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avatar Ubuntu

echo command breaks lines, being used in loops

br flag
Josef Klimuk

In my file 'foo' there is a first line: 'a b', and the second line: 'c d':

$ cat foo
a b 
c d

I want to print in terminal in loop these two lines: one after another:

$ for i in $(cat foo); do echo $i; done

But in the output 'echo' command breaks the order, so instead of having:

a b
c d

I actually have:

a
b
c
d
42
0 + 0
pLumo avatar
in flag
pLumo
`for i in $(cat foo)` does not loop lines but words split by your field separator (`IFS`).
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br flag
Josef Klimuk
So, how I can loop lines / change IFS mode to loop lines, not words?
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pLumo avatar
in flag
pLumo
If you tell us what you actually want to do, we can help you do it properly using `sed` or so.,
1
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Score:4
pLumo avatar Ubuntu
in flag
pLumo

for i in $(cat foo) does not loop lines but words (or fields) split by your field separator $IFS. It default to \n\t (space, newline, tab).

If you change your field separator to newline only, you can use your command as is (Disclaimer: please read below!):

IFS=$'\n'
for i in $(cat foo); do echo $i; done

You might want to make a backup of IFS to restore it later:

OLD_IFS="$IFS"
.
.
.
IFS="$OLD_IFS"

Output:

a b
c d

However, this is considered bad practice.

  • Bit Better: while IFS= read -r line; do ... done < file
  • Much better: Use a dedicated tool for your task such as grep, sed or awk.

Please read Why is using a shell loop to process text considered bad practice?.

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br flag
Josef Klimuk
How I can use 'grep' instead?
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pLumo avatar
in flag
pLumo
We cannot know, because we don't know what you want to do, see my last comment on your question.
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Score:2
rɑːdʒɑ avatar Ubuntu
ph flag
rɑːdʒɑ

You can use while read for this purpose

[/tmp]$ while read line ; do echo $line ; done < foo                                     
a b
c d

Adding a separator for your understanding

[/tmp]$ while read line ; do echo $line ; echo "----" ; done < foo                       
a b
----
c d
----
[/tmp]$
0 + 0
br flag
Josef Klimuk
If I want to go with your solution, but also to add a conditional statement inside my for loop, then what will be "$i"? I mean, something like this: for i in $(while read line ; do echo $line ; done < foo); do if [ grep -c a "$i" -gt 1 ]; then echo "$i";fi;done
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hr flag
steeldriver
Note that `$line` should be quoted `"$line"` for similar reasons (otherwise it will be word split into separate tokens - try with `a b` in place of `a b` for example).
1
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hr flag
steeldriver
... as well, `read` would still remove any leading whitespace and expand escape sequences by default, so more generally if you want to echo lines *exactly* you'd want `while IFS= read -r line; do ...` or (to include a possibly improperly terminated last line) `while IFS= read -r line || [[ -n "$line" ]]; do ...`
3
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bac0n avatar
cn flag
bac0n
@JosefKlimuk ```while read -r; do if ! grep -oe 'a' <<< "$REPLY" | sed -n 2q1; then echo "$REPLY"; fi; done < text```, `-c` only count matched lines, and since you only have one line it will never be more than 1. I like to use sed to count occurrence as it can return an exit status.
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