Few questions about realization of commands using bash scripts

Question

I need to create bash script file, which after its launch does following.

1) Suppose after launching the script I enter the text 'abc'. Then the script must automatically generate commands

cat text.txt
echo '123' > text.txt
echo 'abc' >>text.txt
echo '456' >>text.txt

2) After doing that I need to execute some commands (like compile the program etc.).

How to create bash script which executes these tasks?

Answer 1

If the top and bottom are fixed, it can be something like:

cat top.txt /dev/stdin bottom.txt > text.txt 
# with cat, - works the same as /dev/stdin

or

{ 
  echo 123 #top.txt
  cat
  echo 456 #bottom.txt
} > text.txt 

followed by your compilation commands

cat top.txt /dev/stdin bottom.txt > text.txt
gcc whatever 

The first line should be a shebang line specifying your interpreter, unless you're OK with /bin/sh

 #!/bin/bash
 cat top.txt /dev/stdin bottom.txt > text.txt
 gcc whatever

If you then mark the script executable with chmod +x the_script, ./the_script will be equivalent to /bin/bash ./the_script.

If you want the scrip to abort a command fails, start it with set -e (or make the shebang line (#!/bin/bash -e).

Edit:

cat expects a whole file (until you enter ctr-d -- the end of file marker). If you want just one line, you can do read -r something; printf '%s\n' "$something" or head -n1.

You can read the help pages of the commands with man $command or help $command.