Shell script notes (5) - conditional statements of this

Conditional statement of script

Format of conditional test statement

Format
test < Conditions >
[ < Conditions > ]	In general
[[ < Conditions > ]]	[] Upgraded version , Support Regular expressions
(( < Conditions > ))

If you're not familiar with it, just man test see

File test statement

man test
There are probably the following operations for files

       FILE1 -ef FILE2
              FILE1 and FILE2 have the same device and inode numbers

       FILE1 -nt FILE2
              FILE1 is newer (modification date) than FILE2

       FILE1 -ot FILE2
              FILE1 is older than FILE2

       -b FILE
              FILE exists and is block special

       -c FILE
              FILE exists and is character special

       -d FILE
              FILE exists and is a directory

       -e FILE
              FILE exists

       -f FILE
              FILE exists and is a regular file

       -g FILE
              FILE exists and is set-group-ID

       -G FILE
              FILE exists and is owned by the effective group ID

       -h FILE
              FILE exists and is a symbolic link (same as -L)

       -k FILE
              FILE exists and has its sticky bit set

       -L FILE
              FILE exists and is a symbolic link (same as -h)

       -O FILE
              FILE exists and is owned by the effective user ID

       -p FILE
              FILE exists and is a named pipe

       -r FILE
              FILE exists and read permission is granted

       -s FILE
              FILE exists and has a size greater than zero

       -S FILE
              FILE exists and is a socket

       -t FD  file descriptor FD is opened on a terminal

       -u FILE
              FILE exists and its set-user-ID bit is set

       -w FILE
              FILE exists and write permission is granted

       -x FILE
              FILE exists and execute (or search) permission is granted

#  Test file permissions 
wxl@ubuntu:~/workspace/shellScripts$ [[ -w test222.sh ]] && echo ok || echo error
error
wxl@ubuntu:~/workspace/shellScripts$ chmod 444 test222.sh
wxl@ubuntu:~/workspace/shellScripts$ [[ -w test222.sh ]] && echo ok || echo error
error
wxl@ubuntu:~/workspace/shellScripts$ ls -al test222.sh
-r--r--r-- 1 wxl wxl 0 Apr  4 05:30 test222.sh
wxl@ubuntu:~/workspace/shellScripts$ [[ -r test222.sh ]] && echo ok || echo error
ok

Common formats

#  Execute a command when conditions are met 
[  Conditions  ] &&  command 

#  Execute multiple commands if conditions are met 
[  Conditions  ] && {
  cmd1
  cmd2
  ...
}

#  If the conditions are not met, execute another command 
[  Conditions  ] || cmd
[  Conditions  ] || {
  cmd1
  cmd2
  ...
}

Case study

Advanced version of computer

#! /bin/bash
[ $# -eq 2 ] || {
echo "Usage: Please input 2 numbers!!!"
exit 1
}

x=$1
y=$2

#  Judge whether the current consecutive number is a number 
expr "$x" + "$y" + 666 &>/dev/null
[ $? -eq 0 ] || {
echo "Usage: $0 num1 num2"
exit 2
}

echo "result: "
awk -va=$x -vb=$y 'BEGIN{print a+b,a-b,a*b,a/b}'

wxl@ubuntu:~/workspace/shellScripts$ sh cal.sh 10 10
result:
20 0 100 1
wxl@ubuntu:~/workspace/shellScripts$ sh cal.sh
Usage: Please input 2 numbers!!!
wxl@ubuntu:~/workspace/shellScripts$ sh cal.sh 10 a
Usage: cal.sh num1 num2

String manipulation You need to put double quotes

expression	meaning
-n	Judgment string is not empty , return true
-z	Judge that the string is empty , return true
“str1” = “str2”	Two strings are equal , return true
“str1” != “str2”	Two strings are not equal , return true

