Conditions: test, [ ] and if

An if in the shell does not evaluate a boolean expression. It runs a command and looks at its exit status: 0 means success and takes the then branch; anything else goes to else. That is why if grep -q root /etc/passwd; then … works with no brackets at all. The command most often used as a condition is test, which exists to compare things and exit 0 or 1, and [ is simply another name for test that requires a closing ].

~% test -f /etc/passwd; echo $?
0
~% [ -f /nope ]; echo $?
1
~% [ 5 -gt 3 ] && echo "five is bigger"
five is bigger
~% [5 -gt 3 ]
sh: [5: not found

Because [ is a command, it needs spaces around it like any other word: [ -f file ], never [-f file]. Everything inside is an argument, so variables with spaces must be quoted, [ "$name" = "root" ], or an empty variable makes the command collapse into [ = root ] and fail with a syntax error.

The operators you will actually use

Strings and numbers have different operators on purpose: [ 10 -gt 9 ] is true, but [ 10 \> 9 ] compares as text and is false, because "1" sorts before "9". Combine tests with the shell's own && and || between separate brackets, [ -f f ] && [ -r f ]; it reads better than -a/-o inside one pair.

if, elif, else, fi

~% cat lab/l84/check.sh
#!/bin/sh
target="$1"
if [ -z "$target" ]; then
    echo "usage: check.sh FILE" >&2
    exit 1
elif [ -d "$target" ]; then
    echo "$target is a directory with $(ls "$target" | wc -l) entries"
elif [ -f "$target" ]; then
    echo "$target is a file of $(wc -c < "$target") bytes"
else
    echo "$target does not exist"
fi
~% lab/l84/check.sh /etc
/etc is a directory with 24 entries
~% lab/l84/check.sh /etc/passwd
/etc/passwd is a file of 340 bytes
~% lab/l84/check.sh /nope
/nope does not exist

The structure is if condition; then … elif condition; then … else … fi. The ; before then is just a line separator; you may put then on its own line instead. fi closes the block (it is if backwards). Indentation is for you, the shell ignores it, but keep it consistent: nested blocks without indentation are unreadable within a week.

Any command can be the condition. if cmp -s a b; then echo same; fi compares two files; if ping -c 1 -W 1 10.0.2.2 >/dev/null 2>&1; then … checks the network. Redirecting to /dev/null keeps the output out of the way; the if only cares about the exit status.