Detailed explanation of script debugging mechanism in bash

> bash -x debug.sh 
+ echo 'First line'
First line # The output has no plus sign ++ date # The command is executed first to replace the two plus signs because the command is nested in echo + echo 'Print datetime: Thu 26 Mar 2020 08:21:28 PM CST Done.'
Print datetime: Thu 26 Mar 2020 08:21:28 PM CST Done.

> export PS4='+${BASH_SOURCE}:${LINENO}:${FUNCNAME[0]}: '
> bash -x debug.sh 
+debug.sh:3:: echo 'First line'
First line
++debug.sh:4:: date
+debug.sh:4:: echo 'Print datetime: Thu 26 Mar 2020 08:35:59 PM CST Done.'
Print datetime: Thu 26 Mar 2020 08:35:59 PM CST Done.

trap 'echo "VARIABLE-TRACE> \$variable = \"$variable\""' DEBUG
variable=29
let variable++
let variable*=5
exit 0

# Output as follows VARIABLE-TRACE> $variable = ""
VARIABLE-TRACE> $variable = "29"
VARIABLE-TRACE> $variable = "30"
VARIABLE-TRACE> $variable = "150"

trap 'echo $BASH_COMMAND' ERR

#!/usr/bin/bash

func1 () {
  for i in `seq 0 3`
  do
    echo -e "Level$i\t `caller $i`"
  done
}

func2 () {
  func1
}

func3 () {
  func2
}

func3
caller 0 # must be called in a function, otherwise there will be no output exit 0

set -x
date
set +x

> bash script1.sh # No need to add debug parameters The script starts now.

+ date
Fri 28 Feb 2020 06:23:04 PM CST
+set +x

This is a string: black
And this is a number: 9

> set -v
> date
date 
Fri 28 Feb 2020 06:54:47 PM CST
> set -x # Parameters can be accumulated. The effect of date # -v + the effect of date # -xFri 28 Feb 2020 06:55:37 PM CST
> set +vx # Cancel parameter set +vx

> ls ?
x86_64-pc-linux-gnu-library
> set -f # disable metacharacter matching > ls ?
ls: cannot access '?': No such file or directory
> touch ?
> ls ?
'?'
> rm ?
> set +f -x # Option x can also be used to display detailed error information > aaa
+aaa
+ '[' -x /usr/lib/command-not-found ']'
+ /usr/lib/command-not-found --aaa

Command 'aaa' not found, did you mean:

 command 'aha' from deb aha (0.5-1)
 command 'jaaa' from deb jaaa (0.8.4-4)
 command 'aa' from deb astronomical-almanac (5.6-6)

Try: sudo apt install <deb name>

+ return 127

#!/bin/bash -xv

echo "debug message: now attempting to start w command"; w # Use ; to sort the commands to be executed echo "Variable VARNAME is now set to $VARNAME."

> set -o # View the switch status of all options > set -o | grep xtrace
xtrace off
> set -x # equivalent to set -o xtrace
> set -o | grep xtrace 
+ grep --color=auto xtrace
+ set -o
xtrace on
> set +x # equivalent to set +o xtrace
+set +x
> set -o | grep xtrace
xtrace off

> unset $VAR; echo $VAR

> set -u # equivalent to set -o nounset
> echo $var
bash: var: unbound variable

> set -C # equivalent to set -o noclobber
> touch test
> date > test
bash: test: cannot overwrite existing file

> set -f # equivalent to set -o noglob
> touch *
> ll *
-rw-rw-r-- 1 remilia remilia 0 Mar 1 20:09 '*'