Variables as commands in Bash scripts

Question

I am writing a very simple Bash script that tars a given directory, encrypts the output of that, and then splits the resultant file into multiple smaller files since the backup media doesn’t support huge files.

I don't have a lot of experience with Bash scripting. I believe I’m having issues with quoting my variables properly to allow spaces in the parameters. The script follows:

#! /bin/bash  # This script tars the given directory, encrypts it, and transfers # it to the given directory (likely a USB key).  if [ $# -ne 2 ] then     echo "Usage: `basename $0` DIRECTORY BACKUP_DIRECTORY"     exit 1 fi  DIRECTORY=$1 BACKUP_DIRECTORY=$2 BACKUP_FILE="$BACKUP_DIRECTORY/`date +%Y-%m-%dT%H-%M-%S.backup`"  TAR_CMD="tar cv $DIRECTORY" SPLIT_CMD="split -b 1024m - \"$BACKUP_FILE\""  ENCRYPT_CMD='openssl des3 -salt'  echo "$TAR_CMD | $ENCRYPT_CMD | $SPLIT_CMD"  $TAR_CMD | $ENCRYPT_CMD | $SPLIT_CMD  say "Done backing up" 

Running this command fails with:

split: "foo/2009-04-27T14-32-04.backup"aa: No such file or directory

I can fix it by removing the quotes around $BACKUP_FILE where I set $SPLIT_CMD. But, if I have a space in the name of my backup directory, it doesn't work. Also, if I copy and paste the output from the "echo" command directly into the terminal, it works fine. Clearly there's something I don't understand about how Bash is escaping things.

Answer 1

Simply don't put whole commands in variables. You'll get into a lot of trouble trying to recover quoted arguments.

Also:

1. Avoid using all-capitals variable names in scripts. It is an easy way to shoot yourself in the foot.
2. Don't use backquotes. Use $(...) instead; it nests better.

---

#! /bin/bash  if [ $# -ne 2 ] then     echo "Usage: $(basename $0) DIRECTORY BACKUP_DIRECTORY"     exit 1 fi  directory=$1 backup_directory=$2 current_date=$(date +%Y-%m-%dT%H-%M-%S) backup_file="${backup_directory}/${current_date}.backup"  tar cv "$directory" | openssl des3 -salt | split -b 1024m - "$backup_file" 

Answer 2

eval is not an acceptable practice if your directory names can be generated by untrusted sources. See BashFAQ #48 for more on why eval should not be used, and BashFAQ #50 for more on the root cause of this problem and its proper solutions, some of which are touched on below:

If you need to build up your commands over time, use arrays:

tar_cmd=( tar cv "$directory" ) split_cmd=( split -b 1024m - "$backup_file" ) encrypt_cmd=( openssl des3 -salt ) "${tar_cmd[@]}" | "${encrypt_cmd[@]}" | "${split_cmd[@]}" 

Alternately, if this is just about defining your commands in one central place, use functions:

tar_cmd() { tar cv "$directory"; } split_cmd() { split -b 1024m - "$backup_file"; } encrypt_cmd() { openssl des3 -salt; } tar_cmd | split_cmd | encrypt_cmd