Ultimate Linux Command-Line Guide: Detailed Usage of Common Commands

File Management Commands

The find command searches for files and directories using a path, expression, and optional action, e.g., find /var/log -name "*.log" -size +10M -delete. The cat command displays or concatenates files, with options like -n to number lines and -E to show end‑of‑line markers. touch creates empty files or updates timestamps, for example touch -c -t 202301011200 newfile.txt. The which command locates executables in $PATH, e.g., which bash. expr evaluates arithmetic and string expressions, such as expr 10 + 20. The chmod command changes file permissions using symbolic ( chmod u+x file) or octal ( chmod 755 file) modes.

Text Processing Utilities

less

provides interactive paging with navigation keys and search patterns. awk processes text with pattern‑action statements, for instance awk -F',' '{print $1,$3}' data.csv. sed performs stream editing, such as sed -n '2,5p' file.txt to print lines 2‑5. tr translates or deletes characters, e.g., cat file | tr 'a‑z' 'A‑Z'.

Archive and Compression Tools

tar

creates and extracts archives: tar -cvf archive.tar file1 dir/ and tar -xvf archive.tar. zip compresses multiple files ( zip -r archive.zip dir/) and unzip extracts them. gzip compresses single files ( gzip file.txt) and gzip -d file.txt.gz decompresses.

Disk and Filesystem Utilities

du

reports disk usage with human‑readable output ( du -h /home/user) and total summarisation ( du -sh *). df shows filesystem space, e.g., df -h. stat displays inode information: stat /etc/passwd.

Network Configuration and Monitoring

ifconfig

configures network interfaces ( ifconfig eth0 up, ifconfig eth0 192.168.1.10 netmask 255.255.255.0). telnet opens remote sessions ( telnet 192.168.0.7). netstat lists socket statistics ( netstat -tunap) and ss provides faster socket details ( ss -tuln). ping (not shown in the source but commonly used) tests connectivity.

System Process and Resource Tools

ps

displays processes ( ps aux), while top offers a real‑time view of CPU, memory, and load averages ( top -d 2). kill terminates processes ( kill -9 1234) and free reports memory usage ( free -h). dstat aggregates CPU, disk, network, and memory statistics ( dstat -c -d -n). vmstat and iostat provide additional performance metrics.

Remote Access and Transfer

ssh

securely logs into remote hosts ( ssh user@host -p 2222) with options for identity files, compression, and port forwarding. scp (not listed but related) copies files over SSH. curl and wget retrieve web resources, e.g., curl -O https://example.com/file.tar.gz and wget -c http://example.com/file.iso.

Performance Profiling

The perf suite records hardware events, such as perf stat ls or perf stat -e cycles,instructions ./program. watch repeatedly runs a command and updates the display, for example watch -n 1 free -h to monitor memory.

Shell Variables and Scripting

Variables are assigned without spaces ( var=value) and accessed with $var or ${var}. Integer variables can be declared with declare -i count=10. Arrays are created with arr=(one two three) and associative arrays with declare -A map. Special parameters like $0, $#, and $? provide script name, argument count, and exit status respectively.

Additional Utilities

Manual pages are viewed with man ( man ls). The variable section explains setting, reading, making read‑only, and unsetting variables, as well as string manipulation techniques.