Understanding special characters

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What makes a character special?  If it has a meaning beyond its literal meaning, a meta-meaning, then we refer to it as a special character.

Along with commands and keywords, special characters are building blocks of Bash scripts.

Comments or Hash (#)

This Special character (#) is used for adding lines to the script which will not be executed, a recent example can be Sha-Bang

Below are various scenarios of using Comment with different meaning for each one,

# This line is a comment.

Comment at start of line, making whole line not executable. 

echo "A comment will follow" # Comment here.

Comment at end part of line, making part of line not executable.

echo "The # here does not begin a comment."

Comment used as Text for printing in output, within double quotes

echo The \# here does not begin a comment.

Comment used as Text for printing in output, by escaping it

echo The # here begins a comment.

Comment used without double quotes

echo ${PATH#*:}      

Comment used as parameter substitution

echo $(( 2#101011 )); echo $(( 6#101 )) 

Comment used for conversion

Command separator or Semicolon (;)

Semicolon permits two commands on same line, it’s a command separator to be used in command line as well as inside a script.

echo hello; echo there

Terminator or double Semicolon (;;)

This is mainly used in case statement to show that commands to be executed as a part of a case criteria are finished. i.e. end of logic to be executed when a case is matched as script execution goes in sequence line by line.

case "$variable" in

  abc)  echo "\$variable = abc" ;;

  xyz)  echo "\$variable = xyz" ;;

esac

Dot (.)

When working with filenames, a leading dot is the prefix of a "hidden" file, a file that an ls will not normally show.

bash$ touch .hidden-file

bash$ ls -l      

total 10

 -rw-r--r--    1 sam  sam      4034 Jul 18 22:04 file1

 -rw-r--r--    1 sam  sam      4602 May 25 13:58 file2

 -rw-r--r--    1 sam  sam       877 Dec 17  2000 file3

bash$ ls -al      

total 14

 drwxrwxr-x    2 sam  sam      1024 Aug 29 20:54 ./

 drwx------   52 sam  sam      3072 Aug 29 20:51 ../

 -rw-r--r--    1 sam  sam      4034 Jul 18 22:04 file1

 -rw-r--r--    1 sam  sam      4602 May 25 13:58 file2

 -rw-r--r--    1 sam  sam       877 Dec 17  2000 file3

 -rw-rw-r--    1 sam  sam         0 Aug 29 20:54 .hidden-file

When considering directory names, a single dot represents the current working directory, and two dots denote the parent directory.

bash$ pwd

/home/sam/projects

bash$ cd .

bash$ pwd

/home/sam/projects

bash$ cd ..

bash$ pwd

/home/sam/

The dot often appears as the destination (directory) of a file movement command, in this context meaning current directory. Copy all the "junk" files to $PWD.

$ cp /home/sam/current_work/junk/* .

File name path separator or forward slash (/)

Separates the components of a filename (as in /home/sam/projects/Makefile).

This is also the division arithmetic operator.

Command substitution (`)

The `command` construct makes available the output of command for assignment to a variable. This is also known as backquotes or backticks.

$ echo $value   # NULL

$ pwd

$ value=`pwd`

$ echo $value   # Path obtained in pwd command output

Null command or colon (:)

This is the shell equivalent of a "NOP" (no op, a do-nothing operation).

The ":" command is itself a Bash built-in and its exit status is true (0).

$ :

$ echo $?   # 0

Endless loop using “:” can be constructed as below,

while : ; do echo `date`; sleep 20; done;

Same as that of traditional approach of writing loop as below,

while true ; do echo `date`; sleep 20; done;

Placeholder in if/then test:

if condition

then :   # Do nothing and branch ahead

else     # Or else ...

   some-action

fi

The ":" serves as a field separator in some OS level config files, an example can be /etc/passwd, and in the $PATH from environment variables.

$ echo $PATH

/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/sbin:/usr/sbin:/usr/games

Reverse (or negate) the sense of a test or exit status or exclamation (!)

The (!) operator inverts the exit status of the command to which it is applied.

It also inverts the meaning of a test operator.

