| Advanced Bash-Scripting HOWTO: A guide to shell scripting, using Bash | ||
|---|---|---|
| Prev | Chapter 3. Tutorial / Reference | Next |
Running a shell script launches another instance of the command processor. Just as your commands are interpreted at the command line prompt, similarly does a script batch process a list of commands in a file. Each shell script running is, in effect, a subprocess of the parent shell, the one that gives you the prompt at the console or in an xterm window.
A shell script can also launch subprocesses. These subshells let the script do parallel processing, in effect executing multiple subtasks simultaneously.
( command1; command2; command3; ... )
A command list embedded between parentheses runs as a subshell.
Note: Variables in a subshell are not visible outside the block of code in the subshell. These are, in effect, local variables.
Example 3-62. Variable scope in a subshell
#!/bin/bash echo outer_variable=Outer ( inner_variable=Inner echo "From subshell, \"inner_variable\" = $inner_variable" echo "From subshell, \"outer\" = $outer_variable" ) echo if [ -z $inner_variable ] then echo "inner_variable undefined in main body of shell" else echo "inner_variable defined in main body of shell" fi echo "From main body of shell, \"inner_variable\" = $inner_variable" # $inner_variable will show as uninitialized because # variables defined in a subshell are "local variables". echo exit 0 |
Example 3-63. Running parallel processes in subshells
(cat list1 list2 list3 | sort | uniq > list123) (cat list4 list5 list6 | sort | uniq > list456) # Merges and sorts both sets of lists simultaneously. wait #Don't execute the next command until subshells finish. diff list123 list456 |
Note: A command block between curly braces does not launch a subshell.
{ command1; command2; command3; ... }