Check If Last Command Was Empty in PROMPT_COMMAND

bashprompt

In bash, from inside PROMPT_COMMAND, is there a way to tell if the user just hit 'return' and didn't enter a command?

Best Answer

Check whether the history number was incremented. A cancelled prompt or a prompt where the user just pressed Enter won't increment the history number.

The history number is available in the variable HISTCMD, but this is not available in PROMPT_COMMAND (because what you want there is in fact the history number of the previous command; the command that executes PROMPT_COMMAND itself has no history number). You can get the number from the output of fc.

prompt_command () {
  HISTCMD_previous=$(fc -l -1); HISTCMD_previous=${HISTCMD_previous%%$'[\t ]'*}
  if [[ -z $HISTCMD_before_last ]]; then
    # initial prompt
  elif [[ $HISTCMD_before_last = "$HISTCMD_previous" ]]; then
    # cancelled prompt
  else
    # a command was run
  fi
  HISTCMD_before_last=$HISTCMD_previous
}
PROMPT_COMMAND='prompt_command'

Note that if you've turned on squashing of duplicates in the history (HISTCONTROL=ignoredups or HISTCONTROL=erasedups), this will mistakenly report an empty command after running two identical commands successively.

Related Solutions

Bash Command – Printing and Saving the Last Bash Input Command

You can access the just-executed command line with the history built-in. (I have no idea why history 1 prints the just-executed command line but fc -nl -1 prints the previous commmand, as does fc -nl 0.)

PROMPT_COMMAND='echo -en "\033[38;5;2m"; history 1; echo -en "\033[0m\n"'

This prints a number before the command text. Here's a version that removes the number. (It may be incorrect if you go beyond 99999 history lines, I don't know how bash formats the number then.)

prompt_function () {
  local prompt_history="$(history 1)"
  prompt_history=${prompt_history:7}
  echo -En $'\033[38;5;2m'"$prompt_history"$'\033[0m\n'
}
PROMPT_COMMAND=prompt_function

(Note that echo -en ..."$prompt_history"... would expand backslashes in the command line, so I use echo -E and let the shell expansion generate the control characters with $''.).

Bash – How to a Bash script tell how it was run

From man bash under CONDITIONAL EXPRESSIONS:

-t fd  
    True if file descriptor fd is open and refers to a terminal.

Assuming fd 1 is standard out, if [ -t 1 ]; then should work for you. The Advanced Shell Scripting Guide claims that -t used this way will fail over ssh, and that the test (using stdin, not stdout) should therefore be:

if [[ -t 0 || -p /dev/stdin ]]

-p tests if a file exists and is a named pipe. However, I'd note experientially this is not true for me: -p /dev/stdin fails for both normal terminals and ssh sessions whereas if [ -t 0 ] (or -t 1) works in both cases (see also Gilles comments below about issues in that section of the Advanced Shell Scripting Guide).

If the primary issue is a specialized context from which you wish to call the script to behave in a way appropriate to that context, you can sidestep all these technicalities and save your self some fuss by using a wrapper and a custom variable:

!#/bin/bash

export SPECIAL_CONTEXT=1
/path/to/real/script.sh

Call this live_script.sh or whatever and double click that instead. You could of course accomplish the same thing with command line arguments, but a wrapper would still be needed to make point and click in a GUI file browser work.

Best Answer

Related Solutions

Bash Command – Printing and Saving the Last Bash Input Command

Bash – How to a Bash script tell how it was run

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