command >   file.txt
command >>  file.txt  # to append
command >&  file.txt # re-direct stdout and stderr
command 1>& file.txt # 1-stdout
command 2>& file.txt # 2-stderr
command &>> file.txt # append file with both streams (2-stderr to 1-stdout)
command 2> /dev/null # silence error
command 2>&1         # re-direct 2-stderr to 1-stdout
command 2>&1 >> file.txt # re-direct 2-stderr to 1-stdout and append file with both streams

convert _tmp*.png _tmp.gif

#or in python
os.system("convert _tmp*.png _tmp.gif")