Brackets in if condition: why am I getting syntax errors without whitespace?

Question

I am using the below script to move two days back when script runs at starting two days of the year and also check first and second days of every month and move two days back.

if [$month="01"] && [$day="01"]; then
        date="$last_month/$yes_day/$last_year"
        fulldate="$last_month/$yes_day/$last_year"
else
        if [$month="01"] && [$day="02"]; then
                date="$last_month/$yes_day/$last_year"
                fulldate="$last_month/$yes_day/$last_year"
        else
                if [ $day = "01" ]; then
                        date="$last_month/$yes_day/$year"
                        fulldate="$year$last_month$yes_day"
                else
                        if [ $day = "02" ]; then
                                date="$last_month/$yes_day/$year"
                                fulldate="$year$last_month$yes_day"
                        else
                                date="$month/$yes_day/$year"
                                fulldate="$year$month$yes_day"
                        fi
                fi
        fi
fi

But my bad am getting the below error message

Etime_script.sh: line 19: [06=01]: command not found
Etime_script.sh: line 24: [06=01]: command not found

Solution 1

[ is neither a metacharacter nor a control operator (not even a reserved word; same for ]) thus it needs whitespace around it. Otherwise the shell "sees" the command [01=01] instead of the command [ with the separate parameters 01, =, 01, and ]. Each operator and operand needs to be a separate argument to the [ command, so whitespace is necessary around the operators as well.

if [ "$month" = "01" ]

[$month="01"] is a wildcard pattern matching any of the characters in $month or "01. If it doesn't match anything, it's left alone.

If there is a semicolon after the closing bracket, you don't need a space before it, because the semicolon is always part of a separate token.

if [ "$month" = "01" ]; then

The same goes for bash's (and ksh's and zsh's) double bracket syntax.

More than one condition

There are two ways to combine conditions:

1. within [

2. with separate [ commands combined with && or ||

Grouping with brackets is probably easier within [.

if [ "$month" = "01" -a "$day" = "01" ] # -a for and, -o for or

if [ "$month" = "01" ] && [ "$day" = "01" ]

The first one should be avoided as it's unreliable (try for instance with month='!'). Problems with strange variable content can be avoided by using the safe string (if there is one) first; or by using [[/]] instead of [/]:

if [ "01" = "$month" -a "01" = "$day" ]

Solution 2

Another way to write it:

case $month:$day in
  (01:0[12])
    date="$last_month/$yes_day/$last_year"
    fulldate="$last_month/$yes_day/$last_year"
    ;;
  (*:0[12])
    date="$last_month/$yes_day/$year"
    fulldate="$year$last_month$yes_day"
    ;;
  (*)
    date="$month/$yes_day/$year"
    fulldate="$year$month$yes_day"
esac

Comments

[Shawn J. Goff]

The given answers are correct; you need whitespace after [. Additionally, look into the elif statement; it will help you clean things up. Also, the semicolons after the if statements are not necessary, but are also not incorrect, just strange.

[goldilocks]

@ShawnJ.Goff They are necessary if you concatenate the next line (if [ ... ]; then), so not that unusual.

[Kumar1]

How we can match the two condition in if , like month value == day value if [$month="01"] && [$day="01"]; am expecting 01 && 01 true then print below conditions or move to else statement

[Hauke Laging]

@AshokSanganahalli I have extended my answer.

[Stéphane Chazelas]

[ when -a/-o are not used is always reliable with POSIX compliant shells. There's no advantage whatsoever in using -a/-o over &&/||. I would definitely discourage its use. Note that [[ is not POSIX.

[Jakob Bennemann]

+1 for case statement. If you have more than one elif, you should probably be using case.