Bash iterate over a list of strings

Question

I have this bash script:

for opt in string1 string2 string3 ... string99
do somestuff

It works, but I would like to replace the explicit listing of my strings with a file which actually contains all the strings; something like this:

strings=loadFromFile
for opt in $strings
do somestuff

How should I do this?

• Gilles 'SO- stop being evil': See Why is while IFS= read used so often, instead of IFS=; while read..? and In while IFS= read.., why does IFS have no effect?

Answer 1

while read VAR is probably best here, as it handles per-line input. You can redirect it from a file, e.g.:

while IFS= read -r THELINE; do
 echo "..$THELINE"
done </path/to/file

That'll give you each line prepended with ".."

For your example case:

while IFS= read -r opt; do
 #somestuff $opt
done </path/to/file

See Why is `while IFS= read` used so often, instead of `IFS=; while read..`? for explanations.

• Didi Kohen: Putting cats into pipes is bad behavior from a memory management point of view. especially when using "filter" built-in commands. @rush's answer is better, and what I use on a daily basis.
• glenn jackman: It also puts the while loop into a subshell, which can cause confusing behaviour if update variable values in the loop (the values will disappear when the subshell exits)
• Raphael: If somestuff includes another read, this seems not to work properly.
• IBBoard: @raphael - in what way does it not "work properly"? I just tried nested reads and it works fine.
• Raphael: @IBBoard See this new question.

Answer 2

while IFS= read -r opt
do 
 some_stuff
done < file_with_string

See Why is `while IFS= read` used so often, instead of `IFS=; while read..`? for explanations.

Answer 3

The while IFS= read -r line; do ...; done < aFile is the best answer

If your strings do not contain whitespace or \[*?, you could do

for word in $(< aFile); do something; done

$(< file) is a bash feature that reads the file (like cat except without having to spawn a new process).

Answer 4

Why don't you use the readarray builtin (requires bash >= 4.0)?

readarray < FileNameFromWhereToLoad # push every line of 
 # 'FileNameFromWhereToLoad' onto 
 # $MAPFILE by default
for i in $MAPFILE ; do
 echo $i
done

• Didi Kohen: This also requires newer version of bash.
• user1146332: i wouldn't call bash >= 4 new (release date was February 2009)!
• Didi Kohen: Since he's using 2.05a, it is newer than what the asker has. In addition, some companies have avoided GPLv3 versions of bash, so do not have it in their system.
• user1146332: Do you have a crystal ball around? But yes, i agree with you about your last statement, i will add the version requirement of readarray to my answer.
• user1146332: I didn't read that. Thanks for the hint ;) Maybe my answer is of value for somebody who will search for an answer for a similar question!?

Answer 5

My advice:

cat INPUTFILE| {
 declare -a LINES
 mapfile -t LINES
 for line in "${LINES[@]}"
 do
 somestuff
 done
}

• michelemarcon: Syntax error: bad substitution
• manatwork: Works for me. @michelemarcon, is your bash 4.00 or never?
• michelemarcon: GNU bash, version 2.05a.0(1)-release (arm-unknown-linux-gnu)
• manatwork: That is ancient. Please specify this requirement in the question.
• AnneTheAgile: osx 10.12.2 has ancient bash ; $ bash --version GNU bash, version 3.2.57(1)-release (x86_64-apple-darwin16) Copyright (C) 2007 Free Software Foundation, Inc.