Makes it easy to overlook some of the basics of shell

2010-07-29  来源:本站原创  分类:OS  人气:177 

http://blog.chinaunix.net/u2/86696/showart_2015126.html

Always felt that his own shell scripts can be written, ms is a bit narcissistic, and recently read some expert to write the script, they will have found a bit unusual difficulty. Can not help but could not help but ask yourself, will determine several conditions, several cycle is shell programming expert What? Ever once again came to CU shell version, there are indeed many very basic knowledge, are their own do not understand the need to sink to in-depth study center / review how these basic things:

When you're at the command prompt, type a command / command line after, shell work to be done is:
1. Parsing command line
2. Dealing with wildcard characters (wildcards), redirect (redirection), pipe (pipes) and job control (job control)
3. Search for and execute the command

User login, the user commands to interact with the computer relationship: the command process ---> Shell program ---> UNIX kernel ---> Computer Hardware

Several rare symbol:

cmd &> file # &> that the cmd's stdout and stderr are written to the file

cmd> file 2> & 1 # 2> & 1 basic Ibid, remember that 2> & 1 can not have spaces between

n <> file # open file for reading and writing, and the distribution of the file descriptor n to the document. If the file does not exist, will automatically create

= ~ # Regular expression matching, this must be bash3.0 later version, and must be used in the double in brackets [[]], if [["$ IP" = ~ "^ [0-9] + \. [0-9] + \. [0-9] + \. [0-9] + $ "]] & & echo" ok "| | echo" error "

================================== Pipelines and sub-shell =========== ========================

On the bash, in a pipe brace code segment (block) is run in a sub-shell.
# Ls | (read firstline; read secondline;)
# Echo "First line is $ firstline; second line is $ secondline" # here to see is a blank

Pipe on both sides of the code "block", not just including the "braces" enclosed blocks, but also while, for other loop in the "block", which is why it has frequently been asked why the assignment in the while loop variables, see the entry into force of the loop outside Quemei not cycle itself, but because both sides of this cycle in the pipeline Kazakhstan

As a child process will run the pipeline, not to change the script variables.
1 variable = "initial_value"
2 echo "new_value" | read variable
3 echo "variable = $ variable" # variable = initial_value

Here can read variable <<<"new_value" to replace

LANG = C &, or export LANG = C & it does not affect the current shell, powerless to change the LANG environment variable to a command if the pipeline had an exception and fail in the middle, then the pipeline will be prematurely terminated. This kind of behavior is called the broken pipe, and this state will send a SIGPIPE signal.

bash produced in several sub-shell case

1. Cmd | (cmd_list1; ... cmd_listn;)

(Cmdlist;) | (cmd list;)

Note that these two formats, the process of the relationship between the different, try the next (sleep 10;) | (sleep 20;) and sleep 10 | (sleep 20;) see next process, differences in the relationship between

2. Function func () ()

func &

3. () &

4. (Func &) & This will create two sub-shell

5. (Cmd1; cmd2) # cmd here must be greater than one (ie cmd group) will be subshell

6. Su user

================================== Pipelines and sub-shell =========== ========================

For the location parameter, is greater than or equal to 10 positional parameters must be enclosed with braces, such as echo $ (10). To access the command line, the last parameter, you can ${!#}, or i = $ #, $ (! I), $ (! XX) said indirect reference

====================================== ========== Block ==============================

Use curly braces () implementation of a series of commands, and if we do not wrap, I remember the back of each kicked must add a semicolon; including the last command, otherwise you will get an error such as:

$ (Echo "xyz") # bash will think of you entered is not over

$ Vi test.sh

(Echo "xyz")

$. / Test.sh

. / Test.sh: line 6: syntax error: unexpected end of file. If the commands are separated by a newline, the last exchange between the command line and), you do not need a semicolon

====================================== ========== Block ==============================

============================== Here document ================== ======================

1. Note: the end of the limit string, is the last line here document limit string, a character must begin in the first place. It can not have any front in front of the blank. And in this limit string behind the blank will cause abnormal problem. blank will prevent limit string recognition.

For example, sometimes you to align, nice layout of some:

function test ()
(
cat <<EOF
hello, world
EOF

return 0
)

test

exit 0

When run this script, will get an error: "syntax error: unexpected end of file", there must be EOF on the first line to avoid this error

2. - Option to mark a here document limit string (<<-LimitString), can inhibit the output tab front
(Not spaces). This can increase the readability of a script.

3. Here document supports parameter and command substitution. So here document can also be a message to the body pass different parameters,
This will modify the corresponding output. When the "limit string" is quoted or escaped then disable parameter replacement, such as

cat <<'EOF'

cat <<"EOF"

cat <<\ EOF all have the same effect, this time inside the here document a reference to the corresponding variable substitution and so will not do the replacement, but remember that the end of the EOF does not need to identify with the symbols like these escape

4. Comment blocks of code, each line with # to be more than easy to use more

: <<CommentHere

echo "hello, world"

# Echo "hello"

CommentHere

============================== Here document ================== ======================

In the command line, the exclamation mark "!" In double quotes in the command will error (Translator's Note: For example: echo "hello!"). Because the exclamation mark is interpreted into a historical order. However, in a script file , so writing is correct, because in the script file in Bash history mechanism is disabled.

bash some of the new features:

[["Abc"> "aba"]] & & echo true # [[]] values can be compared not only the size, you can also compare the size of the string

++++++++++ String handling

We can also variable in the string for replacement:

$ (File / dir / path): will replace the first dir path: / path1/dir2/dir3/my.file.txt

$ (File / / dir / path): to replace all the dir path: / path1/path2/path3/my.file.txt

$ (Kk / / [! 0-9] /)

====================== ======================== Bash in the colon ===========

Colon:
Space Command, is the bash built-in command, to do nothing in order, quite and true, return 0
General Usage:
1. Endless loop while: do oper_lists; done
2. In the if / then provide placeholder
if condition
then: # trigger a branch
else
oper_lists
fi
3.: $ (Variable = "xxx") # in a binary command to provide a placeholder
: $ (Variable: = "xxx") # $ (xxx: =) of the different states of the variable assignment, the general wants: combination

4. Combined with comments bash here document block
: <<EOF
echo "hello, world"
......
EOF

The somewhat similar to the c / cpp compilation in the use of pre # if 0 .... # endif to comment a large block of code

====================== ======================== Bash in the colon ===========

ps Why bash the time to see one before - what?
-Bash
This indicated that this is a login shell
If only bash, that a non-login shell, try to use su xxx, and then click ps

=========================== ====================== Function =====

1. In the function, declare the variable, is the local if not specified, and the function can also be used in vitro

2. In the function call another function, called function can be defined after the function call / declaration (bash function declaration is not the concept). For example:

function a ()

(

echo "i'm a"

b

)

function b ()

(

echo "i'm b"

)

3. But if you have a public library, you must call the public library, public library to perform the function, such as:

.. / Functions

func_in_functions

Can not

func_in_functions

.. / Functions

=========================== ====================== Function =====

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