Code example ：

# oldboy It's empty 
wxl@ubuntu:~/workspace/shellScripts$ [ -n "$oldboy" ] && echo ok || echo fail
fail
# oldboy assignment 
wxl@ubuntu:~/workspace/shellScripts$ oldboy=aaa
wxl@ubuntu:~/workspace/shellScripts$ [ -n "$oldboy" ] && echo ok || echo fail
ok
wxl@ubuntu:~/workspace/shellScripts$ [ -z "$oldboy" ] && echo ok || echo fail
fail
wxl@ubuntu:~/workspace/shellScripts$ [ -z "$oldboy11" ] && echo ok || echo fail
ok

#  Determines whether the strings are equal 
wxl@ubuntu:~/workspace/shellScripts$ name=zzz
wxl@ubuntu:~/workspace/shellScripts$
wxl@ubuntu:~/workspace/shellScripts$ [ "$name" = "zzz" ] && echo ok || echo fail
ok
wxl@ubuntu:~/workspace/shellScripts$ [ "$name" = "zz" ] && echo ok || echo fail
fail
wxl@ubuntu:~/workspace/shellScripts$ echo $nae

wxl@ubuntu:~/workspace/shellScripts$ echo $name
zzz
wxl@ubuntu:~/workspace/shellScripts$ [ "$name" != "zz" ] && echo ok || echo fail
ok

Be careful

1. When executing the script, add -x Parameter indicates the execution process of the display script , When there are errors in the script, you can use to debug
2. stay shell There are two ways to get the value of variables in programming $orperson${}
3. ${#aa} Statistical variables aa The number of characters in $# Count the number of script parameters
4. $$ The script pid
5. | Said the pipe
6. $! Of the last script pid
7. #!/bin/bash Specify command interpreter

Comprehensive examples

8. Remove comments and empty lines from the file

#  Use three methods to solve 
1. egrep
wxl@ubuntu:~/workspace/shellScripts$ egrep -v '#|^$' sshd_config
ChallengeResponseAuthentication no
UsePAM yes
X11Forwarding yes
PrintMotd no
AcceptEnv LANG LC_*
Subsystem       sftp    /usr/lib/openssh/sftp-server

#  among ,^$ The corresponding regular expression ,^ Indicates the beginning of a line , such as ^name  Already expressed name Line beginning with ;$ End of expression ,name$ Said to name The line at the end ;|  Represents or 

#  Find the lines beginning with uppercase and lowercase letters in the file 
wxl@ubuntu:~/workspace/shellScripts$ egrep '^[a-Z]' sshd_config
ChallengeResponseAuthentication no
UsePAM yes
X11Forwarding yes
PrintMotd no
AcceptEnv LANG LC_*
Subsystem       sftp    /usr/lib/openssh/sftp-server


2. sed

wxl@ubuntu:~/workspace/shellScripts$ sed -r '/#|^$/d' sshd_config
ChallengeResponseAuthentication no
UsePAM yes
X11Forwarding yes
PrintMotd no
AcceptEnv LANG LC_*
Subsystem       sftp    /usr/lib/openssh/sftp-server

# -r  Indicates support for extended regular ,| Can express or 
# //d  Said to delete 

3. awk
wxl@ubuntu:~/workspace/shellScripts$ awk '!/#|^$/' sshd_config
ChallengeResponseAuthentication no
UsePAM yes
X11Forwarding yes
PrintMotd no
AcceptEnv LANG LC_*
Subsystem       sftp    /usr/lib/openssh/sftp-server

9. obtain “aaa:444:444:/home/user” Medium home route

wxl@ubuntu:~/workspace/shellScripts$ variable="aaa:444:444:/home/user"
wxl@ubuntu:~/workspace/shellScripts$ echo ${variable##*:}  Use ## Delete ： Previous string 
/home/user
wxl@ubuntu:~/workspace/shellScripts$ echo ${variable/*:/}  Use // Replace ： Previous string 
/home/user

10. Suffix the file html Batch change to bak

wxl@ubuntu:~/workspace/shellScripts$ for name in `ls *.html`;do echo $name ${name/.html/.bak} ;done
1.html 1.bak
2.html 2.bak
3.html 3.bak
index.html index.bak

