Use a command like:

git diff file_2.rb

See the git diff documentation for full information on the kinds of things you can get differences for.

Normally, git diff by itself shows all the changes in the whole repository (not just the current directory).

You're looking for

git diff --staged

Depending on your exact situation, there are three useful ways to use git diff:

1. Show differences between index and working tree; that is, changes you haven't staged to commit:

git diff [filename]

2. Show differences between current commit and index; that is, what you're about to commit (--staged does exactly the same thing, use what you like):

git diff --cached [filename]

3. Show differences between current commit and working tree:

git diff HEAD [filename]

git diff works recursively on directories, and if no paths are given, it shows all changes.

This lets Git generate the patches for each log entry:

git log -p -- <filename>

-p in git generates patch text

See git help log for more options — it can actually do a lot of nice things. :)

To get just the diff for a specific commit, use

git show <revision> -- <filename>

or specify any other revision by identifier.

To browse the changes visually:

gitk -- <filename>

To see changes visually without leaving the console, run:

git blame-log <filename>

However, first ensure blame-log command exists, by at least once running:

# Virables: raw=unchanged-line, c=clean, B=Buffer,
git config --global alias.blame-log "! cd -- \"\${GIT_PREFIX:-.}\" && \
( if git grep -qI \".\" \"\$1\"; then \
    git log --oneline --color=always -L 1,\${2:-}:\"\$1\" | awk ' \
    { s=\$0; gsub(/\\033\\[[0-9;]*[mK]/, \"\", s) } \
    s~/^(diff|---|\\+\\+\+)/{next} \
    s~/^@@/{l=1;p=9;bi=0;cm=0;cp=0;next} s!~/^[-+ ]/{print;l=1;p=9;bi=0;cm=0;cp=0;next} \
    /^ /{ cm=0;cp=0; if(p<3){print;p++}else{b[bi%3]=\$0; bl[bi%3]=l; bi++} l++; next } \
    { if(p>=3){ n=(bi<3?bi:3); st=(bi<3?0:bi%3); print \"\\033[36mLine \"(n?bl[st]:l)\":\\033[m\"; for(k=0;k<n;k++)print b[(st+k)%3] } \
      p=0; if(s~/^-/){cm++;if(cm<=12)print;else if(cm==13)print\"...\"} else{cp++;if(cp<=12)print;else if(cp==13)print\"...\";l++} }'; \
else \
    echo \"Detected binary file, hence not showing lines-changed.\"
    git log --oneline --color=always \"\$1\";
fi ) | less -RX #"

Note that blame-log is intentionally limited to showing few lines per change-range, for example, to skip package-lock.json faster.