shell. For example, total=0; seq 3 | while read n; do total=$((total+n)); done; echo $total often prints 0. With lastpipe enabled, the final value can be updated in the current shell: shopt -s lastpipe; total=0; seq 3 | while read n; do total=$((total+n)); done; echo $total prints 6.

Limitations and considerations

While lastpipe can simplify certain data-processing patterns, it changes the expected isolation between pipeline stages. Scripts that rely on the default subshell behavior for variable scope or side effects may behave differently when lastpipe is enabled. It is typically avoided in portable scripts intended to run in environments other than Bash. See also Bash programming and shell pipelines.