Using file descriptors to coordinate processes and paralized tasks in bash.

bash_subprocesses.sh

#!/usr/bin/env bash                                                                                                                                                                          
                                                                                                                                                                                             
function sub_proc_test() {                                                                                                                                                                   
    START=${SECONDS}                                                                                                                                                                         
    echo "[1] pid:${$}"                                                                                                                                                                      
    (                                                                                                                                                                                        
        echo "[2] pid:${$}"                                                                                                                                                                  
        while read line; do [[ -z $line ]] && exit; echo "[2] ${line}" >&100; done                                                                                                           
    ) 100> >(                                                                                                                                                                                
        echo "[3] pid ${$}"                                                                                                                                                                  
        while read line; do echo "[3] ${line}"; done                                                                                                                                         
    ) < <(cat -- \                                                                                                                                                                           
              <(echo "[4]"; echo "[4] test 1"; echo "[4] test 2") \                                                                                                                          
              <(echo "[5]"; echo "[5] sleep test 1"; sleep 1; echo "[5] sleep test 2") \                                                                                                     
              <(echo "[6]";{ sleep 1 && echo "[6] async test 1" & };echo "[6] async test 2")                                                                                                 
         )                                                                                                                                                                                   
                                                                                                                                                                                             
    echo "[1] pid:${$}"                                                                                                                                                                      
    END=${SECONDS}                                                                                                                                                                           
    ELAPSED=$(( END - START ))                                                                                                                                                               
    echo "Time: ${ELAPSED}"                                                                                                                                                                  
}                                                                                                                                                                                            
                                                                                                                                                                                             
# Below we can see the blocking behavior of 'cat' in that processes 4, 5, and 6                                                                                                              
# occur sequentially. Although intra-process we can see that asyncronous behavior                                                                                                            
# is possible as seen in the output of process 6.                                                                                                                                            
                                                                                                                                                                                             
# We can also see that we first call process 2 which reads the input, and passes it                                                                                                          
# into the fd 100, which is then outputted into the process 3, such that process 2                                                                                                           
# is composed with process 3 in a 'pass by reference' style. Where 'pass by value'                                                                                                           
# would be piping.                                                                                                                                                                           
                                                                                                                                                                                             
# time sub_proc_test                                                                                                                                                                         
# [1] pid:18994                                                                                                                                                                              
# [3] pid 18994                                                                                                                                                                              
# [2] pid:18994                                                                                                                                                                              
# [3] [2] [4]                                                                                                                                                                                
# [3] [2] [4] test 1                                                                                                                                                                         
# [3] [2] [4] test 2                                                                                                                                                                         
# [3] [2] [5]                                                                                                                                                                                
# [3] [2] [5] sleep test 1                                                                                                                                                                   
# [3] [2] [5] sleep test 2                                                                                                                                                                   
# [3] [2] [6]                                                                                                                                                                                
# [3] [2] [6] async test 2                                                                                                                                                                   
# [3] [2] [6] async test 1                                                                                                                                                                   
# [1] pid:18994                                                                                                                                                                              
# Time: 1                                                                                                                                                                                    
#                                                                                                                                                                                            
# real  0m1.009s                                                                                                                                                                             
# user  0m0.000s                                                                                                                                                                             
# sys   0m0.008s