Domain 1: Essential Commands (20%)¶

# Navigation
pwd                      # print working directory
cd /path                 # change directory
cd ~                     # go to home directory
cd -                     # go to previous directory
cd ..                    # go up one level

# Listing
ls                       # list files
ls -l                    # long format (permissions, owner, size, date)
ls -la                   # include hidden files
ls -lh                   # human-readable file sizes
ls -ltr                  # sort by time, reversed (newest last)
ls -R                    # recursive listing

# File operations
cp source dest           # copy file
cp -r source/ dest/      # copy directory recursively
cp -a source/ dest/      # copy preserving all attributes
mv source dest           # move or rename
rm file                  # remove file
rm -r directory/         # remove directory recursively
rm -rf directory/        # force remove (be careful!)
mkdir directory          # create directory
mkdir -p path/to/dir     # create parent directories as needed
rmdir directory          # remove empty directory
touch file               # create empty file or update timestamp

# File information
file filename            # determine file type
stat filename            # detailed file information
du -sh /path             # disk usage of directory
df -h                    # disk space of filesystems

cat file                 # display entire file
less file                # page through file (/ to search, q to quit)
more file                # page through file (older, less features)
head -n 20 file          # first 20 lines
tail -n 20 file          # last 20 lines
tail -f file             # follow file updates in real-time

# Basic grep
grep "pattern" file              # search for pattern
grep -i "pattern" file           # case-insensitive
grep -v "pattern" file           # invert match (exclude)
grep -r "pattern" /path          # recursive search
grep -n "pattern" file           # show line numbers
grep -c "pattern" file           # count matches
grep -l "pattern" /path/*.log    # show only filenames with matches
grep -w "word" file              # match whole word only
grep -A 3 "pattern" file         # show 3 lines after match
grep -B 3 "pattern" file         # show 3 lines before match
grep -C 3 "pattern" file         # show 3 lines around match

# Extended regex (egrep)
grep -E "pattern1|pattern2" file # match either pattern
grep -E "^start" file           # lines starting with "start"
grep -E "end$" file             # lines ending with "end"
grep -E "[0-9]{3}" file         # three consecutive digits
grep -E "^$" file               # empty lines

# Common patterns
grep "^root" /etc/passwd         # lines starting with root
grep "bash$" /etc/passwd         # lines ending with bash
grep -v "^#" config.conf         # exclude comments
grep -v "^$" file                # exclude empty lines

# Substitution
sed 's/old/new/' file            # replace first occurrence per line
sed 's/old/new/g' file           # replace all occurrences
sed 's/old/new/gi' file          # replace all, case-insensitive
sed -i 's/old/new/g' file        # in-place edit (modifies file)
sed -i.bak 's/old/new/g' file   # in-place with backup

# Line operations
sed -n '5p' file                 # print only line 5
sed -n '5,10p' file              # print lines 5-10
sed '5d' file                    # delete line 5
sed '/pattern/d' file            # delete lines matching pattern
sed '3i\inserted text' file      # insert text before line 3
sed '3a\appended text' file      # append text after line 3

# Multiple operations
sed -e 's/old1/new1/g' -e 's/old2/new2/g' file
sed '/pattern/s/old/new/g' file  # substitute only on matching lines

# Common tasks
sed 's/^/  /' file               # indent all lines (add 2 spaces)
sed 's/[[:space:]]*$//' file     # remove trailing whitespace
sed '/^$/d' file                 # remove empty lines

# Field extraction (default delimiter: whitespace)
awk '{print $1}' file            # print first field
awk '{print $1, $3}' file       # print first and third fields
awk '{print $NF}' file          # print last field
awk '{print NR, $0}' file       # print line number and entire line

# Custom delimiter
awk -F: '{print $1}' /etc/passwd       # colon-delimited
awk -F, '{print $2}' data.csv         # comma-delimited

# Conditionals
awk '$3 > 1000' /etc/passwd            # UID > 1000 (field 3)
awk -F: '$3 >= 1000 {print $1}' /etc/passwd  # users with UID >= 1000
awk 'NR >= 5 && NR <= 10' file         # lines 5 through 10
awk '/pattern/ {print $0}' file        # lines matching pattern

# Calculations
awk '{sum += $1} END {print sum}' file           # sum of first column
awk '{sum += $1; count++} END {print sum/count}' file  # average
awk 'BEGIN {count=0} /pattern/ {count++} END {print count}' file

# Formatting
awk -F: '{printf "%-20s %s\n", $1, $6}' /etc/passwd  # formatted output

# cut - extract fields
cut -d: -f1 /etc/passwd          # first field, colon-delimited
cut -d: -f1,6 /etc/passwd        # fields 1 and 6
cut -c1-10 file                  # first 10 characters per line

# sort
sort file                         # alphabetical sort
sort -n file                      # numeric sort
sort -r file                      # reverse sort
sort -k2 file                    # sort by second column
sort -t: -k3 -n /etc/passwd     # sort by UID numerically
sort -u file                      # sort and remove duplicates

# uniq (requires sorted input)
sort file | uniq                  # remove adjacent duplicates
sort file | uniq -c               # count occurrences
sort file | uniq -d               # show only duplicates

# wc - word count
wc file                           # lines, words, characters
wc -l file                       # count lines only
wc -w file                       # count words only
wc -c file                       # count bytes

# tr - translate characters
tr 'a-z' 'A-Z' < file           # uppercase
tr -d '\r' < file                # remove carriage returns
tr -s ' '                        # squeeze repeated spaces

# paste and join
paste file1 file2                # merge files side by side
paste -d, file1 file2           # merge with comma delimiter

# diff
diff file1 file2                 # compare files
diff -u file1 file2              # unified format (like git diff)

