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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