For example, change the sense of equal ( = ) to not-equal ( != ).

Wild card or asterisk (*)

The * character serves as a "wild card" for filename expansion. By itself, it matches every filename in a given directory.

$ echo *   # gives name of files and folders in a particular directory

$ ls /*    # gives list of all folders and files within / along with listing of content of subdirectories to one level of /

In the context of arithmetic operations, the * denotes multiplication.

Dollar ($)

Character ($) is used for variable substitution or showing contents of a variable

var1=5

var2=23skidoo

echo $var1     # 5

A $ prefixing a variable name indicates the value the variable holds.

echo $var2     # 23skidoo

$ also indicates end-of-line. In a regular expression, a "$" addresses the end of a line of text.

find   /  -ls  |awk '{print $3, $NF}'  |grep Z$  # files engins with letter Z

${} can be used for parameter substitution.

$' ... ' i.e. Quoted string expansion. This construct expands single or multiple escaped octal or hex values

into ASCII [3] or Unicode characters.

$*, $@ are used as positional parameters.

$? provides exit status variable. The $? Variable holds the exit status of a command, a function, or of the script itself.

$ :

$ echo $?   # 0

$$ is a process ID variable. The $$ variable holds the process ID of the script in which it appears. below steps can be tried to understand its usage inside a script.

Create a file test.sh with below content.

#!/bin.bash

while :

do

echo $$  >> test.out.log

sleep 10

done

Execute file in background and observe the PID obtained when you get it ran in background

# sh test.sh >> test.out.log 2>&1

Once executed in background check output file test.out.log, it will keep on capturing a PID which should match with PID that you observed while getting job added to background for execution.

# cat test.out.log

Redirection (> &> >& >> < <>)

# command > out.log

Redirects the output of command to file out.log. Overwrite filename if it already exists. As error file not specified it will be shown on console.

# command > out.log 2> error.log

Redirects the output of command to file out.log and error of command to file error.log. Overwrite filename if it already exists.

command &> out.log

Redirects both the stdout and the stderr of command to file out.log.

command > out.log 2>&1

Another way to redirect both the stdout and the stderr of command to file out.log.

command >&2  

Redirects stdout of command to stderr. As error file not specified it will be shown on console.

command 2> error.log >&2 

Redirects stdout of command to stderr. As error file not specified it will be shown on console.

command >> out.log

Appends the output of scriptname to file filename. If filename does not already exist, it is created.

[i]<>in.file

Opens file in.file for reading and writing, and assigns file descriptor i to it. If filename does not exist, it is created.

Pipe (|)

This is a very frequently special character. Pipe passes the output (stdout) of a previous command to the input (stdin) of the next one, or to the shell.

echo ls -l | sh

Passes the output of "echo ls -l" to the shell, with the same result as a simple "ls -l".

cat *.lst | sort | uniq

Merges and sorts all ".lst" files, then deletes duplicate lines.

OR (||)

OR logical operator. In a test construct, the || operator causes a return of 0 (success) if either of the linked test conditions is true.

Single and (&)

Run job in background. A command followed by an & will run in the background.

$ sleep 10 &

[1] 850

[1]+ Done sleep 10

Double and (&&)

AND logical operator. In a test construct, the && operator causes a return of 0 (success) only if both the linked test conditions are true.

Option, prefix (-)

Option flag for a command or filter. Prefix for an operator. Prefix for a default parameter in parameter substitution.

COMMAND -[Option1][Option2][...]

ls -al

sort -dfu $filename

Home directory or tilde (~)

This corresponds to the $HOME internal variable. ~sam is sam's home directory, and ls ~sam lists the contents of it. ~/ is the current user's home directory, and ls ~/ lists the contents of it.

$ echo ~bozo

/home/bozo

$ echo ~

/home/bozo

$ echo ~/

/home/bozo/

$ echo ~:

/home/sam:

Escape or backslash (\)

One of the most important special character. In order to use other special characters well in either on command line or within script combined use of Escape and other special character is required. This concept it termed as escaping your special character.

Basic example can be below to escape # and print it in output.

$ echo The \# here does not begin a comment.

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