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1

I have a dash script, and I need to parse $1, which is a string containing two parts separated by ':', such as foo:123. I would like to to save foo in $X and 123 in $Y.

I thought I could use read:

$ echo "foo:123" | tr ':' ' ' | read X Y

but that does not work (no error given)

$ echo $X

gives empty line as output.

Why does my read construct not work? And how could I achieve my goal (any solution, does not have to use read)

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2  
Parameter expansion works: var="foo:123"; echo "${var#*:}"; echo "${var%:*}" –  jasonwryan Apr 26 at 19:36
    
echo doesn't survive the pipe. You can use Process Substitution to achieve what you want: E.g read -r X Y < <(echo "foo:123" | tr ':' ' '); echo "$X" –  val0x00ff Apr 26 at 19:38
    
@val0x00ff There's no process substitution in dash. –  Gilles Apr 26 at 19:40
    
@val0x00ff - process substitution works in bash, but not in dash. –  Martin Vegter Apr 26 at 19:40
    
Oh my mistake. I didn't check that! Now I learned something! THanks –  val0x00ff Apr 26 at 19:41

2 Answers 2

up vote 4 down vote accepted

In dash, each command in Pipelines run in subshell (zsh and AT&T ksh, the rightmost command in pipelines doesn't), so variables X and Y are no longer existed when your command done.

Simply, you can use Parameter Expansion, try:

$ set -- foo:123
$ X=${1%:*}
$ Y=${1#*:}

The example is used for interactive session.

Inside your script, you don't need set -- foo:123.

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@uonglm - thankss, that works. Why do I need the set -- ? –  Martin Vegter Apr 26 at 19:49
    
@MartinVegter: set used for set or unset shell options and positional parameters. In this example, set -- foo:123 set $1 to foo:123. -- marked end of options, let you work with variables start with a dash -. –  cuonglm Apr 26 at 19:54
2  
They don't have to run in subshells. All but one have to run in subshells. In AT&T ksh or zsh the rightmost one doesn't. In bash and dash they all run in subshells. –  Stéphane Chazelas Apr 26 at 22:20
    
@StéphaneChazelas: Thanks, updated my answer. I can find the documentation for zsh, but ksh can't. Can you show me what part of ksh documentation mention this behaviour? –  cuonglm Apr 27 at 3:54
    
@cuonglm, for ksh, I don't think it's documented in the man page, but it's been the case since the 80s so it's fairly wide-spread knowledge. Note that only applies to the AT&T (original) and zsh implementations of ksh, not the pdksh/mksh... one. –  Stéphane Chazelas Apr 27 at 11:09
up vote 3 down vote

You can also use the split+glob operator (leave a variable unquoted) (and which you used for no apparent reason in your question):

IFS=:     # configure the split part to use : as the delimiter
set -f    # disable the glob part
set -- $1 # $1 is split on : and parts are stored in $1, $2...
X=$1 Y=$2

You can also do:

printf '%s\n' "$1" | {
  IFS=: read -r X Y
  printf '%s\n' "$X"
}

In dash like bash (but contrary to AT&T ksh or zsh), all parts of a pipeline run in subshells (they need to run in different processes anyway since they run concurrently). Above, we need the part that uses that $X to be in that subshell that reads the output of printf and sets $X.

Note that those two solutions behave differently if $1 contains a newline character or more than one : character.

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