shell script Learning Guide 2017-03-26 Turn on tracing

set -x

Turn off tracing

set +x

This abbreviation is also too bullish

i18n: internationalization
l10n: localization

Regular expression (absolutely something you don't need to forget) Backward Reference awk (again)

sed, stream editor

sed -n '/Character string/p' file.txt
sed -i '/Character string/d' file.txt

grep，cut，sort，uniq

grep -q: Quiet, not output to standard output -s: no error message is displayed

export: modify or print environment variables readonly: keep variables unmodified

unset: delete variable assignment

Expansion Operator

${varname:-word}:varname exists and is not null, returns its value, otherwise returns word
 ${varname:=world}:varname exists and returns its value if it is not null, otherwise it is set to word
 ${varname:?message}:varname exists and is not null, returns its value, otherwise displays varname:message and exits the command to catch errors caused by undefined variables
 ${varname:+word}:varname exists and returns word if it is not null, otherwise returns NULL to test the existence of a variable

pattern matching

${variable#Pattern}: at the beginning of the pattern matching variable value, the shortest part of the match is deleted and the rest is returned
${variable##Pattern}: at the beginning of the pattern matching variable value, delete the longest part of the match and return the rest

eg: 
[root@localhost shell]# ip0="/etc/sysconfig/network-scripts/ifcfg-eth0"
[root@localhost shell]# echo $ip0
/etc/sysconfig/network-scripts/ifcfg-eth0
[root@localhost shell]# echo ${ip0#/*/}
sysconfig/network-scripts/ifcfg-eth0
[root@localhost shell]# echo ${ip0##/*/}
ifcfg-eth0
${variable%pattern}: If the pattern matches at the end of the variable value, the shortest part of the match is deleted and the rest is returned
${variable%%pattern}: If the pattern matches at the end of the variable value, the longest part of the match is deleted and the rest is returned
eg: 
[root@localhost shell]# echo ${ip0%%/*}

[root@localhost shell]# echo ${ip0%/*}
/etc/sysconfig/network-scripts

Location parameters $, $@: All command line parameters $: Treat all command line parameters as separate strings $@: Treat all command line parameters as separate individuals, separate strings shift command to truncate position parameters from list

eg: 
[root@localhost shell]# vim 1.sh
#!/bin/bash
#
echo "$*"
shift    #Equivalent to shift 1
echo "$*"
shift
echo "$*"
shift
echo "$*"
shift
echo "$*"
[root@localhost shell]# bash 1.sh 1 2 3 4 5 6
1 2 3 4 5 6
2 3 4 5 6
3 4 5 6
4 5 6
5 6

Special variables

#: Number of parameters
@: Command Line Parameters
*: Command Line Parameters
-: Given in Reference shell Options for
?: Exit status
$: shell Process Number
0: shell Program Name
!: The process number of the last background command, which stores the process number by wait Command for later use
ENV: 
HOME: 
IFS: Internal Field Separator
LANG: current locale Default name of, other LC_*Variables will override
LC_ALL: current locale The name of the LANG And others LC_*variable
LC_COLLATE: Current used to sort characters locale Name
LC_CTYPE: Current used to determine character categories during pattern matching locale Name of
LC_MESSAGES: Name of the current language for output information
LINEND: Line number of a line just passed in a script or function
NLSPATH: stay $LC_MESSAGES(XSI)Location of information directory in given information language
PATH: 
PPID: Process number of parent process
PS1: Command prompt string, default is $
PS2: Prompt string for line continuation, default is>
PS4: set -x Set the prompt string to execute the trace, default is+
PWD: 

Exit status code (exit num (can be specified manually)

0: Exit successfully
 > 0: Failed during redirection or word expansion
 1-125: Command unsuccessfully exited
 126: Command found, file could not be executed
 127: Command not found
 > 128: Command dies from receiving a signal

if statement

if
then
elif
then
else

test command

test "$?" -eq 0
 Be equal to
[ "$?" -eq 0 ]

case statement

case $1 in
)
    ;;
)
    ;;
)
    ;;
*)
    ;;
esac

for statement

for in
do
done

the while statement

while
do
done

until statement

until
do
done

break statement Jump out of the current loop, break

continue Statement The continue command is used to start the next repeating loop earlier

function

function name ()
{
}

expr
expr 10 + 20
30

Progress (not carefully watched) Capture, interrupt...