Strings

Everything in bash is a string — so "string manipulation" is really "bash manipulation." The tools come in two tiers: parameter expansion (built into the shell, instant) and delegating to programs (tr, cut, sed — the Unix toolbox). Fluency means knowing both and reaching for the built-in first.

Building strings

Concatenation is just adjacency — no operator at all:

first="Ada"
last="Lovelace"
full="$first $last"           # "Ada Lovelace"
greeting="Hello, ${first}!"   # braces mark the name's end before a letter

Quoting recap with teeth: double quotes expand variables, single quotes don't, and the unquoted form word-splits. For building text, double quotes are your default; you're always one filename-with-spaces away from needing them.

Parameter expansion: the built-in toolkit

Bash packs a surprising string library inside ${ }:

s="hello world"

echo "${#s}"           # 11 — length
echo "${s^^}"          # HELLO WORLD — uppercase (bash 4+)
echo "${s,,}"          # hello world — lowercase
echo "${s/world/bash}" # hello bash — replace first match
echo "${s//o/0}"       # hell0 w0rld — replace ALL matches
echo "${s:0:5}"        # hello — substring: offset 0, length 5
echo "${s:6}"          # world — from offset to end

And the pair that powers half of all real scripts — prefix/suffix stripping, tailor-made for file paths:

file="report.tar.gz"
echo "${file%.gz}"     # report.tar   — strip shortest matching suffix
echo "${file%%.*}"     # report       — strip LONGEST matching suffix
echo "${file#*.}"      # tar.gz       — strip shortest matching prefix

Mnemonic that finally makes these stick: on your keyboard, # is left of $ (prefix), % is right (suffix) — single char = shortest match, doubled = longest. ${file%.txt} ("filename without extension") appears in the own-editor run_tests.sh you may have already read.

When the shell isn't enough: pipe to a program

Some jobs want the toolbox (properly introduced with pipes, next chapter — a preview):

echo "$s" | tr 'a-z' 'A-Z'     # uppercase, the portable way
echo "$s" | cut -d' ' -f1      # first space-delimited field
rev <<< "$s"                   # reverse (<<< feeds a string as stdin)

Rule of thumb: expansion for one variable's worth of text, programs for streams of lines. Spawning a process per string operation inside a tight loop is the classic slow-script mistake; expansions are free.

Comparing strings

Equality tests live in [[ ]] (the test construct, starring next chapter):

if [[ "$answer" == "yes" ]]; then …
if [[ -z "$s" ]]; then …        # -z: is it empty?
if [[ "$file" == *.txt ]]; then … # pattern match, right side unquoted glob

Quote the variable side, always — an empty unquoted variable vanishes from the line and produces syntax errors that point nowhere.

Your exercise: Greeting Card

Assemble a greeting from read-in values with exact spacing and punctuation. It's a quoting-and-adjacency drill: "Hello, $name! Welcome to $place." — every space and comma lives inside the quotes, ${name} braces where a letter follows. When output is one space off, the bug is inside your quoted string, not in bash — same lesson as SQL's string literals, same fix: read your quotes character by character.