Bash Shell usage notes

Bash Shell usage notes

The use of the shell is profound, I comb the basic knowledge, tidy up a simple usage, the specific requirements for self-study.

1.  calculation # echo $ ((1+2)) 3# echo $ ((1+2+3)) 6# echo 3+2+5 |bc102.  If usage if [  $i  -eq 0 ]; then    xxxelse     xxxfi integers compare:-eq  equals, such as:if [  "$a"  -eq  "$b"  ]   -ne  not equal, such as: if  [  "$a"  -ne  "$b"  ]   -gt  greater than, such as:if [  "$a"  -gt  " $b " ]   -ge  greater than equals, such as:if [ " $a " -ge " $b " ]   -lt   Less than, such as:if [  "$a"  -lt  "$b"  ]   -le  less than equals, such as:if [  "$a"  -le  "$b"  ]   <    less (requires double brackets), such as: (("$a"  <  "$b")    <=   is less than or equal (requires double brackets), such as: (("$a"  <=  "$b")    >     is greater than (requires double brackets), such as: (("$a"  >  "$b")    >=   greater than or equal (requires double brackets), such as: (("$a"  >=  "$b"))   stringComparison: This is usually done: if [ x "$test"  = x "Test"  ]=  equals, such as:if [  "$a"  =  "$b"  ]   -d  directory-e  existence-f  file-n  non-empty string-z  empty string  3. for usage array1= (" D.com " " e.com " " f.com ") len=${#array1 [@]}for  ((i=0;i< $len; i++)) Do    echo  ${array1[$i]}donefor a in $ (seq 1 100);d o    echo  $a The usage of     sleep 1sdone4. case case $1 in    start|stop |reload)         $1        ;;     *)         echo  "usage: $0 [ Start|stop|reload] "        ;; Usage of esac5. while while truedo    echo  "abc"     read  -p  "Please enter: "  abc    ddd= "$abC "    if [ ${#ddd} -ne 5 ]; then         echo  "Please re-enter!!!" "    else        break     The fidone6.  parameter ##  $0  is the name of the bash file;#  $?  is the return value of the previous instruction;#  $*  All parameters that are called by the script;#  [email protected]  basically the same as the $* above. The difference is that:#      $*  returns a string separated by a space, and  [email protected]  Returns multiple strings;#  $#  is the number of all positional arguments; # #ab = ($ (LS)) echo  "1: " ${ab[@]}echo  "2: " ${#ab [@]} echo  "3: " [email protected]echo  "4: " $ #echo   "5: " $?7.  regular   regular expression matching " =~ " [[  $XX  =~ ^ $XXX  ]] the =~ regular expression matching  operator within a double brackets test expression.$ [[  "#  Test2 " =~ ^# ]] && echo yes | |  echo noyes8.  Intercept string assumptions: f= "/a/b/c/d/e.name.ext" is:# basename  $fe. name.ext# dirname   $F &NBSP;&NBSP;&NBSP;&NBSP;/A/B/C/D Special usage: Use #,%,* in ${} to output the specified content 1) Remove the first/, and the left string  # echo ${f#* /}A/B/C/D/E.NAME.EXT2) Remove the last 1/, and the left string # echo ${f##*/}e.name.ext3) and remove the last/, and the right string # echo ${ F%/*}&NBSP;/A/B/C/D4) Remove the first/, and the string on the right # echo ${f%%/*}  (empty) above, is based on "/" To do the segmentation, you can also use "." To separate, may wish to try. # echo ${f#*.}  name.ext# echo ${f##*.} ext# echo ${f%.*} /a/b/c/d/e.name# echo ${f%%.*}/a/b/c/d/e9.  script into the background, output to log sh  test.sh >1.log 2>&1 &sh test.sh >/dev/null 2>&1  & Here you need to understand the role of several small things:/dev/null    understand the empty device, which is a special file, where the function is to discard the contents of the output 2>&1         0  input   1  output   2  error   Here is 2 Redirect to 1&             put test.sh into the background to do, think, and what other ways can you put the program in the background? 10.  pipe through the "|"   Import the output into another program's input for processing, such as:echo  ' abc, def '  |cut -d  ', '  -f 111.  Command Trace debug sh -x test.sh12.  shortcut ctrl + a  switch to command line start Ctrl + b - move  back a charCtrl + c  terminate command ctrl + d  exit shell,logoutctrl +  e  switch to the end of the command line ctrl + l  clear the contents of the screen ctrl + k  cut after the clear cursor ctrl + q  Restore Brush screen ctrl + r  in the history command find ctrl + s  can be used to stay in the current ctrl + u  before clearing the contents of the clipping cursor CTRL  + y  paste the characters just removed ctrl + z  into the background to run!!   Repeat the last command ↑ (ctrl+p)   Show previous command ↓ (CTRL + N)   Show next command!$  displays the last parameter of the system 13.  The shell passes Chinese to Python with exception export lang= "en_US. UTF-8 ";/usr/local/bin/python3  xxx.py

Bash Shell usage notes