I need to execute shell command in python program (I have ubuntu). More specifically I want to create graph using graphviz in python script. My code is os.system("dot -Tpng graph.dot -o graph.png") It does not work, but if I just type dot -Tpng graph.dot -o graph.png in command line then everything is fine. Do you know what the problem? Thank you! Edit: Does not work means that nothing happens, there are no errors. Output from dot -v -Tpng graph.dot -o graph.png: dot - graphviz version 2.26.3 (20100126.1600) Activated plugin library: libgvplugin_pango.so.6 Using textlayout: textlayout:cairo Activated plugin library: libgvplugin_dot_layout.so.6 Using layout: dot:dot_layout Using render: cairo:cairo Using device: png:cairo:cairo The plugin configuration file: /usr/lib/graphviz/config6 was successfully loaded. render : cairo dot fig gd map ps svg tk vml vrml xdot layout : circo dot fdp neato nop nop1 nop2 osage patchwork sfdp twopi textlayout : textlayout device : canon cmap cmapx cmapx_np dot eps fig gd gd2 gif gv imap imap_np ismap jpe jpeg jpg pdf plain plain-ext png ps ps2 svg svgz tk vml vmlz vrml wbmp x11 xdot xlib loadimage : (lib) eps gd gd2 gif jpe jpeg jpg png ps svg