How to echo lines of file using bash script and for loop

Question

I have a simple file called dbs.txt I want to echo the lines of that file to the screen using a for loop in bash.

The file looks like this:

db1
db2
db3
db4

The bash file is called test.sh it looks like this

for i in 'cat dbs.txt'; do
echo $i
done
wait

When I run the file by typing:

bash test.sh

I get the terminal output:

cat dbs.txt

instead of the hoped for

db1
db2
db3
db4

The following bash file works great:

cat dbs.txt | while read line
do
echo "$line"
done

Why doesn't the first script work?

fwiw, it's a better practice to use while read instead of for i in $(cat dbs.txt) — Alex
– Alex, Commented Nov 8, 2012 at 21:50
the first one (even when fixed) would not echo line-by-line if any of the lines contain spaces — doubleDown
– doubleDown, Commented Nov 9, 2012 at 8:06
Your single quotes should be back ticks: <pre>for i in cat dbs.txt; do echo $i done wait</pre> — Bryon
– Bryon, Commented Nov 15, 2017 at 18:50

Todor · Accepted Answer · 2012-11-09 13:18:32Z

18

You can use the shell builtin read instead of cat. If you process just a single file and it's not huge, perhaps the following is easier and more portable than most solutions:

#!/bin/sh

while read line
do
    printf "%s\n" "$line"
done < "$1"

I remember reading somewhere that printf is safer than echo in the sense that the options that echo accepts may differ across platforms. So building a habit of using printf may be worthwhile.

For description of the read builtin, check the manual pages of your shell.

answered Nov 9, 2012 at 13:18

Todor

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Comments

Linus Thiel · Accepted Answer · 2012-11-08 21:40:02Z

11

You need to execute a sub-shell and capture the output, like this:

for i in `cat dbs.txt`; do
echo $i
done
wait

Note the backticks ` instead of the single-quotes.

In bash you can also use $(command):

for i in $(cat dbs.txt); do
echo $i
done
wait

answered Nov 8, 2012 at 21:40

Linus Thiel

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1 Comment

Gilles Quénot Over a year ago

See my post, backticks are discouraged.

Gilles Quénot · Accepted Answer · 2012-11-08 23:53:33Z

5

You need command substitution shell feature. This require the POSIX expression $().

Please, don't use backticks as others said.

The backquote (`) is used in the old-style command substitution, e.g.

foo=`command`

The

foo=$(command)

syntax is recommended instead. Backslash handling inside $() is less surprising, and $() is easier to nest. See

http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html -> 2.6.3 Command Substitution
http://mywiki.wooledge.org/BashFAQ/082

Despite of what Linus G Thiel said, $() works in sh, ash, zsh, dash, bash...

edited Nov 8, 2012 at 23:53

answered Nov 8, 2012 at 23:42

Gilles Quénot

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Comments

Pedro del Sol · Accepted Answer · 2012-11-08 21:40:40Z

1

you need backticks rather than single quotes

` vs '

answered Nov 8, 2012 at 21:40

Pedro del Sol

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1 Comment

Gilles Quénot Over a year ago

See my post, backticks are discouraged.

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