Here is the list of all the Bash debugging posts:
When you run a Bash script it will only usually print the output of the commands that you execute. This is fine until you need to figure out why it isn’t doing what you want.
Bash comes with a couple of options that will show you exactly what is being run and also print out the lines of the script in the order of the execution output, including comments.
These options are:
- -v Print shell input lines as they are read.
- -x Print commands and their arguments as they are executed.
These options can either be set at the beginning of the script and will get executed every time you run the script e.g.:
#!/bin/bash
set -vx
Or you can invoke them when you run the script as follows:
$ bash -vx script.sh
What they do will be easier to see with an example. The following script (yes I know the echo is redundant, it’s just something to see working):
$ cat script.sh
#!/bin/bash
# Set a variable for the directory we want to search in
dir="files/"
# Find all the files in the files directory
files=$(find "$dir" -type f)
# Echo the file list
echo "$files"
When this script runs it prints out all the names of all the files in the “files” directory:
$ ./script.sh
files/1.txt
files/2.txt
files/3.txt
files/4.txt
files/5.txt
files/6.txt
files/7.txt
files/8.txt
files/9.txt
files/10.txt
If the script is invoked with the -v command it prints every line of the bash script and also prints their output:
$ bash -v script.sh
#!/bin/bash
# Set a variable for the directory we want to search in
dir="files/"
# Find all the files in the files directory
files=$(find "$dir" -type f)
# Echo the file list
echo "$files"
files/1.txt
files/2.txt
files/3.txt
files/4.txt
files/5.txt
files/6.txt
files/7.txt
files/8.txt
files/9.txt
files/10.txt
The -x option will print the commands as they are executed. It will also, substitute any variables with the values you set:
$ bash -x script.sh
+ dir=files/
++ find files/ -type f
+ files='files/1.txt
files/2.txt
files/3.txt
files/4.txt
files/5.txt
files/6.txt
files/7.txt
files/8.txt
files/9.txt
files/10.txt'
+ echo 'files/1.txt
files/2.txt
files/3.txt
files/4.txt
files/5.txt
files/6.txt
files/7.txt
files/8.txt
files/9.txt
files/10.txt'
files/1.txt
files/2.txt
files/3.txt
files/4.txt
files/5.txt
files/6.txt
files/7.txt
files/8.txt
files/9.txt
files/10.txt
Using both together lets you see each line as it appears in the script and how it is executed:
$ bash -vx script.sh
#!/bin/bash
# Set a variable for the directory we want to search in
dir="files/"
+ dir=files/
# Find all the files in the files directory
files=$(find "$dir" -type f)
++ find files/ -type f
+ files='files/1.txt
files/2.txt
files/3.txt
files/4.txt
files/5.txt
files/6.txt
files/7.txt
files/8.txt
files/9.txt
files/10.txt'
# Echo the file list
echo "$files"
+ echo 'files/1.txt
files/2.txt
files/3.txt
files/4.txt
files/5.txt
files/6.txt
files/7.txt
files/8.txt
files/9.txt
files/10.txt'
files/1.txt
files/2.txt
files/3.txt
files/4.txt
files/5.txt
files/6.txt
files/7.txt
files/8.txt
files/9.txt
files/10.txt
Using -vx is very helpful to see exactly what you have written, and how Bash is executing it.
Finally, you may only want to get the debug output for a specific part of a script. You can do this by setting and then unsetting the -vx switches.
First put set -vx before the problem part and unsetting it with +vx after e.g.:
#!/bin/bash
set -vx
# Set a variable for the directory we want to search in
dir="files/"
set +vx
# Find all the files in the files directory
files=$(find "$dir" -type f)
#Echo the file list
echo "$files"
This now prints the verbose output for the part of the script you are having the problem with e.g.:
$ ./script.sh
# Set a variable for the directory we want to search in
dir="files/"
+ dir=files/
set +vx
+ set +vx
files/1.txt
files/2.txt
files/3.txt
files/4.txt
files/5.txt
files/6.txt
files/7.txt
files/8.txt
files/9.txt
files/10.txt