When working with bash scripts, it’s often necessary to create timestamps for logging or tracking purposes. In this tutorial, we’ll explore how to work with timestamps in bash, including creating timestamp variables and formatting output.
Creating a Timestamp Variable
To create a timestamp variable in a bash script, you can use the date command. The date command allows you to format the output using various specifiers. For example, %T will give you the current time in the format HH:MM:SS.
However, simply assigning the output of date to a variable won’t work as expected:
timestamp="(date +"%T")"
echo $timestamp # prints (date +"%T")
This is because the command substitution is not executed when the variable is assigned. To get around this, you can use command substitution with backticks or $():
timestamp=$(date +"%T")
echo $timestamp # prints the current time
Note that this will give you the time at which the variable was initialized, not the current time.
Getting the Current Timestamp
If you want to get the current timestamp every time you access the variable, you can define a function instead of a variable:
timestamp() {
date +"%T" # current time
}
echo $(timestamp) # prints the current time
This way, every time you call $(timestamp), it will give you the current time.
Formatting Timestamps
The date command allows you to format the output using various specifiers. Here are some common ones:
%Y: year in four digits (e.g., 2022)%m: month as a zero-padded decimal number (e.g., 07)%d: day of the month as a zero-padded decimal number (e.g., 04)%H: hour (24-hour clock) as a zero-padded decimal number (e.g., 14)%M: minute as a zero-padded decimal number (e.g., 30)%S: second as a zero-padded decimal number (e.g., 00)%N: nanosecond as a zero-padded decimal number (e.g., 000000000)
You can combine these specifiers to create custom formats. For example:
timestamp=$(date +"%Y-%m-%d_%H-%M-%S")
echo $timestamp # prints the current timestamp in the format YYYY-MM-DD_HH-MM-SS
Common Timestamp Formats
Here are some common timestamp formats:
- ISO 8601:
YYYY-MM-DDTHH:MM:SSZ(e.g., 2022-07-04T14:30:00Z) - Unix timestamp: seconds since the epoch (January 1, 1970, 00:00:00 UTC)
- YYYYMMDD_HHMMSS:
YYYYMMDD_HHMMSS(e.g., 20220704_143000)
You can use the following commands to get these formats:
iso_timestamp=$(date --utc +"%FT%TZ")
unix_timestamp=$(date +%s)
yyyymmdd_hhmmss=$(date +"%Y%m%d_%H%M%S")
In conclusion, working with timestamps in bash scripts requires understanding how to use the date command and its various specifiers. By defining functions or variables and using custom formats, you can create timestamps that meet your needs.