[bash] When do we need curly braces around shell variables?

In shell scripts, when do we use {} when expanding variables?

For example, I have seen the following:

var=10        # Declare variable

echo "${var}" # One use of the variable
echo "$var"   # Another use of the variable

Is there a significant difference, or is it just style? Is one preferred over the other?

Curly braces are always needed for accessing array elements and carrying out brace expansion.

It's good to be not over-cautious and use {} for shell variable expansion even when there is no scope for ambiguity.

For example:

dir=log
prog=foo
path=/var/${dir}/${prog}      # excessive use of {}, not needed since / can't be a part of a shell variable name
logfile=${path}/${prog}.log   # same as above, . can't be a part of a shell variable name
path_copy=${path}             # {} is totally unnecessary
archive=${logfile}_arch       # {} is needed since _ can be a part of shell variable name

So, it is better to write the three lines as:

path=/var/$dir/$prog
logfile=$path/$prog.log
path_copy=$path

which is definitely more readable.

Since a variable name can't start with a digit, shell doesn't need {} around numbered variables (like $1, $2 etc.) unless such expansion is followed by a digit. That's too subtle and it does make to explicitly use {} in such contexts:

set app      # set $1 to app
fruit=$1le   # sets fruit to apple, but confusing
fruit=${1}le # sets fruit to apple, makes the intention clear

See:

• Allowed characters in Linux environment variable names

You are also able to do some text manipulation inside the braces:

STRING="./folder/subfolder/file.txt"
echo ${STRING} ${STRING%/*/*}

Result:

./folder/subfolder/file.txt ./folder

or

STRING="This is a string"
echo ${STRING// /_}

Result:

This_is_a_string

You are right in "regular variables" are not needed... But it is more helpful for the debugging and to read a script.

The end of the variable name is usually signified by a space or newline. But what if we don't want a space or newline after printing the variable value? The curly braces tell the shell interpreter where the end of the variable name is.

Classic Example 1) - shell variable without trailing whitespace

TIME=10

# WRONG: no such variable called 'TIMEsecs'
echo "Time taken = $TIMEsecs"

# What we want is $TIME followed by "secs" with no whitespace between the two.
echo "Time taken = ${TIME}secs"

Example 2) Java classpath with versioned jars

# WRONG - no such variable LATESTVERSION_src
CLASSPATH=hibernate-$LATESTVERSION_src.zip:hibernate_$LATEST_VERSION.jar

# RIGHT
CLASSPATH=hibernate-${LATESTVERSION}_src.zip:hibernate_$LATEST_VERSION.jar

(Fred's answer already states this but his example is a bit too abstract)

Variables are declared and assigned without $ and without {}. You have to use

var=10

to assign. In order to read from the variable (in other words, 'expand' the variable), you must use $.

$var      # use the variable
${var}    # same as above
${var}bar # expand var, and append "bar" too
$varbar   # same as ${varbar}, i.e expand a variable called varbar, if it exists.

This has confused me sometimes - in other languages we refer to the variable in the same way, regardless of whether it's on the left or right of an assignment. But shell-scripting is different, $var=10 doesn't do what you might think it does!

You use {} for grouping. The braces are required to dereference array elements. Example:

dir=(*)           # store the contents of the directory into an array
echo "${dir[0]}"  # get the first entry.
echo "$dir[0]"    # incorrect

Following SierraX and Peter's suggestion about text manipulation, curly brackets {} are used to pass a variable to a command, for instance:

Let's say you have a sposi.txt file containing the first line of a well-known Italian novel:

> sposi="somewhere/myfolder/sposi.txt"
> cat $sposi

Ouput: quel ramo del lago di como che volge a mezzogiorno

Now create two variables:

# Search the 2nd word found in the file that "sposi" variable points to
> word=$(cat $sposi | cut -d " " -f 2)

# This variable will replace the word
> new_word="filone"

Now substitute the word variable content with the one of new_word, inside sposi.txt file

> sed -i "s/${word}/${new_word}/g" $sposi
> cat $sposi

Ouput: quel filone del lago di como che volge a mezzogiorno

The word "ramo" has been replaced.