rename multiple file

foo001.a
foo002.a
foo003.a
foo004.a
foo005.a


#rename ‘s/foo/bar/’ foo*.a
#ls
------result
bar001.a
bar002.a
bar003.a
bar004.a
bar005.a


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To remove .jpg file extension, you write command as follows:

$ rename 's/\.jpg$//' *.jpg

To convert all uppercase filenames to lowercase:

$ rename 'y/A-Z/a-z/' *

Read the man page of rename command for more information:
man rename


other :
rename space to _

for file in *;do mv $file ${file// /_};done