11. What are the common command interpreters ？

1. bash redhat、centos、ubuntu new edition 
2. dash ubuntu Old version 
2. csh tcsh unix System 

12. Command to get the name of the host hostname,ip,release edition , Kernel version

wxl@ubuntu:~/workspace/shellScripts$ hostname
ubuntu

wxl@ubuntu:~/workspace/shellScripts$ hostname -I|awk '{print $1}'
192.168.1.110

wxl@ubuntu:~/workspace/shellScripts$ cat /etc/os-release
NAME="Ubuntu"
VERSION="18.04.6 LTS (Bionic Beaver)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 18.04.6 LTS"
VERSION_ID="18.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=bionic
UBUNTU_CODENAME=bionic

wxl@ubuntu:~/workspace/shellScripts$ uname -r
5.4.0-84-generic

13. Command to get the current date , Time , working directory , user name

wxl@ubuntu:~/workspace/shellScripts$ date +%F
2022-04-12
wxl@ubuntu:~/workspace/shellScripts$ date +%T
07:37:48
wxl@ubuntu:~/workspace/shellScripts$ whoami
wxl
wxl@ubuntu:~/workspace/shellScripts$ pwd
/home/wxl/workspace/shellScripts

14. Convert lowercase letters in the file to uppercase

wxl@ubuntu:~/workspace/shellScripts$ tr 'a-z' 'A-Z' <test18.cpp
ABCD
DFGH
IJKL
MNOP
QRST
wxl@ubuntu:~/workspace/shellScripts$ cat test18.cpp
abcd
dfgh
ijkl
mnop
qrst
wxl@ubuntu:~/workspace/shellScripts$ awk '{print toupper($0)}' test18.cpp
ABCD
DFGH
IJKL
MNOP
QRST
wxl@ubuntu:~/workspace/shellScripts$ awk '{print tolower($0)}' test18.cpp
abcd
dfgh
ijkl
mnop
qrst

15. Check whether the user is root

wxl@ubuntu:~/workspace/shellScripts$ cat checkName.sh
#!/bin/bash

[ $UID -eq 0 ]||{
  echo "need to be root"
  exit 1
}

[ $UID -eq 0 ]&&{
  echo "Now is root"
}

name=`whoami`
[ "$name" = "root" ]||{
  echo "need to be root"
  exit 1
}

[ "$name" = "root" ]&&{
  echo "Now is root"
}

16. To design a shell Program , Back up and compress on the first day of each month /workspace All the following , Store in /root/bak Directory , And the file name is in the following format yymmdd_bak, yy mm dd On behalf of

#!/bin/bash

dir=/home/wxl/workspace
time=`date +%y%m%d`

[ -d $dir ] || [ mkdir -p $dir ]

tar zcf ${dir}/${time}_bak.tar.gz /home/wxl/workspace/csdn_code

# back /home/wxl/workspace/csdn_code at first day of every month
#  branch   when   Japan   month   Zhou  00 00 01  The first day of each month 0 spot 0 Start the save operation in minutes 
00 00 01 * *  /bin/sh /home/wxl/workspace/shellScripts/backFile.sh &>/home/wxl/workspace/log

17. Count the users currently logged in to the computer

#  It's actually statistics /etc/passwd Below /bin/bash Number of command interpreters 
wxl@ubuntu:~$ grep -c "/bin/bash" /etc/passwd
2

18. Remove all spaces in the string

wxl@ubuntu:~$ str='I am aaa'
wxl@ubuntu:~$ echo ${str// /}
Iamaaa

#  Use tr
wxl@ubuntu:~$ echo ${str}|tr -d ' '
Iamaaa

#  Use sed
echo $str|sed 's# ##g'

19. Redirect standard output and standard error to log.txt

cmd &>/dev/log.txt
cmd >/dev/log.txt 2>&1

20. $val:-10and${val: -10} The difference between

${val:-10}  Said if val If it is undefined or empty , Set it to 10
${val: -10}  Represents the last... Of the display variable 10 Characters