Advanced Bash-shell Guide (Version 10) Learn note one

I like a word
The only-to really learn scripting are to write scripts
The only way to learn a script is to write a script

Better command-line parameter detection
e_wrongargs=85 # non-numerical argument (Bad argument format).
41 #
# case "$" in
# "") lines=50;;
# *[!0-9]*) echo "Usage: ' basename $ ' lines-to-cleanup";
No number in the middle of the argument, invalid argument
# exit $E _wrongargs;;
# *) lines=$1;;
# ESAC

More Efficient directory detection
3 # Cd/var/log | | {
# echo "Cannot change to necessary directory." >&2
# # exit $E _xcd;
66 #}

#!/bin/sh invokes the default shell interpreter, which
Defaults To/bin/bash on a Linux machine.
#!/bin/bash is called Sha-bang magic number.

Whether the number of parameters for the test call is correct
1 e_wrong_args=85
2 script_parameters= "-a-h-m-z"
3 #-A = all,-h = help, etc.
4
5 if [$#-ne $Number _of_expected_args]
6 Then
7 echo "Usage: ' basename ' $script _parameters"
8 # ' basename ' is the script's filename.
9 Exit $E _wrong_args
Ten fi

Invoke script
Bash ScriptName

chmod 555 ScriptName (gives everyone read/execute permission) [9]
Or
chmod +rx scriptname (gives everyone read/execute permission)
chmod U+rx ScriptName (gives only the script owner Read/execute permission)

./scriptname

1 #!/bin/rm
2 # self-deleting script.
echo "This line would never print (betcha!)."
This is a suicide script.
# Nothing much seems-happen when you run the This ...
Except that the file disappears.

Change the document to #!/bin/more and add executable permissions
The result is a self-listing document similar to Cat XX | More


Special characters
# comment, line start # comment, not executed, cancel syntax detection
Inside the quote and escape characters are not comments

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

; Command delimiter
1 echo Hello; echo there
;; Case Option Delimiter
1 case ' $variable ' in
2 ABC) echo "\ $variable = abc";;

. Equivalent to source refresh configuration file, reload

As part of the file name, hide the file

Equivalent to the current directory,.. Equivalent to the parent directory

Match a single character in a regular expression

"Partial reference or weak reference, suppress most of the special characters
' Strong references, suppress all special characters
, linking a sequence of arithmetic operations, returning only the final result
1 let "t2 = ((a = 9, 15/3))"
2 # Set "a = 9" and "t2 = 15/3"

Link string
1 for file in/{,usr/}bin/*calc
2 # ^ Find all executable files ending in "Calc"
3 #+ in/bin and/usr/bin directories.

\ escape character, meaning of expression character literal
/file path delimiter
' Output command result to variable
: Do nothing, placeholder
:> change the file length to 0 without changing the permissions, creating
: > Data.xxx # File "Data.xxx" now empty.
Same effect as Cat/dev/null >data.xxx
Also available as a domain delimiter in/etcpasswd

！ Transition exit status or sense of test
Change the sense of equal (=) to not-equal (! =)
Command history can also be invoked

* General distribution
Match 0 or more characters in a regular
In arithmetic, a single representation of multiplication sign two represents factorial

？ In the double brackets,? As a ternary operator
((var0 = var1<98?9:21))
Represents a single character in a wildcard and regular

$ variable
The value of the variable is represented by the variable name
Represents the end of a line in a regular
[email protected] $* position parameter
$? Variables for exit status
$$ indicates the process ID of the current script
() command group, the command in parentheses is a child shell, not visible to the outside
Array initialization
1 array= (element1 element2 element3)
{} command extension
8 CP File22. {Txt,backup}
9 # Copies "File22.txt" to "File22.backup"
----
echo {file1,file2}\: {\ A, "B", ' C '}
File1:a file1:b file1:c file2:a file2:b
File2:c
----
echo {A.. Z} # A b c d e F g h i j k l m n o p q R S t u v w x y Z
----
8 base64_charset= ({A). Z} {A.. Z} {0..9} +/=)
9 # Initializing An array, using extended brace expansion.
Represents a block of code
Visible to other parts of the script, non-child shell
1 a=123
2 {a=321;}
3 echo "A = $a" # a = 321 (value inside code block)
Also available as a placeholder
Ls. | Xargs-i-t CP./{} $

[] indicates the test
Array elements
1 array[1]=slot_1
2 echo ${array[1]}
Represents a range of characters
$ (()) integer expression
1 a=3
2 b=7
3
4 echo $ (($a + $b)) # 10
> &> >& >> < <> redirect Operators
< > as ASCII code comparison
\<,\> Word anchoring
| Pipeline
Passes the output (stdout) of a previous command to the input (stdin) of the next one
The pipe runs as a child shell, so you cannot modify the variables of the parent shell
>| Force redirection
|| Logical OR
& Background Run Job
&& Logic and
^ Line Head match

Back up files modified in the current directory in the last 24 hours
1 #!/bin/bash
2
3 # Backs up all files in current directory modified within last hours
4 #+ in a "tarball" (tarred and gzipped file).
5
6 backupfile=backup-$ (date +%m-%d-%y)
7 # embeds date in backup filename.
8 # Thanks, Joshua Tschida, for the idea.
9 archive=${1:-$BACKUPFILE}
# If no backup-archive filename specified on command-line,
#+ it would default to "backup-mm-dd-yyyy.tar.gz."
12
Tar cvf-' find. -mtime-1-type f-print ' > $archive. Tar
Gzip $archive. Tar
echo "Directory $PWD backed up in archive file \" $archive. tar.gz\ "."
16
17
# Stephane Chazelas points out that the above code would fail
#+ If there is too many files found
#+ or if any filenames contain blank characters.
21st
# He suggests the following alternatives:
#-------------------------------------------------------------------
# Find. -mtime-1-type f-print0 | Xargs-0 tar rvf "$archive. Tar"
Using the GNU version of "Find".
26
27
# Find. -mtime-1-type f-exec tar rvf "$archive. Tar" ' {} ' \;
# Portable to other UNIX flavors, but much slower.
#-------------------------------------------------------------------
31
32
Exit 0

This article is from the "Linux is belong to You" blog, make sure to keep this source http://jwh5566.blog.51cto.com/7394620/1640293

Advanced Bash-shell Guide (Version 10) Learn note one