5.1.4.1. String comparison

Table 5-1. String comparison operators

Operator	True if...
str1 = str2[4]	str1 matches str2
str1 != str2	str1 does not match str2
str1 < str2	str1 is less than str2
str1 > str2	str1 is greater than str2
-n str1	str1 is not null (has length greater than 0)
-z str1	str1 is null (has length 0)

5.1.4.2. File attribute checking

Table 5-2. File attribute operators

Operator	True if...
-a file	file exists
-d file	file exists and is a directory
-e file	file exists; same as - a
-f file	file exists and is a regular file (i.e., not a directory or other special type of file)
-r file	You have read permission on file
-s file	file exists and is not empty
-w file	You have write permission on file
-x file	You have execute permission on file, or directory search permission if it is a directory
-N file	file was modified since it was last read
-O file	You own file
-G file	file 's group ID matches yours (or one of yours, if you are in multiple groups)
file1 -nt file2	file1 is newer than file2 [6]
file1 -ot file2	file1 is older than file2

^[6] Specifically, the -nt and -ot operators compare modification times of two files.

$* and $@

"$*" is a single string that consists of all of the positional parameters, separated by the first character in the value of the environment variable IFS (internal field separator), which is a space, TAB, and NEWLINE by default. On the other hand, "$@" is equal to "$1" "$2"... "$ N", where N is the number of positional parameters. That is, it's equal to N separate double-quoted strings, which are separated by spaces. If there are no positional parameters, "$@" expands to nothing. We'll explore the ramifications of this difference in a little while.

5.2. for

for x := 1 to 10 do
begin
    statements...
end

for name  [in list ]
do
    statements that can use  
    $name... 
done 

5.1.5. Integer Conditionals

Table 5-3. Arithmetic test operators

Test	Comparison
-lt	Less than
-le	Less than or equal
-eq	Equal
-ge	Greater than or equal
-gt	Greater than
-ne	Not equal

Task 4-2

Task 4-2

You are writing a graphics file conversion utility for use in creating a web page. You want to be able to take a PCX file and convert it to a JPEG file for use on the web page.^[7]

${...}, $(...)

${...}:  String Operator

• String Substitution
•  
• String Pattern Matching
• String Length

$(...):  Command Substitution

Here are some simple examples:

• The value of $(pwd) is the current directory (same as the environment variable $PWD).
• The value of $(ls $HOME) is the names of all files in your home directory.
• The value of $(ls $(pwd)) is the names of all files in the current directory.
• The value of $(< alice) is the contents of the file alice with any trailing newlines removed.^[10]

  ^[10] Not available in versions of bash prior to 2.02.

• To find out detailed information about a command if you don't know where its file resides, type ls -l $(type -path -all command-name). The -all option forces type to do a pathname look-up and -path causes it to ignore keywords, built-ins, etc.
• If you want to edit (with vi) every chapter of your book on bash that has the phrase "command substitution," assuming that your chapter files all begin with ch, you could type:

        vi $(grep -l 'command substitution' ch*)

  
  

• The -l option to grep prints only the names of files that contain matches.