Colon (:) character in shell script and Linux

- The colon character (:) in shell script is considered as NULL command. It means do no operation (Do nothing). The ":" command is itself a Bash builtin, and its exit status is true (0).
Demonstration here,

$ :

$ echo $?
0

- The colon is used in case of while loop to make endless iteration. Example of while endless loop is,

while :
do
  ...
done

which is same as,

while true
do
  ...
done

- Colon (:) can be used as placeholder in if/then test.
Example:

if condition
then :   # Do nothing and branch ahead
else     # Or else ...
  take-some-action
fi

After "then" the colon (:) indicates do nothing if "if condition" is satisfied and ahead branch.

- In case of, "${parameter:-default}" colon (:) makes a difference only when the parameter has been declared, but is set to null. In this case it will echo default. Example.

$ cat >colon_test.sh
# Colon makes to trigger the default optuion
#+if the variable is declared and set to null/

username=
echo "username has been declared, but is set to null."
echo "username = ${username-`whoami`}"
# This will not echo as variable is set to null and is not used colon (:).


echo "username = ${username:-`whoami`}"
# Will echo the execution of `whoami` command as we used colon (:) here.

$ sh colon_test.sh
username has been declared, but is set to null.
username =
username = Arju

$ whoami
Arju

- In case of arithmetic operations, while assigning variable ":" is necessary because otherwise Bash attempts to interpret statement as a command.
Example:

$ cat >arithmetic_colon.sh

n=1; echo -n "$n "
: $((n = $n + 1))
# Here ":" is necessary. If we don't use colon then Bash attempts
#+ to interpret "$((n = $n + 1))" as a command.
echo -n "$n "

$ sh arithmetic_colon.sh
1 2

Note that we could also increment n by 1 using "(( n = n + 1 ))" or "n=$(($n + 1))".

- Colon (:) also can be used to evaluate environmental variable. Before doing any operation it can be checked whether certain environmental variable is set.

$ cat >check_environment.sh
#  Check some of the system's environmental variables.
: ${HOSTNAME?} ${USER?}

$ sh check_environment.sh

$ cat >check_environment2.sh
#  Check some of the system's environmental variables.
: ${HOSTNAME?} ${USER?} ${ORACLE_HOME?}

$ sh check_environment2.sh
check_environment2.sh: line 2: ${$ORACLRACLE_HOME?}: bad substitution

In the above example see if all environmental variables were set then no error returned but as ORACLE_HOME variable was not set and so it returned error.

- In combination with the redirection operator (>) colon, truncates a file to zero length, without changing its permissions. If the file did not previously exist, creates it.
For example

$ : >new_file.txt   # File "new_file.txt" is now empty.       

It have the similar effect as statement cat /dev/null >new_file.txt however, this does not fork a new process, since ":" is a builtin.

If : is used with >> redirection operator, then it has no effect on a pre-existing target file (: >> target_file). If the file did not previously exist, then it creates it.

- Colon (:) can be used to comment a line but not recommended. Because with : error checking is done in the line.

- The ":" also serves as a field separator, in /etc/passwd, and in the $PATH variable.
Example:

$ echo $PATH
/usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin

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