Chapter 4. Special Characters

Special Characters Found In Scripts and Elsewhere

#

Comments. Lines beginning with a # (with the exception of #!) are comments.

1 # This line is a comment.

Comments may also occur at the end of a command.

1 echo "A comment will follow." # Comment here.

Comments may also follow whitespace at the beginning of a line.

1  # A tab precedes this comment.

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, an escaped # in an echo statement does not begin a comment. Likewise, a # appears in certain parameter substitution constructs and in numerical constant expressions. 1 echo "The # here does not begin a comment." 2 echo 'The # here does not begin a comment.' 3 echo The \# here does not begin a comment. 4 echo The # here begins a comment. 5  6 echo ${PATH#*:} # Parameter substitution, not a comment. 7 echo $(( 2#101011 )) # Base conversion, not a comment. 8  9 # 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.

1 echo hello; echo there

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

;;

Terminator in a case option. [Double semicolon]

1 case "$variable" in 2 abc) echo "$variable = abc" ;; 3 xyz) echo "$variable = xyz" ;; 4 esac

.

"dot" command. [period] Equivalent to source (see Example 11-14). This is a bash builtin.

In a different context, as part of a regular expression, a "dot" matches a single character.

In yet another context, a dot is the filename 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

"

partial quoting. [double quote] "STRING" preserves (from interpretation) most of the special characters within STRING. See also Chapter 6.

'

full quoting. [single quote] 'STRING' preserves all special characters within STRING. This is a stronger form of quoting than using ". See also Chapter 6.

,

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

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

\

escape. [backslash] \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 6 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. [backticks] `command` makes available the output of command for setting a variable. This is also known as backticks or backquotes.

:

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 a Bash builtin, and its exit status is "true" (0).

1 : 2 echo $? # 0

Endless loop:

1 while : 2 do 3  operation-1 4  operation-2 5  ... 6  operation-n 7 done 8  9 # Same as: 10 # while true 11 # do 12 # ... 13 # done

Placeholder in if/then test:

1 if condition 2 then : # Do nothing and branch ahead 3 else 4  take-some-action 5 fi

Provide a placeholder where a binary operation is expected, see Example 8-1 and default parameters.

1 : ${username=`whoami`} 2 # ${username=`whoami`} without the leading : gives an error 3 # unless "username" is a command or builtin...

Provide a placeholder where a command is expected in a here document. See Example 17-8.

Evaluate string of variables using parameter substitution (as in Example 9-9).

1 : ${HOSTNAME?} ${USER?} ${MAIL?} 2 #Prints error message if one or more of essential environmental variables not set.

Variable expansion / substring replacement.

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

1 : > data.xxx # File "data.xxx" now empty. 2  3 # Same effect as cat /dev/null >data.xxx 4 # However, this does not fork a new process, since ":" is a builtin.

See also Example 12-10.

In combination with the >> redirection operator, updates a file access/modification time (: >> new_file). If the file did not previously exist, creates it. This is equivalent to touch.

This applies to regular files, not pipes, symlinks, and certain special files.

May be used to begin a comment line, although this is not recommended. Using # for a comment turns off error checking for the remainder of that line, so almost anything may be appear in a comment. However, this is not the case with :.

1 : This is a comment that generates an error, ( if [ $x -eq 3] ).

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

bash$ 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. The ! operator inverts the exit status of the command to which it is applied (see Example 3-2). It also inverts the meaning of a test operator. This can, for example, change the sense of "equal" ( = ) to "not-equal" ( != ). The ! operator is a Bash keyword.

In a different context, the ! also appears in indirect variable references.

*

wild card. [asterisk] The * character serves as a "wild card" for filename expansion in globbing, as well as representing any number (or zero) characters in a regular expression.

A double asterisk, **, is the exponentiation operator.

?

wild card (single character). [question mark] The ? character serves as a single-character "wild card" for filename expansion in globbing, as well as representing one character in an extended regular expression.

Within a double parentheses construct, the ? serves as a C-style trinary operator. See Example 9-21.

$

Variable substitution.

