Access latest command (not args) from PROMPT_COMMAND

Question

I have standalone PROMPT_COMMAND= in my .bash_profile. Every time user input a command, it's being run. I want to get the first word of the last command by user.

For example, if the user runs: $ printf "Hey!", I want to access printf from my PROMPT_COMMAND.

$0 gives me -bash and $1, $2, ... gives me arguments as said on the site below. https://wiki.bash-hackers.org/scripting/posparams#the_first_argument

But how can I access the first word like printf in the above example?

Preferably, I want to use the build-in commands as much and cleanly as possible.

stackoverflow.com/questions/14177700/… has some hints but nothing which really solves this problem. — tripleee
– tripleee, Commented Jan 23, 2019 at 4:57

l'L'l · Accepted Answer · 2019-01-23 23:31:34Z

2

You could use fc:

$ printf "%s %s\n" "foo" "bar"
foo bar
$ fc -ln -1
       printf "%s %s\n" "foo" "bar"

That prints the entire command with arguments and weird spacing, so I suppose you could do:

$ fc -ln -1 | awk '{ print $1 }'
printf

EDIT: In the case you don't want pressing Return to come back as fc you could do:

$ fc -ln -1 | awk '$1 !~ /fc/ {print $1}'

To create a function in ~/.bash_profile:

# Show last command without args
lcm () { fc -ln -1 | awk '$1 !~ /lcm/ {print $1}' ; }

Then for example:

$ date
Wed Jan 23 16:29:14 MST 2019
$ lcm
date

↳ GNU : Bash History Built-ins

edited Jan 23, 2019 at 23:31

answered Jan 23, 2019 at 3:37

l'L'l

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7 Comments

Penguinlay Over a year ago

Nice trick. But, it won't work when the user's last input is nothing (entering return key) though.

Penguinlay Over a year ago

@I'L'l, I've been trying it out. I am seeing two different behaviors and somehow it's not working yet. When I use it inside PROMPT_COMMAND, it prints out the last command as expected but still it's the case as long as user doesn't enter on nothing. When I call just the function, it prints out the last command, not lcm if the last command is not empty. Otherwise, it prints out lcm.

Penguinlay Over a year ago

Actually, calling function also works the same as in PROMPT_COMMAND as described above. Still printing the last command if the user enter nothing. My comment above was wrong as I called lcm after lcm followed by entering nothing a few times.

l'L'l Over a year ago

Did you reload (source) your ~/.bash_profile after making the changes? . ~/.bash_profile, also are wanting to save the command into a variable?

Penguinlay Over a year ago

I did reload ~/.bash_profile. I have two functions, one is where I put the command output to variable like input=$(fc -ln -1 | awk '$1 !~ /fc/ {print $1}'). Another function is just lcm function that you posted above. Both of them give me the same result.

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