Bash script: only echo line to ~/.bash_profile once if the line doesn't yet exist

Something like this should do it:

LINE_TO_ADD=". ~/.git-completion.bash"check_if_line_exists(){    # grep wont care if one or both files dont exist.    grep -qsFx "$LINE_TO_ADD" ~/.profile ~/.bash_profile}add_line_to_profile(){    profile=~/.profile    [ -w "$profile" ] || profile=~/.bash_profile    printf "%s\n" "$LINE_TO_ADD" >> "$profile"}check_if_line_exists || add_line_to_profile

A couple of notes:

• I've used the . command instead of source as source is a bashism, but .profile may be used by non-bash shells. The command source ... is an error in .profile
• I've used printf instead of echo because it's more portable and wont screw up backslash-escaped characters as bash's echo would.
• Try to be a little more robust to non-obvious failures. In this case make sure .profile exists and is writable before trying to write to it.
• I use grep -Fx to search for the string. -F means fixed strings, so no special characters in the search string needs to be escaped, and -x means match the whole line only. The -qs is common grep syntax for just checking the existence of a string and not to show it.
• This is proof of concept. I didn't actually run this. My bad, but it's Sunday morning and I want to go out and play.

if [[ ! -s "$HOME/.bash_profile" && -s "$HOME/.profile" ]] ; then  profile_file="$HOME/.profile"else  profile_file="$HOME/.bash_profile"fiif ! grep -q 'git-completion.bash' "${profile_file}" ; then  echo "Editing ${profile_file} to load ~/.git-completioin.bash on Terminal launch"  echo "source \"$HOME/.git-completion.bash\"" >> "${profile_file}"fi

How about:

grep -q '^source ~/\.git-completion\.bash$' ~/.bash_profile || echo "source ~/.git-completion.bash" >> ~/.bash_profile

or in a more explicit (and readable) form:

if ! grep -q '^source ~/\.git-completion\.bash$' ~/.bash_profile; then    echo "Updating" ~/.bash_profile    echo "source ~/.git-completion.bash" >> ~/.bash_profilefi

EDIT:

You should probably add an additional newline before your one-liner, just in case ~/.bash_profile does not end in one:

if ! grep -q '^source ~/\.git-completion\.bash$' ~/.bash_profile; then    echo "Updating" ~/.bash_profile    echo >> ~/.bash_profile       echo "source ~/.git-completion.bash" >> ~/.bash_profilefi

EDIT 2:

This is a bit easier to modify and slightly more portable:

LINE='source ~/.git-completion.bash'if ! grep -Fx "$LINE" ~/.bash_profile >/dev/null 2>/dev/null; then    echo "Updating" ~/.bash_profile    echo >> ~/.bash_profile       echo "$LINE" >> ~/.bash_profilefi

The -F and -x options are specified by POSIX and were suggested in several other answers and comments.