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35.21 Using IFS to Split Strings

It might not be obvious why the Bourne shell has an IFS (internal field separator) shell variable. By default, it holds three characters: SPACE, TAB, and NEWLINE. These are the places that the shell parses command lines. So what?

If you have a line of text - say, from a database - and you want to split it into fields, the IFS variable can help. Put the field separator into IFS temporarily, use the shell's set ( 44.19 ) command to store the fields in command-line parameters; then restore the old IFS .

For example, the chunk of a shell script below gets current terminal settings from stty -g ( 42.4 ) , which looks like this:

2506:5:bf:8a3b:3:1c:8:15:4:0:0:0:11:13:1a:19:12:f:17:16:0:0

The shell parses the line returned from stty by the backquotes ( 9.16 ) . It stores x in $1 . This trick stops errors if stty fails for some reason - without the x , if stty made no standard output, the shell's set command would print a list of all shell variables. Then 2506 goes into $2 , 5 into $3 , and so on. The original Bourne shell can only handle nine parameters (through $9 ); if your input lines may have more than nine fields, this isn't a good technique. But this script uses the Korn shell, which (along with bash ) doesn't have that limit.

#!/bin/ksh
oldifs="$IFS"
# Change IFS to a colon:
IFS=:
# Put x in $1, stty -g output in $2 thru ${23}:
set x `stty -g`
IFS="$oldifs"
# Window size is in 16th field (not counting the first "x"):
echo "Your window has ${17} rows."

Because you don't need a subprocess to parse the output of stty , this can be faster than using an external command like cut ( 35.14 ) or awk ( 33.11 ) .

There are places where IFS can't be used because the shell separates command lines at spaces before it splits at IFS . It doesn't split the results of variable substitution or command substitution ( 9.16 ) at spaces, though. Here's an example - three different ways to parse a line from /etc/passwd :

% 

cat splitter


#!/bin/sh
IFS=:
line='larry:Vk9skS323kd4q:985:100:Larry Smith:/u/larry:/bin/tcsh'
set x $line
echo "case 1: \$6 is '$6'"
set x `grep larry /etc/passwd`
echo "case 2: \$6 is '$6'"
set x larry:Vk9skS323kd4q:985:100:Larry Smith:/u/larry:/bin/tcsh
echo "case 3: \$6 is '$6'"

% 

./splitter


case 1: $6 is 'Larry Smith'
case 2: $6 is 'Larry Smith'
case 3: $6 is 'Larry'

Case 1 used variable substitution and case 2 used command substitution; the sixth field contained the space. In case 3, though, with the colons on the command line, the sixth field was split: $6 became Larry and $7 was Smith . Another problem would have come up if any of the fields had been empty (as in larry::985:100: etc... )-the shell would "eat" the empty field and $6 would contain /u/larry . Using sed with its escaped parentheses ( 34.10 ) to do the searching and the parsing could solve the last two problems.

- JP


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