Chapter 4 — Searching: grep and find

Time to study: ~60 min You will learn: how to search for text inside files with grep, and how to search for files themselves with find.

← Chapter 3 | back to Terminal 101 | next: Chapter 5 →

before we start, all the examples below are run from inside the Terminal-101 dir, so the file tree is this one:

Terminal-101/
├── README.md
├── CHEATSHEET.md
├── 01-command-line-basics/
│   └── README.md
├── 02-the-linux-file-system/
│   └── README.md
├── 03-files-and-navigation/
│   └── README.md
├── 04-search-grep-find/
│   ├── README.md
│   └── fruits.txt
└── ... (chapters 5 to 9, each with a README.md)

1. the grep command (finding text)

grep is when u as a normal user want to search for something and u use the keymap ctrl+f or the search bar at the top of ur window. grep basically has the same behaviour here. u can think about it like the ctrl+f of the terminal.

• example: i want to search for the word Hello in the file 01-command-line-basics/README.md:

grep "Hello" 01-command-line-basics/README.md

echo "Hello World"
> this is basically the same as writing "Hello World" in a text editor, it prints `Hello World` to the terminal.
Hello World
echo "Hello everyone! My name is $name $surname. I am $age years old, and I work as a $job."

so grep prints back every line that contains the word, not the word alone. this is the thing most people get wrong the first time.

we can also use it to grep from more than one file:

grep "Hello" 01-command-line-basics/README.md 02-the-linux-file-system/README.md

01-command-line-basics/README.md:echo "Hello World"
01-command-line-basics/README.md:Hello World
01-command-line-basics/README.md:echo "Hello everyone! My name is $name $surname..."

notice what changed: when u pass more than one file, grep puts the file name in front of every line so u know where each result came from. with one file it doesn’t, because there is nothing to be confused about.

we can also use the -r flag to grep from all the files in the current dir. the . here means the current dir, so it searches every file under it recursively.

grep -r "Hello" .

./01-command-line-basics/README.md:echo "Hello World"
./01-command-line-basics/README.md:Hello World
./01-command-line-basics/README.md:echo "Hello everyone! My name is $name $surname..."
./03-files-and-navigation/README.md:echo "Hello World" > hello.txt

for the next flags we will use a small file so u can actually see what changes. this is fruits.txt:

cat fruits.txt

apple
Banana
cherry
apple pie

the -i flag makes the search case insensitive:

grep "banana" fruits.txt

(nothing, because the file has "Banana" with a capital B)

grep -i "banana" fruits.txt

Banana

the -v flag inverts the search, it shows all the lines that do not contain the thing u are searching for:

grep -v "apple" fruits.txt

Banana
cherry

the two apple lines are gone and everything else stayed. this is very useful later when u want to filter out noise from a log file.

the -n flag shows u the line number of every match:

grep -n "apple" fruits.txt

1:apple
4:apple pie

• these flags can be combined like any other flags: grep -in "BANANA" fruits.txt searches case insensitively and shows the line number.

2. the find command (finding files)

previously we searched for a word inside files, but now we are trying to find the files themselves.

• example: i want to find the file README.md inside the 02-the-linux-file-system dir:

find 02-the-linux-file-system/ -name "README.md"

02-the-linux-file-system/README.md

the first argument is where to search and the -name is what to search for. so we can pass . to search the current dir instead if we want:

find . -name "README.md"

./README.md
./01-command-line-basics/README.md
./02-the-linux-file-system/README.md
./03-files-and-navigation/README.md
./04-search-grep-find/README.md
./05-permissions-sudo/README.md
./06-text-editors/README.md
./07-scripts-env-path/README.md
./08-io-pipes-processes/README.md
./09-bash-scripting/README.md

now a bit more advanced searching. we can use * to match anything, so we can search for all the files that end with .md:

find . -name "*.md"

./README.md
./01-command-line-basics/README.md
./02-the-linux-file-system/README.md
./03-files-and-navigation/README.md
./04-search-grep-find/README.md
./05-permissions-sudo/README.md
./06-text-editors/README.md
./07-scripts-env-path/README.md
./08-io-pipes-processes/README.md
./09-bash-scripting/README.md

or we can use *word* to match anything that contains that word:

find . -name "*fruit*"

./04-search-grep-find/fruits.txt

or word* to match anything that starts with that word:

find . -name "READ*"

./README.md
./01-command-line-basics/README.md
./02-the-linux-file-system/README.md
./03-files-and-navigation/README.md
./04-search-grep-find/README.md
./05-permissions-sudo/README.md
./06-text-editors/README.md
./07-scripts-env-path/README.md
./08-io-pipes-processes/README.md
./09-bash-scripting/README.md

there is also ? which matches exactly one character, not any number of them like * does.

find . -name "0?-*"

./01-command-line-basics
./02-the-linux-file-system
./03-files-and-navigation
./04-search-grep-find
./05-permissions-sudo
./06-text-editors
./07-scripts-env-path
./08-io-pipes-processes
./09-bash-scripting

so 0? matched 01 up to 09, one character after the zero. if u wrote 0??-* it would match nothing here, because there is no folder with 3 characters before the dash.

one more flag u will use a lot, -type, which filters files or directories only:

find . -type d

.
./01-command-line-basics
./02-the-linux-file-system
./03-files-and-navigation
./04-search-grep-find
./05-permissions-sudo
./06-text-editors
./07-scripts-env-path
./08-io-pipes-processes
./09-bash-scripting

find . -type f -name "*.txt"

./04-search-grep-find/fruits.txt

• d means directory and f means file.

• these are the most used flags and cases u need. for anything else, man grep and man find are there.

NOTES

1. grep searches inside files, find searches for files. this is the whole difference and it is easy to mix them up at the beginning.

2. always put ur search pattern in quotes. if u write find . -name *.md without the quotes, the shell replaces the * before find even sees it, and u get a confusing error.

3. grep and find become much more powerful when u connect them with the | pipe, which we learn in Chapter 8.

Assignment

1. Make a dir ch3-practice and inside it create a file animals.txt containing these 4 lines: cat, Dog, bird, cat food.
2. Find all the lines that contain cat.
3. Find all the lines that contain dog, ignoring the capital letter.
4. Show all the lines that do NOT contain cat.
5. Show the line numbers of every line containing cat.
6. From ur home dir, find every file whose name ends with .txt.
7. From ur home dir, find every directory whose name starts with ch.

stuck, or done and want to check? the solutions are here

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