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https://askubuntu.com/q/939627
Title:
What does $# mean in bash?
ID:
/2017/07/25/What-does-__-mean-in-bash_
Created:
July 25, 2017
Edited: February 11, 2022
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September 15, 2024
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$# is typically used in bash scripts to ensure a parameter is passed. Generally, you check for a parameter at the beginning of your script.
For example, here’s a snippet of a script I was working on today:
if [[ $# -ne 1 ]]; then
echo 'One argument required for the file name, e.g. "Backup-2017-07-25"'
echo '.tar will automatically be added as a file extension'
exit 1
fi
To summarize $# reports the number of parameters passed to a script. In your case, you passed no parameters and the reported result is 0.
Other # uses in Bash
The # is often used in bash to count the number of occurrences or the length of a variable.
To find the length of a string:
myvar="some string"; echo ${#myvar}
returns: 11
To find the number of array elements:
myArr=(A B C); echo ${#myArr[@]}
returns: 3
To find the length of the first array element:
myArr=(A B C); echo ${#myArr[0]}
returns: 1 (The length of A, 0 is the first element as arrays use zero-based indices/subscripts).