Table of Contents
Hide in Bash
When debugging a Bash shell script, we may find that it runs slowly. Therefore, it's important to know some of the details hidden in Bash and learn some tricks.
Is $() or `` Really Necessary?
`` or $() in
shell scripts.
`` and $() create a subshell to run the list of commands
enclosed by them.[a] The subshell is a copy of a shell
process.[b] That is, `` and $() spawn a new
process in the OS and the parent shell has to hang and wait for the new
process to exit. So comes the critical problem, spawning process (1) is very
time-consuming.
Let me verify my claim with the following demos.
Demo 1
Write 0 - 99 to file test.tmp without `` and $().
# !/bin/bash
cat /dev/null > test.tmp
function func() {
echo -n $1 >> test.tmp
}
for (( i=0; i < 100; i++)); do
func $i
done
Time consumed in executing shell scripts.
time ./test.sh
real 0m0.004s
user 0m0.003s
sys 0m0.000s
cat test.tmp
0123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899
Demo 2
Write 0 - 99 to file test.tmp with ``.
# !/bin/bash
cat /dev/null > test.tmp
function func() {
echo -n $1 >> test.tmp
}
for (( i=0; i < 100; i++)); do
a=`func $i`
done
We can see that the time consumed is greater than the time consumed in Demo 1.
time ./test.sh
real 0m0.068s
user 0m0.053s
sys 0m0.010s
cat test.tmp
0123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899
BASHPID
Of course, the Bash shell provides a way to get the current shell process ID.
This is the BASHPID variable [c]. The following demo
uses BASHPID to confirm that a new process has been spawned when `` is used.
# !/bin/bash
function func() {
echo $BASHPID>pid.tmp
}
b=`func $i`
echo current PID: $BASHPID
echo subshell PID: `cat pid.tmp`
As we can see, the PIDs are not the same.
./test.sh
current PID: 14485
subshell PID: 14486