# Standard streams
# 0 = stdin (input)
# 1 = stdout (output)
# 2 = stderr (error)

# Output redirection
command > file           # stdout to file (overwrite)
command >> file          # stdout to file (append)
command 2> file          # stderr to file
command 2>> file         # stderr to file (append)
command &> file          # both stdout and stderr to file
command > file 2>&1      # both stdout and stderr to file (POSIX)
command > /dev/null 2>&1 # discard all output

# Input redirection
command < file           # stdin from file
command << EOF           # here document
line1
line2
EOF

# Pipes
command1 | command2      # stdout of cmd1 to stdin of cmd2
command1 | command2 | command3  # chain multiple commands

# tee - write to file AND stdout
command | tee file       # output to screen and file
command | tee -a file    # append to file
command | tee file1 file2  # write to multiple files

# xargs - build commands from stdin
find . -name "*.log" | xargs rm        # delete found files
find . -name "*.txt" | xargs grep "pattern"  # search in found files
echo "file1 file2" | xargs -n 1 rm    # one argument per command

# Numeric mode
chmod 755 file           # rwxr-xr-x
chmod 644 file           # rw-r--r--
chmod 700 file           # rwx------
chmod 600 file           # rw-------
chmod -R 755 /path       # recursive

# Symbolic mode
chmod u+x file           # add execute for user (owner)
chmod g-w file           # remove write for group
chmod o=r file           # set others to read only
chmod a+r file           # add read for all
chmod u+x,g+r file      # multiple changes

# SUID (Set User ID) - 4xxx
chmod u+s file           # or chmod 4755 file
# When executed, runs as file owner (not executing user)
# Example: /usr/bin/passwd runs as root

# SGID (Set Group ID) - 2xxx
chmod g+s directory      # or chmod 2755 directory
# Files created in directory inherit directory's group
# Useful for shared directories

# Sticky Bit - 1xxx
chmod +t directory       # or chmod 1755 directory
# Only file owner can delete files in directory
# Example: /tmp has sticky bit set

# Identify special permissions in ls output
# SUID: -rwsr-xr-x (s in user execute position)
# SGID: -rwxr-sr-x (s in group execute position)
# Sticky: drwxrwxrwt (t in others execute position)

chown user file          # change owner
chown user:group file    # change owner and group
chown :group file        # change group only
chown -R user:group /path  # recursive
chgrp group file         # change group only

umask                    # show current umask
umask 022                # set umask (files: 644, dirs: 755)
umask 077                # restrictive (files: 600, dirs: 700)
# Default file permission = 666 - umask
# Default directory permission = 777 - umask

ln source hardlink       # create hard link

ln -s source symlink     # create symbolic link

# tar (tape archive)
tar -cf archive.tar /path          # create tar
tar -czf archive.tar.gz /path     # create + gzip compress
tar -cjf archive.tar.bz2 /path   # create + bzip2 compress
tar -cJf archive.tar.xz /path    # create + xz compress
tar -xf archive.tar               # extract tar
tar -xzf archive.tar.gz           # extract gzipped tar
tar -xzf archive.tar.gz -C /dest  # extract to specific directory
tar -tf archive.tar                # list contents
tar -tzf archive.tar.gz           # list contents of gzipped tar

# tar flags summary
# -c  create archive
# -x  extract archive
# -t  list contents
# -f  specify filename
# -z  gzip compression
# -j  bzip2 compression
# -J  xz compression
# -v  verbose output
# -C  change to directory before extracting

# Standalone compression
gzip file                # compress (removes original, creates file.gz)
gunzip file.gz           # decompress
gzip -k file             # compress and keep original
bzip2 file               # bzip2 compress
bunzip2 file.bz2         # decompress
xz file                  # xz compress
unxz file.xz             # decompress

# zip (cross-platform)
zip archive.zip file1 file2        # create zip
zip -r archive.zip /path           # recursive zip
unzip archive.zip                  # extract
unzip -l archive.zip               # list contents

# By name
find /path -name "*.log"           # exact name (case-sensitive)
find /path -iname "*.LOG"          # case-insensitive
find /path -name "file*"           # wildcard

# By type
find /path -type f                 # regular files
find /path -type d                 # directories
find /path -type l                 # symbolic links

# By size
find /path -size +10M              # larger than 10MB
find /path -size -1k               # smaller than 1KB
find /path -size 100c              # exactly 100 bytes

# By time
find /path -mtime -7               # modified in last 7 days
find /path -mtime +30              # modified more than 30 days ago
find /path -mmin -60               # modified in last 60 minutes
find /path -atime -1               # accessed in last 1 day
find /path -newer reference_file   # newer than reference file

# By ownership
find /path -user username          # owned by user
find /path -group groupname        # owned by group
find /path -nouser                 # no owner (orphaned)

# By permissions
find /path -perm 644               # exact permissions
find /path -perm -644              # at least these permissions
find /path -perm /u+x              # any file with user execute

# Actions
find /path -name "*.tmp" -delete            # delete matching files
find /path -name "*.log" -exec rm {} \;     # execute command for each
find /path -name "*.sh" -exec chmod +x {} \;  # make executable
find /path -name "*.conf" -exec grep "pattern" {} +  # efficient exec

# Combining conditions
find /path -name "*.log" -size +1M          # AND (default)
find /path -name "*.log" -o -name "*.txt"   # OR
find /path ! -name "*.log"                  # NOT
find /path -maxdepth 2 -name "*.conf"       # limit depth

locate filename          # fast file finding (uses database)
sudo updatedb            # update locate database
locate -i filename       # case-insensitive
locate -c filename       # count matches

which command            # find command in PATH
whereis command          # find command, source, man page
type command             # determine command type (builtin, alias, file)