Customizing bash completion output: each suggestion on a new line

bash prior to version 4.2 doesn't allow any control over the output format of completions, unfortunately.

^{Bash 4.2+ allows switching to 1-suggestion-per-line output globally, as explained in Grisha Levit's helpful answer, which also links to a clever workaround to achieve a per-completion-function solution.}

The following is a tricky workaround for a custom completion.Solving this problem generically, for all defined completions, would be much harder (if there were a way to invoke readline functions directly, it might be easier, but I haven't found a way to do that).

To test the proof of concept below:

• Save to a file and source it (. file) in your interactive shell - this will:
  • define a command named foo (a shell function)
  • whose arguments complete based on matching filenames in the current directory.
  • (When foo is actually invoked, it simply prints its argument in diagnostic form.)
• Invoke as: foo [fileNamePrefix], then press tab:
  • If between 2 and 9 files in the current directory match, you'll see the desired line-by-line display.
  • Otherwise (1 match or 10 or more matches), normal completion will occur.

Limitations:

• Completion only works properly when applied to the LAST argument on the command line being edited.
• When a completion is actually inserted in the command line (once the match is unambiguous), NO space is appended to it (this behavior is required for the workaround).
• Redrawing the prompt the first time after printing custom-formatted output may not work properly: Redrawing the command line including the prompt must be simulated and since there is no direct way to obtain an expanded version of the prompt-definition string stored in $PS1, a workaround (inspired by https://stackoverflow.com/a/24006864/45375) is used, which should work in typical cases, but is not foolproof.

Approach:

• Defines and assigns a custom completion shell function to the command of interest.
• The custom function determines the matches and, if their count is in the desired range, bypasses the normal completion mechanism and creates custom-formatted output.
• The custom-formatted output (each match on its own line) is sent directly to the terminal >/dev/tty, and then the prompt and command line are manually "redrawn" to mimic standard completion behavior.
• See the comments in the source code for implementation details.

# Define the command (function) for which to establish custom command completion.# The command simply prints out all its arguments in diagnostic form.foo() { local a i=0; for a; do echo "\$$((i+=1))=[$a]"; done; }# Define the completion function that will generate the set of completions# when <tab> is pressed.# CAVEAT:#  Only works properly if <tab> is pressed at the END of the command line,#  i.e.,  if completion is applied to the LAST argument._complete_foo() {  local currToken="${COMP_WORDS[COMP_CWORD]}" matches matchCount  # Collect matches, providing the current command-line token as input.  IFS=$'\n' read -d '' -ra matches <<<"$(compgen -A file "$currToken")"  # Count matches.  matchCount=${#matches[@]}  # Output in custom format, depending on the number of matches.  if (( matchCount > 1 && matchCount < 10 )); then      # Output matches in CUSTOM format:      # print the matches line by line, directly to the terminal.    printf '\n%s' "${matches[@]}" >/dev/tty      # !! We actually *must* pass out the current token as the result,      # !! as it will otherwise be *removed* from the redrawn line,      # !! even though $COMP_LINE *includes* that token.      # !! Also, by passing out a nonempty result, we avoid the bell      # !! signal that normally indicates a failed completion.      # !! However, by passing out a single result, a *space* will      # !! be appended to the last token - unless the compspec      # !! (mapping established via `complete`) was defined with       # !! `-o nospace`.    COMPREPLY=( "$currToken" )      # Finally, simulate redrawing the command line.        # Obtain an *expanded version* of `$PS1` using a trick        # inspired by https://stackoverflow.com/a/24006864/45375.        # !! This is NOT foolproof, but hopefully works in most cases.    expandedPrompt=$(PS1="$PS1" debian_chroot="$debian_chroot" "$BASH" --norc -i </dev/null 2>&1 | sed -n '${s/^\(.*\)exit$/\1/p;}')    printf '\n%s%s' "$expandedPrompt" "$COMP_LINE" >/dev/tty  else # Just 1 match or 10 or more matches?      # Perform NORMAL completion: let bash handle it by       # reporting matches via array variable `$COMPREPLY`.    COMPREPLY=( "${matches[@]}" )      fi }# Map the completion function (`_complete_foo`) to the command (`foo`).# `-o nospace` ensures that no space is appended after a completion,# which is needed for our workaround.complete -o nospace -F _complete_foo -- foo

bash 4.2+ (and, more generally, applications using readline 6.2+) support this with the use of the completion-display-width variable.

The number of screen columns used to display possible matches when performing completion. The value is ignored if it is less than 0 or greater than the terminal screen width. A value of 0 will cause matches to be displayed one per line. The default value is -1.

Run the following to set the behavior for all completions¹ for your current session:

bind 'set completion-display-width 0'

Or modify your ~/.inputrc² file to have:

set completion-display-width 0

to change the behavior for all new shells.

^{1 See here for a method for controlling this behavior for individual custom completion functions.}

^{2 The search path for the readline init file is $INPUTRC, ~/.inputrc, /etc/inputrc so modify the file appropriate for you.}