Linux 系统介绍 (III) 袁华

Transcription

1 Linux 系统介绍 (III) 袁华

2 Command Substitution The backquote ` is different from the single quote. It is used for command substitution: `command` $ LIST=`ls` $ echo $LIST We can perform the command substitution by means of $(command) $ LIST=$(ls) $ echo $LIST $ rm $( find / -name *.tmp ) $ cat > backup.sh #!/bin/bash BCKUP=/home/userid/backup-$(date +%d-%m-%y).tar.gz tar -czf $BCKUP $HOME 3/13/2017 大气科学软件基础与应用 -04 2

3 Conditional Statements Conditionals let us decide whether to perform an action or not, this decision is taken by evaluating an expression. The most basic form is: if [ expression ]; then statements elif [ expression ]; then statements else statements fi the elif (else if) and else sections are optional Put spaces after [ and before ], and around the operators and operands. 3/13/2017 大气科学软件基础与应用 -04 3

4 Expressions An expression can be: String comparison, Numeric comparison, File operators and Logical operators and it is represented by [expression]: String Comparisons: = compare if two strings are equal!= compare if two strings are not equal -n evaluate if string length is greater than zero -z evaluate if string length is equal to zero Examples: [ s1 = s2 ] (true if s1 same as s2, else false) [ s1!= s2 ] (true if s1 not same as s2, else false) [ s1 ] (true if s1 is not empty, else false) [ -n s1 ] (true if s1 has a length greater then 0, else false) [ -z s2 ] (true if s2 has a length of 0, otherwise false) 3/13/2017 大气科学软件基础与应用 -04 4

5 Expressions Number Comparisons: -eq compare if two numbers are equal -ge compare if one number is greater than or equal to a number -le compare if one number is less than or equal to a number -ne compare if two numbers are not equal -gt compare if one number is greater than another number -lt compare if one number is less than another number Examples: [ n1 -eq n2 ] (true if n1 same as n2, else false) [ n1 -ge n2 ] (true if n1 greater then or equal to n2, else false) [ n1 -le n2 ] (true if n1 less then or equal to n2, else false) [ n1 -ne n2 ] (true if n1 is not same as n2, else false) [ n1 -gt n2 ] (true if n1 greater then n2, else false) [ n1 -lt n2 ] (true if n1 less then n2, else false) 3/13/2017 大气科学软件基础与应用 -04 5

6 Expressions Examples $ cat number.sh #!/bin/bash echo -n Enter a number 1 < x < 10: " read num if [ $num -lt 10 ]; then if [ $num -gt 1 ]; then echo $num*$num=$(($num*$num)) else echo Wrong insertion! fi else echo Wrong insertion! fi 3/13/2017 大气科学软件基础与应用 -04 6

7 Expressions Files operators -d check if path given is a directory -f check if path given is a file -e check if file name exists -r check if read permission is set for file or directory -s check if a file has a length greater than 0 -w check if write permission is set for a file or directory -x check if execute permission is set for a file or directory Examples: [ -d fname ] (true if fname is a directory, otherwise false) [ -f fname ] (true if fname is a file, otherwise false) [ -e fname ] (true if fname exists, otherwise false) [ -s fname ] (true if fname length is greater then 0, else false) [ -r fname ] (true if fname has the read permission, else false) [ -w fname ] (true if fname has the write permission, else false) [ -x fname ] (true if fname has the execute permission, else false) 3/13/2017 大气科学软件基础与应用 -04 7

8 Expressions Logical operators! negate (NOT) a logical expression -a logically AND two logical expressions -o logically OR two logical expressions Example: #!/bin/bash echo -n Enter a number 1 < x < 10: read num if [ $num -gt 1 -a $num -lt 10 ]; then echo $num*$num=$(($num*$num)) else echo Wrong insertion! fi 3/13/2017 大气科学软件基础与应用 -04 8

9 Expressions Logical operators && logically AND two logical expressions logically OR two logical expressions Example: #!/bin/bash echo -n "Enter a number 1 < x < 10: " read num if [ $number -gt 1 ] && [ $number -lt 10 ]; then echo $num*$num=$(($num*$num)) else echo Wrong insertion! fi 3/13/2017 大气科学软件基础与应用 -04 9

