Bash Basics

The terminal is your window into the file system. pwd — shows where you are (Print Working Directory). ls — lists files and folders. Add -la for hidden files and details. cd — changes directory. cd .. goes up, cd ~ goes home. mkdir — creates new directories.

touch — creates empty files. The name comes from the Unix idea of "touching" a file's timestamp, but if the file doesn't exist, it creates one. Like tapping something into existence. echo — prints text to the screen. Named because it echoes your words back at you. > — the redirect operator. Instead of printing to screen, it pours output into a file. echo "hello" > file.txt creates (or overwrites) file.txt with "hello". cat — short for "concatenate" (joining files together), but most people use it to simply display a file's contents on screen. cp — copies files: cp source dest. The original stays untouched. mv — moves OR renames files: mv old new. Same command, two uses. rm — removes files permanently. No trash can, no undo. Gone means gone. Tip: use ; to chain commands on one line — echo hi > a.txt; cp a.txt b.txt does both in sequence.

grep — searches for text patterns inside files. Usage: grep "pattern" file.txt. Flags: -i (case-insensitive), -r (recursive through directories), -n (show line numbers). The name stands for Global Regular Expression Print. There's a hacker saying: 'You can't grep dead trees' — meaning digital text is searchable, but paper isn't. That's your superpower. find — locates files by name, size, or type across directories. Usage: find . -name "*.txt" finds all .txt files from the current directory down.

The pipe | is the most powerful concept in bash. It takes the output of one command and feeds it as input to the next. Unlike ; (which just runs commands in sequence), | connects them — data flows through. head -n N — shows the first N lines. tail -n N — shows the last N lines. sort — sorts lines alphabetically or numerically (-n). uniq — removes adjacent duplicate lines (always sort first). wc -l — counts lines.

xargs — the bridge between finding things and acting on them. It takes a list from stdin and passes each item as an argument to another command. Example: find . -name "*.tmp" | xargs rm — finds all .tmp files and deletes them in one sweep. Without xargs, you'd have to delete them one by one. Power pipeline: cat log.txt | grep "ERROR" | awk "{print $1}" | sort | uniq -c — reads a log, filters errors, extracts the first column, sorts, and counts occurrences.

sed (Stream Editor) — performs text substitution on a stream. sed "s/old/new/" replaces the first occurrence per line. Add g for all: sed "s/old/new/g". awk — processes columnar data. It splits each line into fields ($1, $2, etc.) by whitespace. awk "{print $1}" prints the first column. Together with pipes, sed and awk let you transform any text data on the fly.

chmod — changes file permissions. chmod +x script.sh makes a file executable. curl — transfers data from URLs. Great for testing APIs and downloading files. wget — downloads files directly to disk. history — shows your recent commands. Use !42 to re-run command #42. alias — creates shortcuts: alias ll="ls -la" lets you type ll instead. top — shows running processes and resource usage, like a task manager.

A bash script is a file containing a sequence of commands, one per line. #!/bin/bash — the shebang. First line of every script. Tells the system to use bash. Variables — store values: NAME="world". Retrieve with $NAME. read — gets input from the user. echo — prints output. Scripts combine everything you've learned — file ops, pipes, text processing — into reusable programs. Save as .sh, make executable with chmod +x, run with ./script.sh.