in Shell, How to insert a substring if not found in string

Question

I want changes_summary to always be in format <x> files changed, <y> insertion(+), <z> deletions(-) where <x> <y> and <z> are some numbers, but diffstat misses insertions and/or deletions part if <y> and/or <z> is zero, I tried to make it print as <x> files changed 0 insertion(+), 0 deletions(-) always, is there a better or easy way to do this? I would like to change $changes_summary variable so I can use it later part of the script.

changes_summary=`diff -ur ./dir1 ./dir2 | diffstat | tail -1`
  if ! echo $changes_summary | grep -q "insertions" && ! echo $changes_summary | grep -q "deletions" ; then
        echo $changes_summary | awk '{print $1 " " $2 " " $3 " " "0 insertion(+)," " " "0 deletions(-)"}'
 elif ! echo $changes_summary | grep -q "insertions" && echo $changes_summary | grep -q "deletions" ; then
       echo $changes_summary | awk '{print $1 " " $2 " " $3 " " "0 insertion(+), "$4 " " $5  }'
elif echo $changes_summary | grep -q "insertions" && ! echo $changes_summary | grep -q "deletions" ; then
       echo $changes_summary | awk '{print $1 " " $2 " " $3 " " $4 " " $5 "0 deletions(-)" }'

fi

Answer 1

Probably the closest you can get without some serious bash magic or an other language is something like the following.

changes_summary=`diff -ur ./dir1 ./dir2 | diffstat -s`
CC=$(echo "$changes_summary" | sed -n 's:\(.*[0-9]\+ .* changed\).*:\1:p')
II=$(echo "$changes_summary" | sed -n 's:.*\([0-9]\+ insertions\?\).*:\1:p')
DD=$(echo "$changes_summary" | sed -n 's:.*\([0-9]\+ deletions\?\).*:\1:p')
echo "${CC}, ${II:-0 insertions}(+), ${DD:-0 deletions}(-)"

Sed strips out the message corresponding to each stat. The -n suppresses the normal output, p prints only if a match is found. If not, then CC, II, DD will be empty, in which case the ${II:-...} pattern substitutes a default value.

From man bash:

${parameter:-word} Use Default Values. If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.

Note that keeping the (s) with s\? might be an overkill for you.

The other option is that in bash you can check for containment with [[ $a =~ "b" ]] and use your original approach. It spares you the greps at least and "b" here can also be regex if you drop the quotes.

if ! [[ "$changes_summary" =~ "insert" ]]; then
  awk ...
fi

You can also find the =~ in man bash.