Bash - Creating a string with a string variable

Question

I have a variable called CURRENTDATE=20151105.

I want to create a string as below:

abc_20151105_20151105

I tried the following variations:

echo "abc_$CURRENTDATE_$CURRENTDATE" 
This gave abc_20151105

echo "abc_'$CURRENTDATE'_'$CURRENTDATE'"
This gave abc_'20151105'_'20151105'

What am I missing here? Thanks in advance!

The first version tries to use the CURRENTDATE_ variable. Use ${CURRENTDATE}_. The second version does exactly what you'd expect which is expand your variables and leave the single quotes intact. — Etan Reisner
– Etan Reisner, Commented Nov 12, 2015 at 18:50
As Etan pointed out, the variable name it's first expanding has an underscore appended to it. You can use his suggestion or use \ like so echo abc_$CURRENT_DATE\_$CURRENT_DATE I've also found this unix.stackexchange.com/questions/88452/… -- it's worth a read. — Lando
– Lando, Commented Nov 12, 2015 at 19:20
You can even split with quotes. echo "abc_$CURRENTDATE""_$CURRENTDATE". That said, I use curly braces almost whenever possible, to reduce unknowns. I also prefer printf. printf 'abc_%s_%s\n' "$CURRENTDATE" "$CURRENTDATE" — ghoti
– ghoti, Commented Nov 12, 2015 at 19:25

miken32 · Accepted Answer · 2015-11-12 19:20:57Z

3

The problem is that the underscore is a valid character for a variable name. Try one of these:

echo "abc_"$CURRENT_DATE"_"$CURRENT_DATE
echo "abc_${CURRENT_DATE}_$CURRENT_DATE"

Bash doesn't have a concatenation operator, so you concatenate strings by smashing them together in the command; this is what the first example is doing. The second uses braces to explicitly point out the variable name.

answered Nov 12, 2015 at 19:20

miken32

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Denio Mariz · Accepted Answer · 2015-11-14 18:27:20Z

1

You must surround the variable name with ${} in order to isolate it from other valid characters. Like this:

echo "abc_${CURRENT_DATE}_${CURRENT_DATE}"

answered Nov 14, 2015 at 18:27

Denio Mariz

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