test

$ if [ -f notes.txt ]; then echo "notes.txt is a regular file"; fi
> test -d data && echo "data is a directory"
> [ -e nothing.txt ] || echo "nothing.txt does not exist"

notes.txt is a regular file
data is a directory
nothing.txt does not exist

$ [ 5 -gt 3 ]; echo "5 -gt 3: $?"
> [ 5 -lt 3 ]; echo "5 -lt 3: $?"
> [ 5 -gt ]; echo "malformed: $?"

5 -gt 3: 0
5 -lt 3: 1
/bin/bash: line 3: [: 5: unary operator expected
malformed: 2

$ [ 10 = 10.0 ] && echo "= equal" || echo "= different"
> [ 010 -eq 10 ] && echo "-eq equal" || echo "-eq different"
> [[ 9 > 10 ]] && echo "[[ > ]] says 9 is greater (text order)"
> (( 9 > 10 )) || echo "(( > )) says 9 is smaller"

= different
-eq equal
[[ > ]] says 9 is greater (text order)
(( > )) says 9 is smaller

$ name=""
> [ -z "$name" ] && echo "name is empty"
> name="alice"
> [ -n "$name" ] && echo "name is set to $name"
> [ "$name" != "root" ] && echo "not root"

name is empty
name is set to alice
not root

$ name=""
> if [ $name = "admin" ]; then echo admin; else echo "not admin"; fi
> if [ -n $name ]; then echo "-n says non-empty"; fi
> if [ -n "$name" ]; then echo "never"; else echo "quoted -n says empty"; fi

/bin/bash: line 2: [: =: unary operator expected
not admin
-n says non-empty
quoted -n says empty

$ file="file with spaces.txt"
> [ -f $file ] || echo "[ with unquoted var: failed"
> [[ -f $file ]] && echo "[[ with unquoted var: works"
> [[ $file == *.txt ]] && echo "pattern match: ends in .txt"
> [[ $file =~ ^file\ (.*)\.txt$ ]] && echo "regex group: ${BASH_REMATCH[1]}"

/bin/bash: line 2: [: too many arguments
[ with unquoted var: failed
[[ with unquoted var: works
pattern match: ends in .txt
regex group: with spaces

$ for p in notes.txt data link-to-notes scripts/greet.sh missing; do
>   printf '%-17s' "$p"
>   [ -e "$p" ] && printf ' exists' || printf ' missing'
>   [ -f "$p" ] && printf ' file'
>   [ -d "$p" ] && printf ' dir'
>   [ -L "$p" ] && printf ' symlink'
>   [ -x "$p" ] && printf ' executable'
>   echo
> done

notes.txt         exists file
data              exists dir executable
link-to-notes     exists file symlink
scripts/greet.sh  exists file executable
missing           missing

$ : > empty.log
> for f in empty.log logs/error.log; do
>   if [ -s "$f" ]; then echo "$f has data"; else echo "$f is empty"; fi
> done

empty.log is empty
logs/error.log has data

$ touch -d '1 year ago' app.bin
> [ src/main.c -nt app.bin ] && echo "main.c changed, rebuild"
> [ app.bin -ot src/main.c ] && echo "app.bin is older than main.c"
> [ src/main.c -nt never-built.bin ] && echo "never-built.bin is missing, build it"

main.c changed, rebuild
app.bin is older than main.c
never-built.bin is missing, build it

$ n=7
> [ "$n" -ge 1 ] && [ "$n" -le 10 ] && echo "$n is between 1 and 10"
> [[ $n -ge 1 && $n -le 10 ]] && echo "same test inside [["
> [ ! -d "$n" ] && echo "and there is no directory called $n"

7 is between 1 and 10
same test inside [[
and there is no directory called 7

$ count="12 items"
> [ "$count" -gt 10 ] && echo "more than ten"
> echo "status: $?"

/bin/bash: line 2: [: 12 items: integer expression expected
status: 2

$ x=abc
> [[ $x == a* ]] && echo match
> [ "$x" = abc ] && echo "[ works in sh"

/bin/sh: 2: [[: not found
[ works in sh