Text Search — grep

Finding a String — grep

grep is the command that pulls out and shows only the lines in a file that contain a given string.

Use it as grep search-string filename, and if there are matching lines, they print to the screen as-is.

Use it to find just the lines you need out of a large log.

echo 'error: disk full' > log.txt   # create line 1
echo 'info: started' >> log.txt     # append
echo 'error: timeout' >> log.txt    # append
grep error log.txt                  # the 2 lines containing error
grep info log.txt                   # the 1 line containing info

① Create line 1 with echo 'error: disk full' > log.txt, then append with echo 'info: started' >> log.txt and echo 'error: timeout' >> log.txt to make a 3-line material file.

② Use grep to show only the lines containing error from log.txt.

③ Confirm that every line shown contains error. (Run it correctly and an explanation will appear.)

Ignore Case — -i / Line Numbers — -n

grep distinguishes uppercase from lowercase.

Add -i to search without that distinction, so you can catch both Error and error.

Add -n and the line number is shown in front of each matching line, so you can tell where in the file it is.

-i and -n can be combined and applied at once, like grep -in error app.txt.

echo 'Error in disk' > app.txt   # create line 1
echo 'error in app' >> app.txt   # append
echo 'ok' >> app.txt             # append
grep -i error app.txt            # the 2 lines Error and error
grep -n error app.txt            # 2:error in app (with line number)

① Create line 1 with echo 'Error in disk' > app.txt, then append with echo 'error in app' >> app.txt and echo 'ok' >> app.txt to make a material with mixed case.

② First search for error with grep and the option that ignores case, and confirm the Error line is caught too.

③ Next run the same search with the option that shows line numbers, and confirm a line number appears in front of each match.

Excluding Non-Matching Lines — -v

Add -v and grep shows the lines that don't contain the given string.

Whereas ordinary grep keeps the matching lines, -v removes the matching lines and outputs the rest.

Use it when you want to read a log with unneeded lines (like info or debug) removed.

echo 'error: disk full' > log.txt   # create line 1
echo 'info: started' >> log.txt     # append
echo 'error: timeout' >> log.txt    # append
grep -v info log.txt                # the 2 lines that don't contain info

① Create line 1 with echo 'error: disk full' > log.txt, then append with echo 'info: started' >> log.txt and echo 'error: timeout' >> log.txt to make a 3-line material file.

② Use grep with the option that keeps non-matching lines to show only the lines from log.txt that don't contain info.

③ Confirm that the lines shown are the 2 error lines and the info line is gone.

Searching a Directory Recursively — -r

When you want to search not just one file but every file under a directory, add -r and pass a directory instead of a filename.

It walks down into subdirectories recursively and shows matching lines in the form filename:line.

mkdir -p src/sub                       # create the material directory
echo 'TODO: fix later' > src/a.txt     # 1 file in the parent
echo 'TODO: review' > src/sub/b.txt    # 1 file in the subdirectory
grep -r TODO src                       # search recursively under src

① Create a material tree with a subdirectory using mkdir -p src/sub.

② With echo 'TODO: fix later' > src/a.txt and echo 'TODO: review' > src/sub/b.txt, create files in both the parent and the subdirectory.

③ Search for TODO under the src directory with grep and the recursive-search option, and confirm the lines from both files appear.