This is a simple shell function to calculate factorial.
#!/bin/bash
function factorial()
{
if (( $1 < 2 ))
then
echo 1
else
echo $(( $1 * $(factorial $(( $1 - 1 ))) ))
fi
}
factorial $1
But I find that this script will create many sub-shells when given a very big input. It is not necessary and not efficient. Is there any way to call recursive functions without creating new sub-shells?
My goal is not how to write a factorial function in shell, but how to avoid creating sub-shells when call recursively defined functions.
In fact, even a simple function call will cause creation of sub-shells:
#!/bin/bash
function fac0() {
ps >> log
echo $1
}
function fac1() {
ps >> log
echo $(( $1 * $(fac0 $(( $1 - 1 ))) ))
}
touch log
fac1 $1
After run the script, the log
file's content is: (It still creates sub-shells)
PID TTY TIME CMD
9205 pts/8 00:00:00 bash
9245 pts/8 00:00:00 call_subshell.s
9247 pts/8 00:00:00 ps
PID TTY TIME CMD
9205 pts/8 00:00:00 bash
9245 pts/8 00:00:00 call_subshell.s
9248 pts/8 00:00:00 call_subshell.s
9249 pts/8 00:00:00 ps
Because of sub-shell creation, other unwanted aspects exist.
#!/bin/bash
declare -i i
i=0
function factorial() {
i=$(($i + 1))
if (( $1 < 2 ))
then
echo 1
else
local c=$(( $1 - 1 ))
echo $(( $1 * $(factorial $c) ))
fi
}
factorial $1
echo $i
This script will print $i=1
no matter what number the argument is!
Avoid the recursion:
#!/bin/bash
factorial() {
local f=1
for ((i=2; i<=$1; i++)) ; do
(( f *= i ))
done
echo $f
}
factorial $1
The subshell is spawned because of command substitution. Use a "argument as a result" way to avoid it:
#!/bin/bash
# set -xv
factorial() {
# echo $BASH_SUBSHELL
if [[ $2 ]] ; then
if (( $2 == 1 )) ; then
echo $1
else
factorial $(( $1 * $2 )) $(( $2 - 1 ))
fi
else
factorial $1 $(( $1 - 1 ))
fi
}
factorial $1
One need only separate the calculation from the function call and use the 'let' builtin. If there were commands in the parameters, use the 'eval' builtin rather than $(). Not as elegant, perhaps, but avoids subshells.
if [[ $2 ]] ; then
if (( $2 == 1 )) ; then
echo $1
else
let "p1=$1*$2"
let "p2 = $2-1"
factorial $p1 $p2
fi
else
let "p2 = $1-1"
factorial $1 $p2
fi
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With