Linux Filesystem and Navigation

Every Linux system follows a standard directory layout called the Filesystem Hierarchy Standard (FHS). Once we learn where things live, navigating any Linux box feels familiar.

The FHS Tree

The key takeaway: config goes in /etc, logs go in /var/log, and our stuff lives in /home.

Essential Navigation Commands

pwd                    # print current directory
ls -la                 # list all files with details (including hidden)
cd /var/log            # change to a specific directory
cd ~                   # go home (same as just "cd")
cd -                   # go back to the previous directory
mkdir -p app/src/utils # create nested directories in one shot

Viewing and Searching Files

cat config.json        # dump entire file to screen
head -20 access.log    # first 20 lines
tail -f app.log        # follow log output in real-time (Ctrl+C to stop)
wc -l data.csv         # count lines in a file

grep is our best friend for searching inside files.

grep "error" app.log              # find lines containing "error"
grep -i "warning" app.log         # case-insensitive search
grep -r "TODO" src/               # recursive search through directories
grep -n "function" index.js       # show line numbers

find searches for files by name, type, or age.

find /var/log -name "*.log"       # find all .log files
find . -type f -mtime -1          # files modified in the last 24 hours
find . -name "*.tmp" -delete      # find and delete .tmp files

Pipes and Redirection

Pipes (|) send the output of one command into another. Think of it like an assembly line.

cat access.log | grep "404" | wc -l   # count 404 errors
ps aux | grep nginx                     # find nginx processes
history | grep "docker"                 # search command history

Redirection sends output to files instead of the screen.

echo "hello" > file.txt     # write to file (overwrites!)
echo "world" >> file.txt    # append to file (safe)
cat missing.txt 2> err.log  # redirect errors (stderr) to a file
sort data.txt > sorted.txt  # sort and save to a new file

Wildcards

ls *.log          # all files ending in .log
ls app.???        # app followed by exactly 3 characters
rm temp_[0-9]*    # remove files starting with temp_ followed by a digit

Quick awk Intro

awk is great for pulling columns out of structured text.

# print the 1st and 3rd column from a space-separated file
awk '{print $1, $3}' access.log

# print lines where the 5th column is greater than 500
awk '$5 > 500' access.log

In simple language, the Linux filesystem is like a well-organized filing cabinet. Once we know which drawer (/etc, /var, /home) holds what, we can find anything fast.