Advanced Bash-Scripting Guide - Part 2. Basics

continued from Advanced Bash-Scripting Guide - Part 1. Introduction

Chapter 3. Special Characters

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.

Special Characters Found In Scripts and Elsewhere

# Comments - Lines beginning with a # (with the exception of #!) are comments and will not be executed.

# This line is a comment.

Comments may also occur following the end of a command.

echo "A comment will follow." # Comment here.
#                            ^ Note whitespace before #

Comments may also follow whitespace at the beginning of a line. # A tab precedes this comment.

Comments may even be embedded within a pipe.

initial=( `cat "$startfile" | sed -e '/#/d' | tr -d '\n' |\
# Delete lines containing '#' comment character.
           sed -e 's/\./\. /g' -e 's/_/_ /g'` )
# Excerpted from life.sh script

A command may not follow a comment on the same line. There is no method of terminating the comment, in order for "live code" to begin on the same line. Use a new line for the next command.

Of course, a quoted or an escaped # in an echo statement does not begin a comment. Likewise, a # appears in certain parameter-substitution constructs and in numerical constant expressions.

echo "The # here does not begin a comment."
echo 'The # here does not begin a comment.'
echo The \# here does not begin a comment.
echo The # here begins a comment.

echo ${PATH#*:}       # Parameter substitution, not a comment.
echo $(( 2#101011 ))  # Base conversion, not a comment.

# Thanks, S.C.

The standard quoting and escape characters (" ' \) escape the #.

Certain pattern matching operations also use the #.

; Command separator [semicolon]. Permits putting two or more commands on the same line.

echo hello; echo there


if [ -x "$filename" ]; then    #  Note the space after the semicolon.
#+                   ^^
  echo "File $filename exists."; cp $filename $filename.bak
else   #                       ^^
  echo "File $filename not found."; touch $filename
fi; echo "File test complete."

Note that the ";" sometimes needs to be escaped.

;; Terminator in a case option [double semicolon].

case "$variable" in
  abc)  echo "\$variable = abc" ;;
  xyz)  echo "\$variable = xyz" ;;
esac

;;&, ;& Terminator in a case option [double semicolon].

case "$variable" in
  abc)  echo "\$variable = abc" ;;
  xyz)  echo "\$variable = xyz" ;;
esac

;;&, ;& Terminators in a case option (version 4+ of Bash).

. "dot" command [period]. Equivalent to source (see Example 15-22). This is a bash builtin.

. "dot", as a component of a filename. 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 bozo      4034 Jul 18 22:04 data1.addressbook
 -rw-r--r--    1 bozo      4602 May 25 13:58 data1.addressbook.bak
 -rw-r--r--    1 bozo       877 Dec 17  2000 employment.addressbook


bash$ ls -al	      
total 14
 drwxrwxr-x    2 bozo  bozo      1024 Aug 29 20:54 ./
 drwx------   52 bozo  bozo      3072 Aug 29 20:51 ../
 -rw-r--r--    1 bozo  bozo      4034 Jul 18 22:04 data1.addressbook
 -rw-r--r--    1 bozo  bozo      4602 May 25 13:58 data1.addressbook.bak
 -rw-r--r--    1 bozo  bozo       877 Dec 17  2000 employment.addressbook
 -rw-rw-r--    1 bozo  bozo         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/bozo/projects

bash$ cd .
bash$ pwd
/home/bozo/projects

bash$ cd ..
bash$ pwd
/home/bozo/

The dot often appears as the destination (directory) of a file movement command, in this context meaning current directory.

bash$ cp /home/bozo/current_work/junk/* .

Copy all the "junk" files to $PWD.

. "dot" character match. When matching characters, as part of a regular expression, a "dot" matches a single character.

" partial quoting [double quote]. "STRING" preserves (from interpretation) most of the special characters within STRING. See Chapter 5.

' full quoting [single quote]. 'STRING' preserves all special characters within STRING. This is a stronger form of quoting than "STRING". See Chapter 5.

, comma operator. The comma operator links together a series of arithmetic operations. All are evaluated, but only the last one is returned.

let "t2 = ((a = 9, 15 / 3))"
# Set "a = 9" and "t2 = 15 / 3"

The comma operator can also concatenate strings.

for file in /{,usr/}bin/*calc
#             ^    Find all executable files ending in "calc"
#+                 in /bin and /usr/bin directories.
do
        if [ -x "$file" ]
        then
          echo $file
        fi
done

# /bin/ipcalc
# /usr/bin/kcalc
# /usr/bin/oidcalc
# /usr/bin/oocalc


# Thank you, Rory Winston, for pointing this out.

,, , Lowercase conversion in parameter substitution (added in version 4 of Bash).

\/ escape [backslash]. A quoting mechanism for single characters.

\X escapes the character X. This has the effect of "quoting" X, equivalent to 'X'. The \ may be used to quote " and ', so they are expressed literally.

See Chapter 5 for an in-depth explanation of escaped characters.

/ Filename path separator [forward slash]. Separates the components of a filename (as in /home/bozo/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.

: null command [colon]. This is the shell equivalent of a "NOP" (no op, a do-nothing operation). It may be considered a synonym for the shell builtin true. The ":" command is itself a Bash builtin, and its exit status is true (0).

:
echo $?   # 0

Endless loop:

while :
do
   operation-1
   operation-2
   ...
   operation-n
done

# Same as:
#    while true
#    do
#      ...
#    done

Placeholder in if/then test:

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

Provide a placeholder where a binary operation is expected, see Example 8-2 and default parameters.

: ${username=`whoami`}
# ${username=`whoami`}   Gives an error without the leading :
#                        unless "username" is a command or builtin...

: ${1?"Usage: $0 ARGUMENT"}     # From "usage-message.sh example script.

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