Grep and ignoring leading whitespace

bash grep regular-expression

6,065

Solution 1

Just use awk (using grep seems redundant to me, since awk already can match a regular expression):

awk '$0~/\s*\#define\s*\[.*\]\s*.*/ {print $3}' *.h

Going through the expression in more detail:

$0 ~ /regexp/          # look for the regular expression in the record
      \s*              # whitespace, any number of times
         \#define      # literal string, '#' has to be scaped
                 \s*   # same as above
                    .* # any character, any number of times, this is
                       # your hex code and you can refine the regex here
{ print $3 }           # print the third field if the record matches

To have this run recursively, e.g.

mkdir -p a/b/c
echo "   #define [name] 0x0001" > a/a.h
echo "   #define [name] 0x0002" > a/b/b.h
echo "   #define [name] 0x0003" > a/b/c/c.h
tree
.
└── a
    ├── a.h
    └── b
        ├── b.h
        └── c
            └── c.h

3 directories, 3 files

since awk needs to be given a list of files to operate on, you could:

find . -type f -name "*.h" \
  -exec awk '$0~/\s*\#define\s*\[.*\]\s*.*/ {print $3}' {} \;
0x0002
0x0003
0x0001

Solution 2

Use grep -o to print only the matching part of the line.

Obviously take the \s part out again, because you don't want that part.

6,065

Gammerx

Updated on September 18, 2022

Comments

Gammerx over 1 year
I've been working on a bash script to search for names of defines, and then grab the hex values for them and put them in the list. Once I have the list of names I'll attempt to search for "#define [name]" using -w to ensure exact match, and then awk '{ print $3 }' to grab the hex value.

However it works if the line in the header file is similar to
```
a.h:#define [name] 0x0001
```
But it does NOT work if it is similar to
```
a.h:    #define [name] 0x0001
```
How can I get around this? I have tried this
```
grep -nrw "\s*#define[[:space:]]*$p" . --include=*.h | awk '{ print $3 }'
```
I thought the \s* would ignore the leading whitespace before #define but it doesn't. Am I doing something wrong?
- Matej Vrzala M4 almost 9 years
  
  I think you need to use extended grep or the -E flag to use \s
- Gammerx almost 9 years
  
  I am, but I believe the issue comes from leading white space causing #define to be shifted from $1 to $2
- jimmij almost 9 years
  
  You should describe the problem better, it is hard to understand what do you want to achieve.
Gammerx almost 9 years

Problem is that with -o, now I don't get the hex value since it only grabs #define and $p
Gammerx almost 9 years

can this handle recursive as well? (checking *.h from subdirs)
Gammerx almost 9 years

Very well done, this is awesome. Based on some suggestions and testing I have found that grep -Ro '#define\s*$.*$' *.h works. -o was tricky to get the entire line but if you just tell it to include anything that comes after, I can now just use awk to get element 3. since grep includes the header file name, if there was a space before #define it would mess it up.