1 var1=5 2 var2=23skidoo 3  4 echo $var1 # 5 5 echo $var2 # 23skidoo

In a regular expression, a $ matches the end of a line.

${}

Parameter substitution.

$*, $@

positional parameters.

()

command group.

1 (a=hello; echo $a)

A listing of commands within parentheses starts a subshell. Variables inside parentheses, within the subshell, are not visible to the rest of the script. The parent process, the script, cannot read variables created in the child process, the subshell. 1 a=123 2 ( a=321; ) 3  4 echo "a = $a" # a = 123 5 # "a" within parentheses acts like a local variable.

array initialization.

1 Array=(element1 element2 element3)

{xxx,yyy,zzz,...}

Brace expansion.

1 grep Linux file*.{txt,htm*} 2 # Finds all instances of the work "Linux" 3 # in the files "fileA.txt", "file2.txt", "fileR.html", "file-87.htm", etc.

A command may act upon a comma-separated list of file specs within braces. [1] Filename expansion (globbing) applies to the file specs between the braces.

No spaces allowed within the braces unless the spaces are quoted or escaped. echo {file1,file2}\ :{\ A," B",' C'} file1 : A file1 : B file1 : C file2 : A file2 : B file2 : C

{}

Block of code. [curly brackets] Also referred to as an "inline group", this construct, in effect, creates an anonymous function. However, unlike a function, the variables in a code block remain visible to the remainder of the script.

bash$ { local a; a=123; } bash: local: can only be used in a function

1 a=123 2 { a=321; } 3 echo "a = $a" # a = 321 (value inside code block) 4  5 # Thanks, S.C.

The code block enclosed in braces may have I/O redirected to and from it.

---

Example 4-1. Code blocks and I/O redirection

1 #!/bin/bash 2 # Reading lines in /etc/fstab. 3  4 File=/etc/fstab 5  6 { 7 read line1 8 read line2 9 } < $File 10  11 echo "First line in $File is:" 12 echo "$line1" 13 echo 14 echo "Second line in $File is:" 15 echo "$line2" 16  17 exit 0

---

---

Example 4-2. Saving the results of a code block to a file

1 #!/bin/bash 2 # rpm-check.sh 3  4 # Queries an rpm file for description, listing, and whether it can be installed. 5 # Saves output to a file. 6 # 7 # This script illustrates using a code block. 8  9 SUCCESS=0 10 E_NOARGS=65 11  12 if [ -z "$1" ] 13 then 14  echo "Usage: `basename $0` rpm-file" 15  exit $E_NOARGS 16 fi 17  18 { 19  echo 20  echo "Archive Description:" 21  rpm -qpi $1 # Query description. 22  echo 23  echo "Archive Listing:" 24  rpm -qpl $1 # Query listing. 25  echo 26  rpm -i --test $1 # Query whether rpm file can be installed. 27  if [ "$?" -eq $SUCCESS ] 28  then 29  echo "$1 can be installed." 30  else 31  echo "$1 cannot be installed." 32  fi 33  echo 34 } > "$1.test" # Redirects output of everything in block to file. 35  36 echo "Results of rpm test in file $1.test" 37  38 # See rpm man page for explanation of options. 39  40 exit 0

---

Unlike a command group within (parentheses), as above, a code block enclosed by {braces} will not normally launch a subshell. [2]

{} \;

pathname. Mostly used in find constructs. This is not a shell builtin.

The ";" ends the -exec option of a find command sequence. It needs to be escaped to protect it from interpretation by the shell.

[ ]

test.

Test expression between [ ]. Note that [ is part of the shell builtin test (and a synonym for it), not a link to the external command /usr/bin/test.

[[ ]]

test.

Test expression between [[ ]] (shell keyword).

See the discussion on the [[ ... ]] construct.

(( ))

integer expansion.

Expand and evaluate integer expression between (( )).

See the discussion on the (( ... )) construct.

> >& >> <

redirection.

scriptname >filename redirects the output of scriptname to file filename. Overwrite filename if it already exists.

command >&2 redirects output of command to stderr.

scriptname >>filename appends the output of scriptname to file filename. If filename does not already exist, it will be created.

process substitution.

