How to get tab completion in your bash aliases

So did you learn about the magical world of aliases and functions in bash?

So do you now type `ag` instead of `sudo apt-get` and found out that pressing tab doesn’t show options in apt-get? What started out as a crusade to save yourself from typing 10 letters, ended up with you typing 20?

Luckily there’s a way out for you and here goes:

sudo nano /usr/local/bin/tab-completion.sh

Now add the following code in this file:

# Automatically add completion for all aliases to commands having completion functions
function alias_completion {
 local namespace=”alias_completion”
# parse function based completion definitions, where capture group 2 => function and 3 => trigger
 local compl_regex=’complete( +[^ ]+)* -F ([^ ]+) (“[^”]+”|[^ ]+)’
 # parse alias definitions, where capture group 1 => trigger, 2 => command, 3 => command arguments
 local alias_regex=”alias ([^=]+)=’(\”[^\”]+\”|[^ ]+)(( +[^ ]+)*)’”
# create array of function completion triggers, keeping multi-word triggers together
 eval “local completions=($(complete -p | sed -Ene “/$compl_regex/s//’\3'/p”))”
 (( ${#completions[@]} == 0 )) && return 0
# create temporary file for wrapper functions and completions
 rm -f “/tmp/${namespace}-*.tmp” # preliminary cleanup
 local tmp_file; tmp_file=”$(mktemp “/tmp/${namespace}-${RANDOM}XXX.tmp”)” || return 1
local completion_loader; completion_loader=”$(complete -p -D 2>/dev/null | sed -Ene ‘s/.* -F ([^ ]*).*/\1/p’)”
# read in “<alias> ‘<aliased command>’ ‘<command args>’” lines from defined aliases
 local line; while read line; do
 eval “local alias_tokens; alias_tokens=($line)” 2>/dev/null || continue # some alias arg patterns cause an eval parse error
 local alias_name=”${alias_tokens[0]}” alias_cmd=”${alias_tokens[1]}” alias_args=”${alias_tokens[2]# }”
# skip aliases to pipes, boolean control structures and other command lists
 # (leveraging that eval errs out if $alias_args contains unquoted shell metacharacters)
 eval “local alias_arg_words; alias_arg_words=($alias_args)” 2>/dev/null || continue
 # avoid expanding wildcards
 read -a alias_arg_words <<< “$alias_args”
# skip alias if there is no completion function triggered by the aliased command
 if [[ ! “ ${completions[*]} “ =~ “ $alias_cmd “ ]]; then
 if [[ -n “$completion_loader” ]]; then
 # force loading of completions for the aliased command
 eval “$completion_loader $alias_cmd”
 # 124 means completion loader was successful
 [[ $? -eq 124 ]] || continue
 completions+=($alias_cmd)
 else
 continue
 fi
 fi
 local new_completion=”$(complete -p “$alias_cmd”)”
# create a wrapper inserting the alias arguments if any
 if [[ -n $alias_args ]]; then
 local compl_func=”${new_completion/#* -F /}”; compl_func=”${compl_func%% *}”
 # avoid recursive call loops by ignoring our own functions
 if [[ “${compl_func#_$namespace::}” == $compl_func ]]; then
 local compl_wrapper=”_${namespace}::${alias_name}”
 echo “function $compl_wrapper {
 (( COMP_CWORD += ${#alias_arg_words[@]} ))
 COMP_WORDS=($alias_cmd $alias_args \${COMP_WORDS[@]:1})
 (( COMP_POINT -= \${#COMP_LINE} ))
 COMP_LINE=\${COMP_LINE/$alias_name/$alias_cmd $alias_args}
 (( COMP_POINT += \${#COMP_LINE} ))
 $compl_func
 }” >> “$tmp_file”
 new_completion=”${new_completion/ -F $compl_func / -F $compl_wrapper }”
 fi
 fi
# replace completion trigger by alias
 new_completion=”${new_completion% *} $alias_name”
 echo “$new_completion” >> “$tmp_file”
 done < <(alias -p | sed -Ene “s/$alias_regex/\1 ‘\2’ ‘\3’/p”)
 source “$tmp_file” && rm -f “$tmp_file”
}; alias_completion

Code adapted from here, here and here.

Type Ctrl-O and return to save the file and Ctrl-X to exit. Now you need to source this shell script to your .bashrc file so that the functions get loaded.

sudo nano ~/.bashrc

Scroll down all the way to the bottom (it’s important), and then append the following code:

if [ -f ~/.bashrc ]; then
 source /usr/local/bin/tab-completion.sh
fi

Again save and exit using Ctrl-O and Ctrl-X. Now we need to reload bash for the changes to work!

source ~/.bashrc

Now try something like `ag ` (don’t forget the space) and press tab and Voila, tab completion is back!