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I know that the SHELL allows variable assignment to take place immediately before a command, such that IFS=":" read a b c d <<< "$here_string" works...

What I was wondering is do such assignments not work when done with compound statements such as loops? I tried something like IFS=":" for i in $PATH; do echo $i; done but it results in a syntax error. I could always do something like oldIFS="$IFS"; IFS=":"; for....; IFS="$oldIFS", but I wanted to know if there was any way I could make such inline assignments work for compound statements like for loops?

computronium
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2 Answers2

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for is a reserved word and as such follows special rules:

The following words shall be recognized as reserved words:

! { } case do done elif else esac fi for if in then until while

This recognition shall only occur when none of the characters is quoted and when the word is used as:

  • The first word of a command

  • The first word following one of the reserved words other than case, for, or in

  • The third word in a case command (only in is valid in this case)

  • The third word in a for command (only in and do are valid in this case)

If you try

IFS=":" for i in $PATH; do echo $i; done

then by the rules above that is not a for loop, as the keyword is not the first word of the command. But you can get the desired output with

tr ':' '\n' <<< "$PATH"               #Bash, Ksh, Zsh
printf "%s\n" "$PATH" | tr ':' '\n'   #Any standard shell

where tr replaces each : by a newline.

You may be familiar with this valid approach:

while IFS= read -r line; do

while is the first word of the command, and the IFS assignment applies to read, so all is OK.

Quasímodo
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  • Regarding the while loop solution... For POSIX sh: Use a here document (<<) to input a variable into the loop. Don't do echo "$example" | while ... because that creates a subshell and won't maintain variable state out of the loop. Bash has here string (<<<) support so use that if available. – Elliot Killick Jul 24 '23 at 14:17
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Shell macro assignements are permitted before simple commands (such as read) but not before compound statements like for.

It is recommanded to use:

OIFS=$IFS
IFS=:
some complex commands
IFS=$OIFS
schily
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