if runs a command and checks its exit status: 0 counts as true. The usual command to check is [ ], the test command, which compares things and checks files. Spaces inside the brackets are required.
#!/bin/bash
if [ -f "$1" ]; then
echo "$1 is a file"
elif [ -d "$1" ]; then
echo "$1 is a folder"
else
echo "No such thing: $1"
fiThe common tests: -f (a file exists), -d (a folder exists), -e (anything exists), -z (text is empty). Text is compared with = and !=. Numbers use letters: -eq (equal), -ne (not equal), -lt (less than), -gt (greater than), -le and -ge (or equal). ! turns a test around: [ ! -d backups ].
if [ "$#" -lt 1 ]; then
echo "Usage: $0 name"
exit 1
fiFor quick checks, chain commands. a && b runs b only if a succeeded; a || b runs b only if a failed. mkdir -p out && cd out changes into the folder only if it was made, and cd /data || exit 1 stops the script if the folder isn't there.
Bash also has [[ ]], which is more forgiving about quotes and adds pattern matching ([[ $file == *.jpg ]]). It's fine to use in Bash scripts; [ ] works in every shell.
Which test is true when notes.txt exists and is an ordinary file?
| if cmd; then … elif … else … fi | Run a block when cmd succeeds. |
|---|---|
| [ -f x ], [ -d x ], [ -e x ] | File, folder, anything exists. |
| [ -z "$s" ], [ "$a" = "$b" ] | Empty text; equal text. |
| -eq -ne -lt -gt -le -ge | Number comparisons. |
| ! | Not. |
| a && b, a || b | b if a worked; b if a failed. |