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JanezKranjski
Asked: 2019-11-13 00:30:53 +0800 CST

Dealing with dates in bash

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In the bash script, I want to check if it is the first day of the month. Now I have:

if [ $DAY -eq 1 ]; then
        rclone copy /tmp/$MONTH-$YEAR.tar.gz.gpg /server2/archive
fi

and I get an error: 

[: -eq: unary operator expected

I tried many different syntaxes from the web, but I can't succeed. Any idea what is wrong? Ubuntu Server 18.04

bash

1 Answers

  1. Bash does not predefine DAY. As Jos says, the error message you're getting is the same as the message obtained when it is omitted altogether:

    ek@Kip:~$ [ -eq 1 ]
-bash: [: -eq: unary operator expected

    The command date +%d outputs the current day of the month. If you want DAY to hold that value, you can use:

    DAY="$(date +%d)"

    But you can also just place the command substitution in the [ command:

    if [ "$(date +%d)" -eq 1 ]; then
        rclone copy "/tmp/$MONTH-$YEAR.tar.gz.gpg" /server2/archive
fi

    date +%d always outputs two digits, so when it is the first day of the month it outputs 01, but that is no problem. Arithmetically, 01 is equal to 1, and you're rightly using -eq, which performs numerical comparison (unlike =, which performs textual comparison).

    You will notice that I have enclosed $(date +%d) in double quotes in the [ command. This prevents globbing and word splitting.

    It is a good idea always to enclose parameter expansion ("$DAY"), arithmetic expansion ("$((3 + 4))"), and command substitution ("$(date +%d)" or the less preferred form `date +%d`) in double quotes except in the fairly uncommon case that you actually want globbing and word splitting to be performed.

    If you had written "$DAY" originally, instead of $DAY, then you would have gotten this error from [:

    -bash: [: : integer expression expected

    That error makes much more sense.

    I've also enclosed your path, /tmp/$MONTH-$YEAR.tar.gz.gpg, which contains the parameter expansions $MONTH and $YEAR, in double quotes.

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