1.2Shell Scripting

Shell scripts automate routine work and carry weight 6—the highest of any subtopic on exam 102. The goal is building parts that even non-Linux users can run safely; questions probe the behavior of arguments, return values, and branching, not rote syntax.

1.2.1The skeleton of a script

• The shebang (#!/bin/bash) names the interpreter for this script—the same mechanism serves sh, python, etc.
• Running needs read + execute permission (chmod +x script.sh → ./script.sh); manage ownership with chown. Without execute permission, bash script.sh still works (read is enough).
• Positional parameters: $0 (script name), $1–$n (nth argument), $# (argument count), $*/$@ (all arguments). shift moves them left by one ($2 becomes $1).
• Exit status: $? holds the last command's result (0 = success, non-zero = failure). A script sets its own with exit N.

1.2.2Branching, loops, functions

• if: if [ "$1" = "start" ]; then …; elif …; else …; fi ([ ] is the test command; closes with fi).
• case branches one value by patterns: case "$1" in start) …;; stop) …;; *) …;; esac (closes with esac; default is *)).
• for iterates a list (for f in *.log; do …; done); while repeats while true (while read line; do …; done < file). Both close with done.
• A shell function names a routine (backup() { tar czf "$1.tar.gz" "$1"; }; the function keyword also works). Inside, $1 refers to the function's own argument.
• PATH-independent scripts call commands by absolute path or set PATH themselves at the top (so user environments cannot break them). Debug with bash -x (prints each expanded command as executed) and bash -v (echoes lines as read).

Assemble a 30-line practical script mentally—a service wrapper svc.sh start nginx: start with the shebang; then validate arguments with if [ $# -ne 2 ]; then echo "Usage: $0 {start|stop} name"; exit 1; fi (wrong count → print usage, fail). Then admit only allowed operations: case "$1" in start|stop) systemctl "$1" "$2";; *) exit 2;; esac. Callers can chain on $? via svc.sh start nginx && echo OK || echo NG—honoring "0 = success" is what makes a script a reusable part. Process many files with for f in "$@"; do process "$f"; done; consume arguments with while [ $# -gt 0 ]; do …; shift; done. When it misbehaves, bash -x svc.sh start nginx shows what each variable expanded to, line by line—the fastest route to the bug.

1.2.3Section summary

• Skeleton = #! shebang + chmod +x; arguments via $0/$1../$#/$@ and shift; status via $? (0 = success) and exit N
• Branch with if…fi / case…esac, loop with for/while…done, modularize with functions; debug with bash -x (trace)