Automation using Expect

Pexpect on Mac

$ sudo easy_install pexpect $ sudo easy_install ptyprocess

$ python -c “import pexpect” $ echo $? // to check if pexpect module is installed

Tips n Tricks

1. Command line input using -c option $ expect -c ‘expect “” {send “pressed enter”}’

2. Execute expect script interactively using -i option $ expect -i arg1 arg2 arg3

3. Print debug messages while executing expect script $ expect -d sample.exp

4. Enable expect debugger using -D $ expect -D 1 sample.exp

5. Execute expect script line by line $ expect -b sample.exp

6. To provide options to expect script (not to be interpreted by expect itself) $ expect sample.exp -d -c

7. Reading command line arguments by indexes. lindex can be used for this. Ex. set arg0 [lindex $argv 0]

8. Reading command line arguments by range. lrange is used. It specifies start arg index and stop arg index. Hence to read same arg0 using lrange, it is set arg0 [lrange $argv 0 0]

9. Printing logs to user console. Use 'send_user' instead of ‘puts’. For ex. send_user "string"

10. Square brackets [] have different meaning in Expect (used to execute commands). Hence, when using them in strings, they should be escaped using
    character.

11. In order to provide default values to some arguments of a function, use proc justdoit {a {b 1} {c -1}} { }

12. To pass variable number of arguments to a proc/function, ‘args’ is used. proc justdoit {args} { }

This can be combined with variable names to get proc arguments in which first few arguments are fixed and remaining ones are variable. proc justdoit {firstarg secondarg args} { }

13. Pre-defined command line arguments $argc - number items of arguments passed to a script. $argv - list of the arguments. $argv0 - name of the script.

14. Few more debugging pearls: To see where the error was triggered in interactive mode, use set errorInfo

This variable shows index of the faulty line inside the procedure body - FP (for the beginners) info body the_name_of_the_procedure

15. These seem to be the major issues people hit:
    1. expecting less than is advisable and too much in other places: “Funny Users Password:” in contrast to “assword:”
    2. underestimating the greediness of REs “.*" in contrast to “.*?" or building really brilliant and convoluted REs that break in really brilliant and convoluted ways.
    3. using spawn where exp_send would be the proper command: spawn telnet .. login stuff .. exp_send “MySuperDuperRemoteScript with some args”
    4. being too optimistic and constructing scripts that continue unfazed due to not handling “timeout” and “eof”
    5. Would making un“expect”ed timeout or eof a catchable error help?
16. ‘autoexpect’ creates script.exp by recording different commands entered on command prompt.

http://www.thegeekstuff.com/2010/12/5-expect-script-command-line-argument-examples/