How to extract the first two characters of a string in shell scripting?


For example, given:


I want to extract just:

8/7/2018 6:34:35 AM

Probably the most efficient method, if you're using the bash shell (and you appear to be, based on your comments), is to use the sub-string variant of parameter expansion:

pax> long="USCAGol.blah.blah.blah"
pax> short="${long:0:2}" ; echo "${short}"

This will set short to be the first two characters of long. If long is shorter than two characters, short will be identical to it.

This in-shell method is usually better if you're going to be doing it a lot (like 50,000 times per report as you mention) since there's no process creation overhead. All solutions which use external programs will suffer from that overhead.

If you also wanted to ensure a minimum length, you could pad it out before hand with something like:

pax> long="A"
pax> tmpstr="${long}.."
pax> short="${tmpstr:0:2}" ; echo "${short}"

This would ensure that anything less than two characters in length was padded on the right with periods (or something else, just by changing the character used when creating tmpstr). It's not clear that you need this but I thought I'd put it in for completeness.

Having said that, there are any number of ways to do this with external programs (such as if you don't have bash available to you), some of which are:

short=$(echo "${long}" | cut -c1-2)
short=$(echo "${long}" | head -c2)
short=$(echo "${long}" | awk '{print substr ($0, 0, 2)}'
short=$(echo "${long}" | sed 's/^\(..\).*/\1/')

The first two (cut and head) are identical for a single-line string - they basically both just give you back the first two characters. They differ in that cut will give you the first two characters of each line and head will give you the first two characters of the entire input

The third one uses the awk sub-string function to extract the first two characters and the fourth uses sed capture groups (using () and \1) to capture the first two characters and replace the entire line with them. They're both similar to cut - they deliver the first two characters of each line in the input.

None of that matters if you are sure your input is a single line, they all have an identical effect.

6/29/2017 1:38:44 AM

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