How to read the output from git diff

Question

The man page for git-diff is rather long  and explains many cases which don t seem to be necessary for a beginner  For example   git diff origin master

User · Answer

Here s the simple example   diff --git a file b file  index 10ff2df  84d4fa2 100644 --- a file     b file    -1 5  1 5     line1  line2 -this line will be deleted  line4  line5  this line is added   Here s an explanation  see details here      --git is not a command  this means it s a git version of diff  not unix  a  b  are directories  they are not real  it s just a convenience when we deal with the same file  in my case a  is in index and b  is in working directory  10ff2df  84d4fa2 are blob IDs of these 2 files 100644 is the    mode bits     indicating that this is a regular file  not executable and not a symbolic link  --- a file     b file minus signs shows lines in the a  version but missing from the b  version  and plus signs shows lines missing in a  but present in b   in my case --- means deleted lines and     means added lines in b  and this the file in the working directory     -1 5  1 5    in order to understand this it s better to work with a big file  if you have two changes in different places you ll get two entries like    -1 5  1 5     suppose you have file line1     line100 and deleted line10 and add new line100 - you ll get        -7 7  7 6    line6  line7  line8  line9 -this line10 to be deleted  line11  line12  line13    -98 3  97 4    line97  line98  line99  line100  this is new line100

User · Answer

-1 2  3 4    part of the diff  This part took me a while to understand  so I ve created a minimal example   The format is basically the same the diff -u unified diff   For instance   diff -u  lt  seq 16   lt  seq 16   grep -Ev    2 3 14 15       Here we removed lines 2  3  14 and 15  Output      -1 6  1 4     1 -2 -3  4  5  6    -11 6  9 4     11  12  13 -14 -15  16      -1 6  1 4    means    -1 6 means that this piece of the first file starts at line 1 and shows a total of 6 lines  Therefore it shows lines 1 to 6   1 2 3 4 5 6   - means  old   as we usually invoke it as diff -u old new   1 4 means that this piece of the second file starts at line 1 and shows a total of 4 lines  Therefore it shows lines 1 to 4     means  new    We only have 4 lines instead of 6 because 2 lines were removed  The new hunk is just   1 4 5 6       -11 6  9 4    for the second hunk is analogous    on the old file  we have 6 lines  starting at line 11 of the old file   11 12 13 14 15 16  on the new file  we have 4 lines  starting at line 9 of the new file   11 12 13 16   Note that line 11 is the 9th line of the new file because we have already removed 2 lines on the previous hunk  2 and 3    Hunk header  Depending on your git version and configuration  you can also get a code line next to the    line  e g  the func1     in      -4 7  4 6    func1       This can also be obtained with the -p flag of plain diff   Example  old file   func1         1      2      3      4      5      6      7      8      9      If we remove line 6  the diff shows      -4 7  4 6    func1          3       4       5  -    6       7       8       9    Note that this is not the correct line for func1  it skipped lines 1 and 2   This awesome feature often tells exactly to which function or class each hunk belongs  which is very useful to interpret the diff   How the algorithm to choose the header works exactly is discussed at  Where does the excerpt in the git diff hunk header come from

