Needed simple script/loop for input command, execute and output within textfiles

Much simpler one,

$ while read -r line; do whatis "$line"; done < cmdlist.txt > cmdinfo.txt
cut (1)              - remove sections from each line of files
lshw (1)             - list hardware
top (1)              - display Linux processes
awk (1)              - pattern scanning and text processing language
sensors (1)          - print sensors information

Use the below command to write the result into a file.

while read -r line; do whatis "$line"; done < cmdlist.txt > cmdinfo.txt

Something like this:

#! /bin/bash
in_file=$1
out_file=$2

while read command
do
    whatis $command 
done < $in_file >> $out_file

I have chosen to leave stderr untouched, since if you have a command in there for which whatis information is not available, you should be made aware of that (and look elsewhere, if needed).

I think that you can get properly result with following command.

$ for i in `cat cmdlist.txt`;do whatis $i 2>&1;done | sed "s,: nothing appropriate.,,g" > cmdinfo.txt

In fact,

$ for i in `cat cmdlist.txt`;do whatis $i 2>&1;done

The command, first command's one part will show outputs as following .

cut (1)              - remove sections from each line of files
lshw (1)             - list hardware
top (1)              - display Linux tasks
.: nothing appropriate.
.: nothing appropriate.
.: nothing appropriate.
tr (1)               - translate or delete characters

you can do it with whatis $(cat cmdlist.txt), but it's output includes following lines.

.: nothing appropriate.
.: nothing appropriate.
.: nothing appropriate.

above sed command remove some output lines you needless.

Question is changed as now. If all of the lines of cmdlist.txt can be listed from whatis, Can use following command as the simple way.

whatis `cat 1.txt` 2>/dev/null > cmdinfo.txt

If you just need the lines that can be completely listed from whatis, Can use following command as the simpleway.

whatis `cat 1.txt` > cmdinfo.txt

However you can choose one way of various ones.

Very simply:

for i in $(cat cmdlist.txt); do whatis $i ; done > cmdinfo.txt

This loops through every entry (i) in the output of $(cat cmdlist.txt), and runs a whatis on very entry. Example output:

cut (1)              - remove sections from each line of files
lshw (1)             - list hardware
top (1)              - display Linux processes
awk (1)              - pattern scanning and processing language
sensors (1)          - print sensors information

N.B. even though in many examples you will find i used, you don't have to use it - you can use most plain alphanumeric strings- e.g.:

for jnka127nsdn in $(cat input.txt); do whatis $jnka127nsdn ; done

To do it without parsing cat:

while read -r i ; do whatis $i ; done < cmdlist.txt