how to grep a value stored in a variable

Question

I am writing a bash script in which I am storing current date in a variable and then I am greping that variable. Issue is it's not working

currentdate= $(date +%b\ %d)
echo "$currentdate"
last |grep -E '$currentdate'>> /usr/IBM/HTTPServer7/logs/alert/users.txt

users.txt is showing empty. If I write manually the current date then it works. What am I doing wrong?

Single quotes `'` vs. double quotes `"`. The former won't expand variables. — Sparhawk, Dec 04 '18 at 05:58
remove that space between = and $ in currentdate. and date syntax in last is like `DEC 1` . that date command returns `DEC 01` — BlackCrystal, Dec 04 '18 at 06:26
Possible duplicate of [When is double-quoting necessary?](https://unix.stackexchange.com/questions/68694/when-is-double-quoting-necessary) — roaima, Dec 04 '18 at 08:56

score 1 · Accepted Answer · answered Dec 04 '18 at 09:30

In addition to the quotes, you also need to account for the date format used by the last command, which looks like the following:

Dec  3    # Note the padding to the left of '3'.
Nov 23

This requires a slightly different date command:

date "+%b %_d"
Dec  4

The underscore instructs date to pad the field with spaces. You can also use %e as an alternative.

Putting these together, you can modify your script as shown below:

currentdate=$(date "+%b %_d")
last | grep "$currentdate" >> /usr/IBM/HTTPServer7/logs/alert/users.txt

score 0 · Answer 2 · answered Dec 04 '18 at 06:18

0

@Sparhawk is correct.

Double quotes cause the shell to expand variables.

$ VAR=blahblah
$ echo "$VAR"
blahblah

Single quotes cause the shell to use the text literally.

$ VAR=blahblah
$ echo '$VAR'
$VAR

You would use this if you wanted to prevent the shell from thinking a dollar sign plus some other text was a variable.

echo 'This script is terminating because you didn't set $IMPORTANT_VARIABLE'

Without single quotes the above message wouldn't output properly.

answered Dec 04 '18 at 06:18

LawrenceC

Issue is with this command all though currentdate has correct value which is today Dec 04 but in users.txt the file is empty . last |grep -E '$currentdate'>> /usr/IBM/HTTPServer7/logs/alert/users.txt if i use actual value Dec 04 in the above command the result is fine . – Black Virus Dec 04 '18 at 06:29

2 Answers2