User · Answer

Lets take a look at example advanced diff from git history  in commit 1088261f in git git repository    diff --git a builtin-http-fetch c b http-fetch c similarity index 95  rename from builtin-http-fetch c rename to http-fetch c index f3e63d7  e8f44ba 100644 --- a builtin-http-fetch c     b http-fetch c    -1 8  1 9      include  cache h    include  walker h   -int cmd http fetch int argc  const char   argv  const char  prefix   int main int argc  const char   argv             const char  prefix          struct walker  walker          int commits on stdin   0          int commits     -18 6  19 8    int cmd http fetch int argc  const char   argv  const char  prefix          int get verbosely   0          int get recover   0           prefix   setup git directory              git config git default config  NULL            while  arg  lt  argc  amp  amp  argv arg  0      -       Lets analyze this patch line by line    The first line diff --git a builtin-http-fetch c b http-fetch c is a  git diff  header in the form diff --git a file1 b file2   The a  and b  filenames are the same unless rename copy is involved  like in our case    The --git is to mean that diff is in the  git  diff format  Next are one or more extended header lines   The first threesimilarity index 95  rename from builtin-http-fetch c rename to http-fetch ctell us that the file was renamed from builtin-http-fetch c to http-fetch c and that those two files are 95  identical  which was used to detect this rename    The last line in extended diff header  which is index f3e63d7  e8f44ba 100644 tell us about mode of given file  100644 means that it is ordinary file and not e g  symlink  and that it doesn t have executable permission bit   and about shortened hash of preimage  the version of file before given change  and postimage  the version of file after change    This line is used by git am --3way to try to do a 3-way merge if patch cannot be applied itself  Next is two-line unified diff header--- a builtin-http-fetch c     b http-fetch cCompared to diff -U result it doesn t have from-file-modification-time nor to-file-modification-time after source  preimage  and destination  postimage  file names   If file was created the source is  dev null  if file was deleted  the target is  dev null If you set diff mnemonicPrefix configuration variable to true  in place of a  and b  prefixes in this two-line header you can have instead c   i   w  and o  as prefixes  respectively to what you compare  see git-config 1  Next come one or more hunks of differences  each hunk shows one area where the files differ   Unified format hunks starts with line like   -1 8  1 9   or   -18 6  19 8    int cmd http fetch int argc  const char   argv       It is in the format    from-file-range to-file-range     header    The from-file-range is in the form - lt start line gt   lt number of lines gt   and to-file-range is   lt start line gt   lt number of lines gt    Both start-line and number-of-lines refer to position and length of hunk in preimage and postimage  respectively   If number-of-lines not shown it means that it is 0   The optional header shows the C function where each change occurs  if it is a C file  like -p option in GNU diff   or the equivalent  if any  for other types of files  Next comes the description of where files differ   The lines common to both files begin with a space character  The lines that actually differ between the two files have one of the following indicator characters in the left print column        --    A line was added here to the first file   -  --    A line was removed here from the first file    So  for example  first chunk    include  cache h    include  walker h   -int cmd http fetch int argc  const char   argv  const char  prefix   int main int argc  const char   argv             const char  prefix          struct walker  walker          int commits on stdin   0          int commits    means that cmd http fetch was replaced by main  and that const char  prefix  line was added   In other words  before the change  the appropriate fragment of then  builtin-http-fetch c  file looked like this    include  cache h   include  walker h   int cmd http fetch int argc  const char   argv  const char  prefix           struct walker  walker         int commits on stdin   0         int commits    After the change this fragment of now  http-fetch c  file looks like this instead    include  cache h   include  walker h   int main int argc  const char   argv           const char  prefix         struct walker  walker         int commits on stdin   0         int commits   There might be   No newline at end of fileline present  it is not in example diff     As Donal Fellows said it is best to practice reading diffs on real-life examples  where you know what you have changed   References    git-diff 1  manpage  section  Generating patches with -p   diff info Detailed Unified node   Detailed Description of Unified Format

User · Answer

The default output format  which originally comes from a program known as diff if you want to look for more info  is known as a    unified diff     It contains essentially 4 different types of lines    context lines  which start with a single space  insertion lines that show a line that has been inserted  which start with a    deletion lines  which start with a -  and metadata lines which describe higher level things like which file this is talking about  what options were used to generate the diff  whether the file changed its permissions  etc    I advise that you practice reading diffs between two versions of a file where you know exactly what you changed  Like that you ll recognize just what is going on when you see it

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On my mac    info diff then select  Output formats -  Context -  Unified format -  Detailed Unified    Or online man diff on gnu following the same path to the same section      File  diff info   Node  Detailed   Unified   Next  Example Unified   Up    Unified Format      Detailed Description of Unified Format                                               The unified output format starts   with a two-line header  which looks   like this    --- FROM-FILE FROM-FILE-MODIFICATION-TIME      TO-FILE TO-FILE-MODIFICATION-TIME       The time stamp looks like  2002-02-21   23 30 39 942229878 -0800  to indicate   the date  time with fractional   seconds  and time zone       You can change the header s content   with the  --label LABEL  option  see    Note Alternate Names         Next come one or more hunks of   differences  each hunk shows one area   where the files differ   Unified   format hunks look like this       FROM-FILE-RANGE TO-FILE-RANGE      LINE-FROM-EITHER-FILE   LINE-FROM-EITHER-FILE          The lines common to both files   begin with a space character   The   lines that actually differ between the   two files have one of the following   indicator characters in the left print   column                  A line was added here to the first file        -         A line was removed here from the first file

User · Answer

It s unclear from your question which part of the diffs you find confusing  the actually diff  or the extra header information git prints  Just in case  here s a quick overview of the header   The first line is something like diff --git a path to file b path to file - obviously it s just telling you what file this section of the diff is for  If you set the boolean config variable diff mnemonic prefix  the a and b will be changed to more descriptive letters like c and w  commit and work tree    Next  there are  mode lines  - lines giving you a description of any changes that don t involve changing the content of the file  This includes new deleted files  renamed copied files  and permissions changes   Finally  there s a line like index 789bd4  0afb621 100644  You ll probably never care about it  but those 6-digit hex numbers are the abbreviated SHA1 hashes of the old and new blobs for this file  a blob is a git object storing raw data like a file s contents   And of course  the 100644 is the file s mode - the last three digits are obviously permissions  the first three give extra file metadata information  SO post describing that    After that  you re on to standard unified diff output  just like the classic diff -U   It s split up into hunks - a hunk is a section of the file containing changes and their context  Each hunk is preceded by a pair of --- and     lines denoting the file in question  then the actual diff is  by default  three lines of context on either side of the - and   lines showing the removed added lines

[git] How to read the output from git diff?

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