10 Iteration Statements The for structure is used when you are looping through a range of variables. for var in list do statements done Example: #!/bin/bash let sum=0 for num in do let sum = $sum + $num done echo $sum 3/13/2017 大气科学软件基础与应用

11 Iteration Statements An alternative form of the for structure is: A C-like for loop for (( EXPR1 ; EXPR2 ; EXPR3 )) do statements done Example: #!/bin/bash echo n Enter a number: ; read x let sum=0 for (( i=1 ; $i<=$x ; i=$i+1 )) ; do let sum = $sum + $i done echo the sum of the first $x numbers is: $sum 3/13/2017 大气科学软件基础与应用

12 Debugging Bash provides two options which will give useful information for debugging -x : displays each line of the script with variable substitution and before execution -v : displays each line of the script as typed before execution Usage: #!/bin/bash v or #!/bin/bash x or #!/bin/bash xv $ /bin/bash -v/-x/-xv scriptfile 3/13/2017 大气科学软件基础与应用

13 Process Management top Display processes dynamically ps Report a snapshot of current processes jobs List active jobs bg Place a job in the background fg Place a job in the foreground kill Send a signal to a process killall Kill processes by name Ctrl + c Send a interrupt single to a process Ctrl + z Send a stop single to a process 3/13/2017 大气科学软件基础与应用

14 top: Process viewer, find the CPU-intensive programs currently running $ top Interactive commands: Command means Viewing Processes Space/Enter Update display immediately P M u Sort tasks by CPU usage (default) Sort tasks by resident memory usage View a specific user s processes (blank mean all users) 1 View all CPUs status f h q Add/remove/order/sort fields, and see the each field s mean Help Exit 3/13/2017 大气科学软件基础与应用

15 Viewing Processes ps: Process status, information about processes running in memory $ ps $ ps ux $ ps aux grep command Jobs: Print currently running jobs and their status $ jobs 3/13/2017 大气科学软件基础与应用

16 Controlling Processes Interrupting a process Ctrl + c Putting a process in the background $ command & $ bg %num Stopping (pausing) a process Ctrl + z Returning a process to the foreground $ fg %num 3/13/2017 大气科学软件基础与应用

17 Sending Signals To Processes With kill kill: Kill a process by specifying its PID $ kill -9 PID killall: kill processes by name $ Killall xclock Signal name Signal value Effect SIGHUP 1 Hangup SIGINT 2 Interrupt from keyboard SIGQUIT 3 Quit SIGABRT 6 Abort SIGKILL 9 Kill signal SIGTERM 15 Termination signal - allow an orderly shutdown SIGSTOP 17,19,23 Stop the process 3/13/2017 大气科学软件基础与应用

18 View and Set Environmental Variables env: Display environment variables $ env grep name set: Environment variables and local variables in current shell export/unset: Set/remove an environment variable $ export HISTSIZE=1000 $ Unset name alias: Create an alias, aliases allow a string to be substituted for a word when it is used as the first word of a simple command. $ alias $ alias rm= rm -i 3/13/2017 大气科学软件基础与应用

19 Environmental Variables Set by System /etc/profile /etc/profile.d/userdefined.sh ~/.bash_profile or ~/.bash_login or ~/.profile ~/.bashrc 3/13/2017 大气科学软件基础与应用

20 Differing Features Between Shells Differences between the standard shell (sh), Bourne Again SHell (bash), Korn shell (ksh) and the C shell (csh) 3/13/2017 大气科学软件基础与应用

21 Command History ~/.bash_history $ less ~/.bash_history $ export HISTSIZE=number Command line history $ history $ history grep command $!command_line_number 3/13/2017 大气科学软件基础与应用

22 What we have learned Move around in the Linux system Manipulate files and directories Find files and commands View files and some text processes A Shell Bash, and shell script writing Environmental and local variables Process management Get help Concluding Remarks What starts here is just a beginning. Many things are needing your Tab command to be completed. 按需所学, 由小及大 ; 经验是多次尝试才有的, 不然那只是经历 3/13/2017 大气科学软件基础与应用