If I had a sleep command
or something else slowing down execution of .bashrc,
I can profile it to see the slow parts like this:
$ printf "set -x\nPS4='+\\\\t '\nexport HISTFILE=empty\nsleep 4\ntrue\n" > mybashrc
$ > empty
$ time bash --rcfile mybashrc -i <<< exit
+ PS4='+\t '
+19:13:47 export HISTFILE=empty
+19:13:47 HISTFILE=empty
+19:13:47 sleep 4
+19:13:51 true
$ exit
+19:13:51 exit
exit
real 0m4.022s
user 0m0.010s
sys 0m0.010s
with
other
options
for higher precision using GNU date.
But suppose I have a large bash history file, something like this:
$ yes ls | head -n 9999999 > big-hist
$ du -sh big-hist
29M big-hist
Now the shell startup is also slow (about four seconds on my machine),
but I can't see where the time is being spent,
only that it's some time after the last command of the .bashrc.
$ printf "set -x\nPS4='+\\\\t '\nexport HISTSIZE=-1\nexport HISTFILESIZE=-1\nexport HISTFILE=big-hist\ntrue\n" > mybashrc
$ time bash --rcfile mybashrc -i <<< exit
+ PS4='+\t '
+19:12:23 export HISTSIZE=-1
+19:12:23 HISTSIZE=-1
+19:12:23 export HISTFILESIZE=-1
+19:12:23 HISTFILESIZE=-1
+19:12:23 export HISTFILE=big-hist
+19:12:23 HISTFILE=big-hist
+19:12:23 true
$ exit
+19:12:27 exit
exit
real 0m4.184s
user 0m3.641s
sys 0m0.536s
The best guess I can think of is to run this:
$ time bash -i <<< exit
$ SHLVL=2 $ exit
exit
real 0m0.400s
user 0m0.326s
sys 0m0.075s
then add the last line of the .bashrc:
HISTFILE=/dev/null
and run again:
$ time bash -i <<< exit
$ exit
exit
real 0m0.081s
user 0m0.059s
sys 0m0.023s
although I'm not entirely sure this measures only the time spent processing the history file.
(Note: if you try this for yourself,
it is highly important to remember to remove the last line of the .bashrc
once the timing is complete!)
This is of particular interest to me
because I have for several years
enabled
unlimited history
in my .bashrc
and have noticed the startup time of new interactive shells has been impacted;
by more than 300 ms, if the timing above is accurate.
