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====== Hide in Bash ======

{{template>meta:template:pageinfo#tpl|
desc=<wrap heart_beat>Oops!</wrap> why is my Bash shell script running
so slow?}}

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? =====

<callout type="warning">
In the bash shell, we need to avoid frequent use of ''``'' or ''$()'' in
shell scripts. 
</callout>

''``'' and ''$()'' __create a subshell__ to run the list of commands
enclosed by them.((:bash5.2-c3.5.4)) The subshell is a copy of a shell
process.((:bash5.2-c3.7.3)) That is, ''``'' and ''$()'' __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 ((note:>
todo: Introduce what happens when a process is spawned.)) is very
time-consuming. 

Let me verify my claim with the following demos.

==== Demo 1 ====

Write 0 - 99 to file <wrap file>test.tmp</wrap> without ''``'' and ''$()''.

<WRAP group>
<WRAP column half>
<codeprism lang=bash>
# !/bin/bash

cat /dev/null > test.tmp

function func() {
        echo -n $1 >> test.tmp
}

for (( i=0; i < 100; i++)); do
        func $i
done
</codeprism>
</WRAP>

<WRAP column half>
Time consumed in executing shell scripts.

<codeprism lang=bash cmdout=3-5,7 hl=3-5>
time ./test.sh

real    0m0.004s
user    0m0.003s
sys     0m0.000s
cat test.tmp
0123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899
</codeprism>
</WRAP>
</WRAP>

==== Demo 2 ====

Write 0 - 99 to file <wrap file>test.tmp</wrap> with ''``''.

<WRAP group>
<WRAP column half>
<codeprism lang=bash hl=10>
# !/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
</codeprism>

</WRAP>

<WRAP column half>
We can see that the time consumed is greater than the time consumed in [[#demo_1|Demo 1]].

<codeprism lang=bash cmdout=3-5,7 hl=3-5>
time ./test.sh

real    0m0.068s
user    0m0.053s
sys     0m0.010s
cat test.tmp
0123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899
</codeprism>
</WRAP>
</WRAP>

==== BASHPID ====

Of course, the Bash shell provides a way to get the current shell process ID.
This is the ''BASHPID'' variable ((:bash5.2-index-BASHPID)). The following demo
uses ''BASHPID'' to confirm that a new process has been spawned when ''``'' is used.

<WRAP group>
<WRAP column half>
<codeprism lang=bash hl=4,7,9>
# !/bin/bash

function func() {
        echo $BASHPID>pid.tmp
}

b=`func $i`

echo current PID: $BASHPID
echo subshell PID: `cat pid.tmp`
</codeprism>

</WRAP>

<WRAP column half>
As we can see, the PIDs are not the same.

<codeprism lang=bash cmdout=2-3 hl=2-3>
./test.sh 
current PID: 14485
subshell PID: 14486
</codeprism>
</WRAP>
</WRAP>

{{template>meta:template:refnote#note}}
{{template>meta:template:refnote#ref}}

===== Further Reading =====

  * [[https://www.gnu.org/software/bash/manual/bash.html|Bash Reference Manual]]