There comes a time in every shell script where a decision has to be made.
To make a decision in bash the following if then else syntax can be used:
if [ condition ]
[ elif [ condition ]
Anything in non bold [ brackets ] is optional, and they should not be typed but the bold in them is required. Anything in bold must be typed. Statements are generally other commands or flow control statements.
To give an example of how to use bash if then else statements in the real world take the following scenario:
A system administrator has a strict habit of firing people that have too many .png files. He checks the systems regularly and makes sure that nobody has too many. The following script will display a message depending on the number of .png’s that are in the directory.
gif_count=$(ls | grep .png | wc -l)
echo “Number of PNG’s: $gif_count”
if [ $gif_count -lt 10 ]
echo “He will be happy, you have less than 10 files”
elif [ $gif_count -gt 10 ] && [ $gif_count -lt 20 ]
echo “Consider deleting some files”
echo “you have too many files, DELETE SOME BEFORE HE FINDS OUT!”;
Cases are similar to if statements in that they match a expression and perform a task if the pattern is matched.
bash case syntax:
case expression in
pattern1 ) statements ;;
pattern2 ) statements ;;
This is fairly simple and some people find this easier than doing if statements for simple logic. Take the following real world example:
The system administrator has recently gone on a bigger power trip than before. Since people got wise about using png’s and started saving images in other file formats he is now monitoring png’s gif’s and jpg’s. To combat the problem, you can use a case to count how many files you have of each type. (This is intended as an example, there are many ways to accomplish this task, this is just to demonstrate how cases work)
#set all variables to 0
# start loop
for wc in $(ls); do
case “$wc” in
*png ) let png=$png+1 ;;
*gif ) let gif=$gif+1 ;;
*jpg ) let jpg=$jpg+1 ;;
# end loop
echo “Png’s $png”;
echo “gif’s $gif”;
echo “jpg’s $jpg”;
There you have it, two ways to make basic decisions in bash. Just figure out what you want to do then use an if then else, or a case statement to do the logic. I myself prefer if statements over cases as they make more sense to me and I find it easier to perform logic within ifs.
Programs used in this post
ls, echo, grep, wc