Shell Scripting: RegEx in if Statement

Question

I can't seem to figure out how to write a regex correctly in an if statement. I wanted it to print out all the lines with "End Date" in it.

NUMBERS contains a text file with the following content:

End Date    ABC ABC ABC ABC ABC ABC
05/15/13    2   7   1   1   4   5  
04/16/13    4   3   0   1   3   6  
03/17/13    6   9   3   8   5   9  
02/18/13    8   2   7   1   0   1  
01/19/13    1   9   2   2   5   2  
12/20/12    7   2   7   1   0   1

Here is a snippet of my code that I am having problems with:

if [ -f $NUMBERS ]
then
        while read line
        do
                if [ $line = ^End ]
                then
                        echo "$line"
                else
                        echo "BROKEN!"
                        break                   
                fi
        done < $NUMBERS
else
        echo "===== $NUMBERS failed to read  ====="
fi

The output is:

Broken!

You don't need regular expressions for something this simple: [[ $line == End* ]] — glenn jackman
– glenn jackman, Commented May 16, 2013 at 0:21
Does your file actually contain the > characters at the start of each line? — glenn jackman
– glenn jackman, Commented May 16, 2013 at 0:22
@glennjackman I was doing the regex for the ones with dates, but it wasn't working, so I did it for the first line to make sure nothing was going wrong. The actual file does not contain the ">". — dalawh
– dalawh, Commented May 16, 2013 at 2:07

Liam · Accepted Answer · 2022-09-21 07:47:45Z

14

if you're using bash, try =~:

...
if [[ $line =~ ^End ]]

Note that the following will NOT work:1

if [[ "$line" =~ "^End" ]]

edited Sep 21, 2022 at 7:47

Liam

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answered May 15, 2013 at 23:22

datguy

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3 Comments

Dietrich Epp Over a year ago

Note that this is not portable to all shells.

dalawh Over a year ago

@datguy What does "=~" do?

Kevin Over a year ago

@dalawh =~ tells bash to match against a regex instead of a literal string.

tripleee · Accepted Answer · 2019-03-23 06:18:59Z

4

The portable solution is to use case which supports wildcards (glob wildcards; not actual regular expressions) out if the box. The syntax is slightly freaky, but you get used to it.

while read -r line; do
    case $line in
        End*) ... your stuff here 
            ... more your stuff here
            ;;   # double semicolon closes branch
    esac
done

edited Mar 23, 2019 at 6:18

answered May 16, 2013 at 3:43

tripleee

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6 Comments

tripleee Over a year ago

@cnst Indeed, this should be portable all the way back to the original Bourne sh. (No guarantees for csh, which some *BSD variants still - inexplicably - seem to cherish.)

jimmymcheung Over a year ago

Is it possible to use [0-9]* or equivalent? I want to check if the variable is 0 or positive integer.

tripleee Over a year ago

@jimmymcheung Try stackoverflow.com/a/18620446/874188

jimmymcheung Over a year ago

Thanks for your suggestion @tripleee , in the meantime, I found [ -z "${inter##*[!0-9]*}" ] would also work, never think of ## in if statement!

tripleee Over a year ago

The ## parameter expansion should be reasonably portable, i.e. it's in POSIX but might not be supported by legacy shells like Solaris sh. I really don't see how this discussion belongs here, though.

|

Bill · Accepted Answer · 2013-05-16 02:31:35Z

-1

You can use the following to check if the line begins with End

if [[ "$line" == End* ]]

You can use the following if you want to use regex.

if [[ "$line" =~ ^End* ]]

http://tldp.org/LDP/abs/html/comparison-ops.html

Also note that it is recommended to always quote your variables.

http://tldp.org/LDP/abs/html/quotingvar.html

edited May 16, 2013 at 2:31

answered May 15, 2013 at 23:27

Bill

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6 Comments

dalawh Over a year ago

I thought there was no difference between = and ==. Is there? Also, what is the difference from using one [] and two []?

Bill Over a year ago

if [ "$a" = "$b" ] -- does a string comparison, but == with [[ ]] can be used for regex searching. Pls look at the link I gave in the answer. Hope it helps. Copied from that link -- "The == comparison operator behaves differently within a double-brackets test than within single brackets."

Bill Over a year ago

can you show me your code, it should work...i have tested it.

Bill Over a year ago

@dalawh I had a small typo. I updated the answer. Pls use End* and see if that works.

tripleee Over a year ago

[ is the basic old Bourne sh command test which does not support regex. The [[ variant is a Bash (and ksh etc) extension with improved robustness around corner cases as well as more capabilities, including =~ regex matching. The correct string equality operator is = though Bash leniently permits == as an alias.

|

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