(command)>

<(command)

In a different context, the "<" and ">" characters act as string comparison operators.

In yet another context, the "<" and ">" characters act as integer comparison operators. See also Example 12-6.

<<

redirection used in a here document.

|

pipe. Passes the output of previous command to the input of the next one, or to the shell. This is a method of chaining commands together.

1 echo ls -l | sh 2 # Passes the output of "echo ls -l" to the shell, 3 #+ with the same result as a simple "ls -l". 4  5  6 cat *.lst | sort | uniq 7 # Merges and sorts all ".lst" files, then deletes duplicate lines.

A pipe, as a classic method of interprocess communication, sends the stdout of one process to the stdin of another. In a typical case, a command, such as cat or echo, pipes a stream of data to a filter for processing. cat $filename | grep $search_word

The output of a command or commands may be piped to a script.

1 #!/bin/bash 2 # uppercase.sh : Changes input to uppercase. 3  4 tr 'a-z' 'A-Z' 5 # Letter ranges must be quoted 6 #+ to prevent filename generation from single-letter filenames. 7  8 exit 0

Now, let us pipe the output of ls -l to this script.

bash$ ls -l | ./uppercase.sh -RW-RW-R-- 1 BOZO BOZO 109 APR 7 19:49 1.TXT -RW-RW-R-- 1 BOZO BOZO 109 APR 14 16:48 2.TXT -RW-R--R-- 1 BOZO BOZO 725 APR 20 20:56 DATA-FILE

The stdout of each process in a pipe must be read as the stdin of the next. If this is not the case, the data stream will block, and the pipe will not behave as expected. 1 cat file1 file2 | ls -l | sort 2 # The output from "cat file1 file2" disappears. A pipe runs as a child process, and therefore cannot alter script variables. 1 variable="initial_value" 2 echo "new_value" | read variable 3 echo "variable = $variable" # variable = initial_value If one of the commands in the pipe aborts, this prematurely terminates execution of the pipe. Called a broken pipe, this condition sends a SIGPIPE signal.

>|

force redirection (even if the noclobber option is set). This will forcibly overwrite an existing file.

&

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

bash$ sleep 10 & [1] 850 [1]+ Done sleep 10

A command run in the background within a script may cause the script to hang, waiting for a keystroke. Fortunately, there is a remedy for this.

-

redirection from/to stdin or stdout. [dash]

1 (cd /source/directory && tar cf - . ) | (cd /dest/directory && tar xpvf -) 2 # Move entire file tree from one directory to another 3 # [courtesy Alan Cox <a.cox@swansea.ac.uk>, with a minor change] 4  5 # 1) cd /source/directory Source directory, where the files to be moved are. 6 # 2) && "And-list": if the 'cd' operation successful, then execute the next command. 7 # 3) tar cf - . The 'c' option 'tar' archiving command creates a new archive, 8 # the 'f' (file) option, followed by '-' designates the target file as stdout, 9 # and do it in current directory tree ('.'). 10 # 4) | Piped to... 11 # 5) ( ... ) a subshell 12 # 6) cd /dest/directory Change to the destination directory. 13 # 7) && "And-list", as above 14 # 8) tar xpvf - Unarchive ('x'), preserve ownership and file permissions ('p'), 15 # and send verbose messages to stdout ('v'), 16 # reading data from stdin ('f' followed by '-'). 17 # 18 # Note that 'x' is a command, and 'p', 'v', 'f' are options. 19 # Whew! 20  21  22  23 # More elegant than, but equivalent to: 24 # cd source-directory 25 # tar cf - . | (cd ../target-directory; tar xzf -) 26 # 27 # cp -a /source/directory /dest also has same effect.

1 bunzip2 linux-2.4.3.tar.bz2 | tar xvf - 2 # --uncompress tar file-- | --then pass it to "tar"-- 3 # If "tar" has not been patched to handle "bunzip2", 4 # this needs to be done in two discrete steps, using a pipe. 5 # The purpose of the exercise is to unarchive "bzipped" kernel source.

