Conditionals and test

if in bash doesn't test anything itself. It runs a command and branches on that command's exit status. [ (also spelled test) and [[ are the commands almost always used for that check.

`[` is a command, not syntax

count=5
if [ "$count" -gt 3 ]; then
  echo "more than 3"
fi

more than 3

[ is an actual program (/usr/bin/[, though bash also has it built in), and ] is its final argument, required so the command knows where its argument list ends. if just runs [ "$count" -gt 3 ] and checks whether it exited 0. This is why spacing around [ and ] is mandatory: they're separate words to the shell, not special syntax it parses differently.

String comparison uses = (or !=), numeric comparison uses -eq, -ne, -gt, -lt, -ge, -le:

name="deb1"
if [ "$name" = "deb1" ]; then
  echo "matched"
fi

Why `[` needs its arguments quoted

unset myvar
if [ $myvar = "x" ]; then echo yes; fi

bash: line 1: [: =: unary operator expected

With myvar unset and unquoted, [ $myvar = "x" ] expands to [ = "x" ]: only two arguments where [ expected three, so it reports a syntax error instead of doing the comparison you meant.

if [ "$myvar" = "x" ]; then echo yes; else echo "no match, safely"; fi

no match, safely

Quoted, "$myvar" expands to an empty string that still counts as an argument, so [ sees [ "" = "x" ], three arguments, and evaluates the comparison correctly. This is the same quoting rule from the previous lesson, showing up again in a new place: unquoted variable expansions inside [ ] are a common source of scripts that work in testing and break the moment a variable happens to be empty.

`[[` is a bash keyword, not a command, and more forgiving

if [[ $myvar = "x" ]]; then echo yes; else echo "no match, still safe"; fi

no match, still safe

[[ ]] is bash syntax handled directly by the parser, not a separate program, so it doesn't word-split or glob-expand its contents the way [ does. The same unquoted $myvar that broke [ works fine inside [[ ]]. It also supports pattern matching and combined conditions directly:

file="report.txt"
if [[ "$file" == *.txt ]]; then echo "is a text file"; fi

count=5
if [[ "$count" -gt 3 && "$count" -lt 10 ]]; then echo "in range"; fi

The equivalent with [ needs -a (deprecated and best avoided) or two separate [ ] tests joined with &&:

if [ "$count" -gt 3 -a "$count" -lt 10 ]; then echo "in range via -a"; fi
# or, preferred:
if [ "$count" -gt 3 ] && [ "$count" -lt 10 ]; then echo "in range"; fi

[[ ]] is bash-only, not portable to a POSIX sh script (#!/bin/sh). Still quote variable expansions inside it out of habit; it protects against word splitting, but not against every edge case, and consistent quoting is easier to maintain than remembering which rule applies where.

File test operators

if [ -f exists.txt ]; then echo "file exists"; fi
if [ -d /tmp ]; then echo "directory exists"; fi
if [ ! -f nope.txt ]; then echo "file does not exist"; fi

file exists
directory exists
file does not exist

-f tests for a regular file, -d for a directory, -e for either existing at all, -x for executable, -r/-w for readable/writable. ! negates any test, in both [ and [[.

`case`: pattern matching without repeated `if`

ext="txt"
case "$ext" in
  txt|md) echo "text-like" ;;
  jpg|png) echo "image" ;;
  *) echo "unknown" ;;
esac

text-like

Each pattern is a glob, not a regex; | separates alternatives within one pattern, and *) acts as the default case, matched only if nothing above it did. case reads more clearly than a long if/elif chain once there are more than two or three branches.

Exercises

Write a test that prints "empty" if a variable is unset or an empty string, using [[ ]].
Answer
```
if [[ -z "$myvar" ]]; then
  echo "empty"
fi
```
-z tests for a zero-length string; -n is its opposite, testing for a non-empty string.
Given a variable path, write a check using [ that safely handles path being unset, testing whether it equals /tmp.
Answer
```
if [ "$path" = "/tmp" ]; then
  echo "matched"
fi
```
The quotes around "$path" are what make this safe when path is unset; without them, [ would see too few arguments and error out instead of comparing.

Write a case statement that prints "weekday" for mon through fri and "weekend" for sat or sun.

Answer

day="sat"
case "$day" in
  mon|tue|wed|thu|fri) echo "weekday" ;;
  sat|sun) echo "weekend" ;;
  *) echo "not a day" ;;
esac

What's next

The next lesson covers loops: iterating over lists, files, and command output safely.

Conditionals and test

[ is a command, not syntax

Why [ needs its arguments quoted

[[ is a bash keyword, not a command, and more forgiving