File System & Navigation

Navigation & File Operations

# Navigation
pwd                          # print working directory
cd /var/log                  # absolute path
cd ~/projects                # home directory
cd -                         # previous directory
cd ..                        # parent directory

# List files
ls                           # basic listing
ls -la                       # long format + hidden files
ls -lh                       # human-readable sizes
ls -lt                       # sorted by modification time
ls -lS                       # sorted by size

# File operations
cp file.txt backup.txt       # copy file
cp -r dir/ newdir/           # copy directory recursively
mv old.txt new.txt           # move/rename
rm file.txt                  # delete file
rm -rf dir/                  # delete directory (DANGER: no undo)
mkdir -p a/b/c               # create nested directories
touch file.txt               # create empty file or update timestamp
ln -s /path/to/target link   # create symlink

# View files
cat file.txt                 # print file contents
less file.txt                # paginated viewer (q to quit)
head -n 20 file.txt          # first 20 lines
tail -n 50 file.txt          # last 50 lines
tail -f /var/log/app.log     # follow log in real time
wc -l file.txt               # count lines
wc -w file.txt               # count words

# Find files
find /var/log -name "*.log"              # find by name
find . -name "*.ts" -not -path "*/node_modules/*"
find . -type f -size +100M              # files larger than 100MB
find . -mtime -7                         # modified in last 7 days
find . -name "*.log" -delete             # find and delete

# Disk usage
df -h                        # disk space by filesystem
du -sh /var/log/*             # size of each item
du -sh .                     # size of current directory
ncdu                         # interactive disk usage browser (install separately)

Text Processing

# grep — search text
grep "error" app.log                    # search for pattern
grep -i "error" app.log                 # case-insensitive
grep -n "error" app.log                 # show line numbers
grep -r "TODO" ./src                    # recursive search
grep -v "debug" app.log                 # invert match (exclude)
grep -E "error|warning" app.log        # extended regex (OR)
grep -l "pattern" *.txt                # only filenames

# sed — stream editor
sed 's/foo/bar/g' file.txt             # replace all foo with bar
sed 's/foo/bar/2' file.txt             # replace 2nd occurrence only
sed -i 's/foo/bar/g' file.txt          # edit in-place
sed -i.bak 's/foo/bar/g' file.txt      # edit with backup
sed '1,5d' file.txt                    # delete lines 1-5
sed -n '10,20p' file.txt              # print lines 10-20

# awk — column processing
awk '{print $1}' file.txt              # print first column
awk '{print $NF}' file.txt            # print last column
awk -F: '{print $1}' /etc/passwd       # colon-separated, field 1
awk '{sum+=$1} END{print sum}' nums    # sum first column
awk 'NR==5' file.txt                  # print line 5

# sort, uniq, cut
sort file.txt                          # sort alphabetically
sort -n numbers.txt                    # sort numerically
sort -rn numbers.txt                   # sort numerically, descending
sort -k2 file.txt                     # sort by 2nd field
sort file.txt | uniq                   # unique lines
sort file.txt | uniq -c                # count occurrences
cut -d: -f1,3 /etc/passwd             # cut fields 1 and 3 (colon delimiter)
cut -c1-10 file.txt                   # cut first 10 characters

Permissions

# Permission format: -rwxrwxrwx (type, owner, group, others)
# r=4, w=2, x=1

chmod 755 script.sh          # rwxr-xr-x (owner all, others read+exec)
chmod 644 file.txt           # rw-r--r-- (owner read+write, others read)
chmod +x script.sh           # add execute to all
chmod -w file.txt            # remove write from all
chmod u+x,g-w file.txt       # add exec to owner, remove write from group

chown user:group file.txt    # change owner and group
chown -R user:group dir/     # recursive

# Special permissions
chmod +s binary              # setuid/setgid
chmod +t /tmp                # sticky bit (only owner can delete)

# sudo
sudo command                 # run as root
sudo -u postgres psql        # run as different user
sudo su -                    # switch to root shell
visudo                       # edit sudoers safely