Note that in this context the "-" is not itself a Bash operator, but rather an option recognized by certain UNIX utilities that write to stdout, such as tar, cat, etc.

Where a filename is expected, - redirects output to stdout (sometimes seen with tar cf), or accepts input from stdin, rather than from a file. This is a method of using a file-oriented utility as a filter in a pipe.

bash$ file Usage: file [-bciknvzL] [-f namefile] [-m magicfiles] file...

By itself on the command line, file fails with an error message.

bash$ file - #!/bin/bash standard input: Bourne-Again shell script text executable

This time, it accepts input from stdin and filters it.

The - can be used to pipe stdout to other commands. This permits such stunts as prepending lines to a file.

Using diff to compare a file with a section of another:

grep bash file1 | diff file2 -

Finally, a real-world example using - with tar.

---

Example 4-3. Backup of all files changed in last day

1 #!/bin/bash 2  3 # Backs up all files in current directory modified within last 24 hours 4 # in a "tarball" (tarred and gzipped file). 5  6 NOARGS=0 7 E_BADARGS=65 8  9 if [ $# = $NOARGS ] 10 then 11  echo "Usage: `basename $0` filename" 12  exit $E_BADARGS 13 fi 14  15 tar cvf - `find . -mtime -1 -type f -print` > $1.tar 16 gzip $1.tar 17  18  19 # Stephane Chazelas points out that the above code will fail 20 # if there are too many files found 21 # or if any filenames contain blank characters. 22  23 # He suggests the following alternatives: 24 # ------------------------------------------------------------- 25 # find . -mtime -1 -type f -print0 | xargs -0 tar rvf "$1.tar" 26 # using the GNU version of "find". 27  28 # find . -mtime -1 -type f -exec tar rvf "$1.tar" '{}' \; 29 # portable to other UNIX flavors, but much slower. 30  31  32 exit 0

---

Filenames beginning with - may cause problems when coupled with the - redirection operator. A script should check for this and pass such filenames as ./-FILENAME or $PWD/-FILENAME. If the value of a variable begins with a -, this may likewise create problems. 1 var="-n" 2 echo $var 3 # Has the effect of "echo -n", and outputs nothing.

-

previous working directory. [dash] cd - changes to previous working directory. This uses the $OLDPWD environmental variable.

This is not to be confused with the "-" redirection operator just discussed. The interpretation of the "-" depends on the context in which it appears.

-

Minus. Minus sign in an arithmetic operation.

=

Equals. Assignment operator

1 a=28 2 echo $a # 28

In a different context, the "=" is a string comparison operator.

+

Plus. Addition arithmetic operator.

In a different context, the + is a Regular Expression operator.

%

modulo. Modulo (remainder of a division) arithmetic operation.

In a different context, the % is a pattern matching operator.

~

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

bash$ echo ~bozo /home/bozo bash$ echo ~ /home/bozo bash$ echo ~/ /home/bozo/ bash$ echo ~: /home/bozo: bash$ echo ~nonexistent-user ~nonexistent-user

~+

current working directory. This corresponds to the $PWD internal variable.

~-

previous working directory. This corresponds to the $OLDPWD internal variable.

Control Characters

change the behavior of the terminal or text display. A control character is a CONTROL + key combination.

• Ctl-C

  Terminate a foreground job.

• Ctl-D

  Log out from a shell (similar to exit).

  "EOF" (end of file). This also terminates input from stdin.

• Ctl-G

  "BEL" (beep).

• Ctl-H

  Backspace.

• Ctl-J

  Carriage return.

• Ctl-L

  Formfeed (clear the terminal screen). This has the same effect as the clear command.

• Ctl-M

  Newline.

• Ctl-U

  Erase a line of input.

• Ctl-Z

  Pause a foreground job.

Whitespace

functions as a separator, separating commands or variables. Whitespace consists of either spaces, tabs, blank lines, or any combination thereof. In some contexts, such as variable assignment, whitespace is not permitted, and results in a syntax error.

Blank lines have no effect on the action of a script, and are therefore useful for visually separating functional sections.

$IFS, the special variable separating fields of input to certain commands, defaults to whitespace.